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Cover images: Federal Highway Administration
Technical Report Documentation Page

1. Report No. 

FHWA-HOP-17-003

2. Government Accession 

No.

3. Recipient’s Catalog No.

4. Title and Subtitle 

Synthesis of Variable Speed Limit Signs

5. Report Date

May 2017

6. Performing Organizations Code

7. Authors

 Bryan Katz, Jiaqi Ma, Heather Rigdon, Kayla Sykes, 

Zhitong Huang, Kelli Raboy

8. Performing Organization 

Report No.

9. Performing Organization Name and Address

Leidos

11251 Roger Bacon Drive 

Reston, VA 20190

ToXcel, LLC

7140 Heritage Village Plaza

Gainesville, VA 20155

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

 Contract No. 

DTFH61-12-D-00045

Task T-5009

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period 

Covered

Research Synthesis, 

March 2016–December 2016

14. Sponsoring Agency Code

HOTM

15. Supplementary Notes

Jimmy Chu, HOTM, Government Task Manager (GTM)

16. Abstract

Variable speed limit (VSL) systems utilize information on traffic speed, occupancy, and volume 

detection, weather, and road surface conditions to determine the appropriate speeds at which drivers 

should be traveling, given current roadway and traffic conditions. The use of VSL during less than 

ideal conditions, such as heavy traffic and adverse weather, can improve safety by decreasing the 

risks associated with traveling at speeds that are higher than appropriate for the conditions and by 

reducing speed variance in traffic. In addition, VSL can be used to dynamically manage speeds 

during planned (rush hour congestion) and unplanned (incidents) events. Used in conjunction with 

managed lanes and other active traffic management strategies, VSL can respond to downstream 

congestion to eliminate or delay bottlenecks and mitigate the possibility of crashes.

Based on a comprehensive literature review along with agency interviews to gather information on 

existing, deactivated and planned VSL systems, this synthesis provides a comprehensive review of 

current practices on VSL operations, particularly experiences from deployments in the United States, 

and to identify successful and best practices from the following perspectives: planning and policy, 

design, deployment, and standards, operations and maintenance, and outcomes.

17. Key Words

Variable speed limit; planning and policy; design; deployment; 

standards; operations and maintenance; active traffic 

management

18. Distribution Statement 

 No restrictions. 

19. Security Classif. (of this report) 

Unclassified

20. Security Classif. (of this 

page) Unclassified

21. No of Pages

 100

22. Price 

 N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.

iii

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

APPROXIMATE CONVERSIONS TO SI UNITS

SYMBOL WHEN YOU KNOW MULTIPLY BY TO FIND SYMBOL

LENGTH

In. inches 25.4 millimeters mm

ft feet 0.305 meters m

yd yards 0.914 meters m

mi miles 1.61 kilometers km

AREA

in.2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME

fl oz fluid ounces 29.57 milliliters mL

gal gallons 3.785 liters L

ft3 cubic feet 0.028 cubic meters m3

yd3 cubic yards 0.765 cubic meters m3

NOTE: volumes greater than 1,000 L shall be shown in m3

MASS

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t")

TEMPERATURE (exact degrees)

°F Fahrenheit 5 (F-32)/9 or 

(F-32)/1.8 

Celsius °C

ILLUMINATION

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS

lbf poundforce 4.45 newtons N

lbf/in.2 poundforce 

per square inch

6.89 kilopascals kPa

SI* (MODERN METRIC) CONVERSION FACTORS

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003).
iv

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

APPROXIMATE CONVERSIONS TO SI UNITS

SYMBOL WHEN YOU KNOW MULTIPLY BY TO FIND SYMBOL

LENGTH

mm millimeters 0.039 inches in.

m meters 3.28 feet ft

m meters 1.09 yards yd

km kilometers 0.621 miles mi

AREA

mm2 square millimeters 0.0016 square inches in.2

m2 square meters 10.764 square feet ft2

m2 square meters 1.195 square yards yd2

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2

VOLUME

mL milliliters 0.034 fluid ounces fl oz

L liters 0.264 gallons gal

m3 cubic meters 35.314 cubic feet ft3

m3 cubic meters 1.307 cubic yards yd3

MASS

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton")1.103 short tons (2000 lb) T

TEMPERATURE (exact degrees)

°C Celsius 1.8C+32 Fahrenheit °F

ILLUMINATION

lx lux 0.0929 foot-candles fc

cd/m2 candela/m2 0.2919 foot-lamberts fl

FORCE and PRESSURE or STRESS

N newtons 0.225 poundforce lbf

kPa kilopascals 0.145 poundforce 

per square inch 

lbf/in2

SI* (MODERN METRIC) CONVERSION FACTORS (CONTINUED)

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003).
v

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Table of Contents

EXECUTIVE SUMMARY.................................................................................................................. 1

PURPOSE OF REPORT ......................................................................................................................1

METHODOLOGY ...............................................................................................................................1

KEY RESULTS......................................................................................................................................2

CHAPTER 1. INTRODUCTION........................................................................................................ 3

SHORT DESCRIPTION.......................................................................................................................3

HISTORY AND PREVIOUS RESEARCH............................................................................................4

CHAPTER 2. RESEARCH METHODOLOGY.................................................................................... 5

LITERATURE REVIEW........................................................................................................................5

AGENCY OUTREACH........................................................................................................................5

SYNTHESIS AND ANALYSIS.............................................................................................................7

CHAPTER 3. VARIABLE SPEED LIMIT STATE OF THE PRACTICE ................................................. 9

REPRESENTATIVE VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES ......................9

VARIABLE SPEED LIMIT SYSTEMS USED FOR CONGESTION-BASED ACTIVE TRAFFIC 

MANAGEMENT................................................................................................................................10

VARIABLE SPEED LIMIT SYSTEMS USED FOR WORK ZONES..................................................12

VARIABLE SPEED LIMIT SYSTEMS USED FOR WEATHER.........................................................12

DEACTIVATED VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES..........................13

LESSONS LEARNED ........................................................................................................................14

CHAPTER 4. FINDINGS ................................................................................................................ 17

PLANNING AND POLICY................................................................................................................17

DESIGN, DEPLOYMENT AND STANDARDS ................................................................................21

OPERATIONS AND MAINTENANCE.............................................................................................24

OUTCOMES ......................................................................................................................................32
vi

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 5. CONCLUSION – BENEFITS AND CHALLENGES OF VARIABLE 

SPEED LIMIT SYSTEMS................................................................................................................ 39

KEY BENEFITS..................................................................................................................................39

KEY CHALLENGES.......................................................................................................................... 40

VARIABLE SPEED LIMIT KEY CONSIDERATIONS .......................................................................41

FUTURE OF VARIABLE SPEED LIMITS..........................................................................................42

APPENDIX A. BIBLIOGRAPHY .................................................................................................... 45

APPENDIX B. AGENCY INTERVIEW SUMMARY......................................................................... 51

Table of Contents (Continued)
vii

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

List of Figures

Figure 1. Photo. New Jersey Turnpike variable speed limit and variable message signing. . . . . . . 22

Figure 2. Chart. Weather responsive system for Oregon Route 217. . . . . . . . . . . . . . . . . . . . . . . . . . 28

List of Tables

Table 1. Summary of variable speed limit data sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Table 2. Description of the variable speed limit systems considered in the report. . . . . . . . . . . . . . . 9

Table 3. Lessons learned by State agencies from variable speed limit implementations. . . . . . . . . . 14

Table 4. Fundamental variable speed limit infrastructure requirements. . . . . . . . . . . . . . . . . . . . . . 21

Table 5. Washington State Department of Transportation speed limit reference. . . . . . . . . . . . . . . . 29

Table 6. Variable speed limit system practices and field results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

ix

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

List of Acronyms

ATM active traffic management

ATMS active/advanced traffic management system

ATSMS active traffic safety management system

CAV connected automated vehicle

CMS changeable message sign

CCTV closed-circuit television

CV connected vehicle 

DOT department of transportation

FHP Florida Highway Patrol

ITS intelligent transportation system

HOT high occupancy toll 

HOV high occupancy vehicle

MOE measures of effectiveness

MUTCD Manual on Uniform Traffic Control Devices

RWIS road weather information system

TMC transportation management center

TMC PFS Transportation Management Center Pooled Fund Study

VMS variable message signs

VSL variable speed limit

1

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

EXECUTIVE SUMMARY

Variable speed limit (VSL) systems utilize information on traffic speed, occupancy, volume 

detection, weather, and road surface conditions to determine the appropriate speeds at which 

drivers should be traveling, given current roadway and traffic conditions. The use of VSL during 

less than ideal conditions, such as heavy traffic and adverse weather conditions, can improve 

safety by decreasing the risks associated with traveling at speeds that are higher than appropriate 

for the conditions and by reducing speed variance among vehicles. In addition, VSL can be used 

to dynamically manage speeds during planned (rush hour congestion) and unplanned (incidents) 

events. Used in conjunction with managed lanes and other active traffic management (ATM) 

strategies, VSL can respond to downstream congestion to eliminate or delay bottlenecks and 

mitigate the possibility of crashes.

VSL has been successfully implemented in Europe since the 1960s, with deployments in countries 

such as the Netherlands and Germany generating significant benefits. While the United States has 

deployed VSL systems for safety purposes over a few decades, during the past 15 years there has been 

a renewed interest in expanding VSL use among the States in order to achieve operational benefits.

PURPOSE OF REPORT

The purpose of this synthesis report is to provide a comprehensive review of current practice on 

VSL operations, particularly experiences from deployments in the United States, and to identify 

successful and best practices from the following perspectives:

•	 Planning and policy.

•	 Design, deployment, and standards.

•	 Operations and maintenance.

•	 Outcomes.

METHODOLOGY

The research team conducted a comprehensive literature review along with agency interviews to 

gather information on existing, deactivated and planned VSL systems. Literature reviewed included 

published research, policy, and operating documentation from departments of transportation (DOT) 

and cooperating law enforcement agencies, and public-facing outreach material, such as websites. 

Thirteen agencies were identified for interview; the research team established a list of questions and 

conducted the interviews via telephone.
2

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

KEY RESULTS

Several VSL systems have been implemented successfully in the United States for congestionbased active traffic management, work zones, and weather. The research team identified the 

current state of the practice, lessons learned, and best practices from implementations to date. 

Key findings:

•	 VSL planning processes should use the systems engineering process to clearly identify and 

communicate objectives, requirements, and anticipated costs/benefits.

•	 VSL infrastructure requirements generally include changeable speed limit signs, weather/

environmental sensors, traffic speed/volume sensors, and communications equipment to 

transmit data. Infrastructure repair and replacement is a considerable cost that should be 

recognized early. Durable signing is especially important to maintaining a functional 

system.

•	 Selection of speed control algorithms greatly depends on the primary functional 

requirements. The success of VSL systems relies to a significant degree on driver 

compliance, and therefore it is essential that regulatory systems are consistently enforced. 

However, in real-world deployments, particularly those in the United States, many systems 

are still advisory or cannot be enforced as intended. 

•	 Dynamic speed limit setting control algorithms can be difficult to calibrate due to data 

quality or delays and driver behavior. Agencies should anticipate the need for periodic 

adjustment and enhancement of algorithms over time based on observed roadway and 

driver characteristics.

•	 In most cases, VSL implementations can generate preferential system benefits in terms of 

traffic efficiency and safety. Because VSL systems have different deployment goals and 

corresponding system design, varying system benefits result. Speed homogenization 

projects usually use simple algorithms in response to real-time traffic, road, and other 

conditions (e.g., weather, work zone, incidents, visibility, etc.), and they usually focus more 

on safety. Multi-objective projects, mostly as a part of ATM systems, report positive effects 

on mobility, safety, and even environmental impacts.

•	 State and local statutes and agency policies should ensure that a VSL system is enforceable 

if a regulatory speed limit is desired. It is also beneficial to begin meeting with law 

enforcement partners early to discuss concerns and processes for enforcing the VSL 

system, if enforcement is required.

•	 When calculating the system’s cost, maintenance, operations, staffing, evaluations, and 

end-of-life replacement costs must be considered.

•	 Future deployments of VSL/speed harmonization could incorporate emerging connected/

automated vehicle technologies; real-time collection, storage, processing, and decisionmaking using emerging big data sources will be necessary for the next generation of VSL/

speed harmonization systems.
3

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 1. INTRODUCTION

SHORT DESCRIPTION

The advancement of technological solutions in surveillance and control systems for traffic operations 

has led to significant reductions in the cost of implementing actively managed components of 

transportation systems. The benefit of these systems, particularly on highway-grade facilities, can be 

substantial. A few deployments have demonstrated the success of active traffic management (ATM) 

strategies. Recent studies conducted in Europe found that ATM applications have resulted in up to 22 

percent of capacity increases and 30 percent of incident reductions. The key component in many of 

these actively managed systems is the variable speed limit (VSL) system. While the United States has 

installed VSL systems as far back as the 1960s on systems such as the New Jersey Turnpike, there has 

been a renewed interest in expanding their use in the United States during the past 15 years in order to 

achieve both operational benefits as well as proven safety benefits. The level and amount of positive 

impacts, however, vary from site to site, and there is a great interest in understanding the actual 

benefits under different operational scenarios.

Variable speed limits are typically installed on interstate highways or high-speed arterials and are 

used for three primary functions that can improve safety and operations: reducing congestion, 

reducing speeds during inclement weather, and managing speeds during traffic events such as work 

zones and incidents. Depending on State statutes and policies, speed limits can be either regulatory 

or advisory. Agencies use various data to inform the appropriate speed limit for current conditions. 

Using VSL, agencies can take into account a variety of conditions such as traffic volume, operating 

speeds, weather information, sight distance, and roadway surface conditions when posting speed 

limits. This data is typically transmitted to a transportation management center (TMC) and analyzed 

automatically with an algorithm or reviewed by agency personnel who make decisions about the 

speed limit. Some agencies use systems that will automatically change the speed limit based on 

the data received and others use data monitoring by personnel to change the speed limit manually. 

A majority of agencies use a hybrid approach with the VSL updating automatically supplemented 

with oversight by agency staff that have the option of overriding the automated system to manually 

change the speed limit when warranted.

VSL provides many benefits for improving roadway safety and operations. The use of VSL 

systems to manage speed during inclement weather or other challenging driving conditions can 

improve safety by decreasing the risks associated with traffic moving at speeds that are higher 

than appropriate for the conditions. In addition, VSL can dynamically manage speeds during 

planned (rush hour congestion) and unplanned (incidents) circumstances. Used in conjunction with 

managed lanes and other ATM strategies, VSL can help eliminate or delay bottlenecks and mitigate 

the possibility of rear-end, sideswipe, and other collisions generally associated with slowed traffic 

on high-speed roadways.
4

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

HISTORY AND PREVIOUS RESEARCH

VSL has been successfully implemented in European countries for several decades, and 

deployments in countries such as the Netherlands and Germany have shown significant benefits. 

In Germany, VSL (i.e., speed harmonization) systems have been deployed since the 1960s, with 

installations on about 124 miles of highway. Germany’s experiences show that VSL has the 

potential to decrease crash rates and increase road capacity by five to 10 percent. The Netherlands 

has implemented VSL since the 1970s in order to manage traffic speed, mitigate effects of extreme 

weather, and improve safety. The United Kingdom has implemented VSL and hard shoulder 

running on the M-42 motorway. An evaluation over a 12-month period showed the application of 

VSL and hard shoulder running resulted in seven percent increase in capacity, a 

4 to 10 percent decrease in pollutants, and a 4 percent drop in fuel consumption.

In the United States, Michigan and New Jersey were the first two States to implement VSL. Speed 

limits at these two pioneer VSL sites were adjusted manually according to traffic conditions 

observed by traffic operations staff. Since 1990, VSL use in the United States has increased 

dramatically, with a renewed interest in expanding functionality to achieve operational benefits. 

VSL system complexity, in terms of infrastructure, signing, real-time detection and control 

algorithms, has significantly increased. 

Over the years, agencies and researchers have published reports evaluating the effectiveness of 

VSL systems. Lu et al. (2014) and Ma et al. (2016) are two examples of comprehensive reviews 

of VSL speed control algorithms and resulting benefits from deployed systems. Generally, these 

systems have been proven effective in one or multiple performance measures on traffic efficiency, 

safety, and environmental impacts, depending on project characteristics such as deployment goals 

and speed control algorithms. These reviews, however, rely only on published reports and focus 

heavily on academic research. Many important issues such as VSL planning, policy, standards, 

design, and maintenance are not addressed systematically in these or other existing literature on 

VSL or ATM. This synthesis has been developed to provide a more comprehensive review of VSL 

by using agency interviews and additional internal agency documents to complement material from 

published reports.
5

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 2. RESEARCH METHODOLOGY

The research team used two methods to collect information for analysis on existing policies, 

procedures, and practices by highway transportation agencies on variable speed limit (VSL) 

systems. A literature review compiled information from resources such as published research, 

dissertations, presentations, guidelines, and other relevant publications. To supplement the 

literature review, agencies operating VSL were interviewed to gather descriptions of their systems 

and background information on lessons learned. This report reviews all VSL systems but uses 

those that have been investigated through both a literature review and agency interviews as 

examples when discussing different VSL system components.

LITERATURE REVIEW

The research team conducted a comprehensive literature review using a variety of resources. This 

literature review included published research related to VSL planning, policy, and operations; 

resources obtained from operating agencies, policy manuals, operating documentation from 

operating and law enforcement agencies, and documentation directly related to VSL-instigated 

legislative action on speed limits; and additional public information campaign materials, such as 

web sites, that are used to interact with the public. The team reviewed VSL sites in the United States 

and other countries. A list of most relevant references is shown in Appendix A. 

AGENCY OUTREACH

In addition to the literature review, the research team selected representative VSL systems that are 

currently active and collected VSL data from corresponding agencies directly. The team documented 

how agencies are operating their respective VSL systems and their experiences using this particular 

traffic management strategy by interviewing agency representatives and reviewing various documents 

provided by highway transportation agencies. During the process of contacting agencies, the team 

identified some States that are considering VSL systems as well as those that have deactivated their 

VSL systems. Information collected from these agencies was documented as well. 

The data collection methods included phone interviews with agency staff and a review of various 

VSL documents and materials (e.g., video clips, reports, presentations, operations documents, etc.) 

provided by the agencies. The research team used the following resources to identify agencies using 

VSL systems:

•	 Results of the literature review.

•	 Historical information from Guidelines for the Use of Variable Speed Limit Systems in Wet 

Weather.

•	 Members of the Transportation Management Center Pooled Fund Study (TMC PFS).

•	 Personal knowledge of the research team.
6

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The research team interviewed 13 agencies about their VSL systems, 9 of which currently have 

active VSL systems. In addition to the phone interviews, the team also obtained VSL documents 

from nine agencies. These documents include VSL activation procedures, operations manuals, 

equipment/software specifications, signing protocols, algorithms, checklists, and internal/external 

educational materials. Table 1 summarizes the data sources.

Table 1. Summary of variable speed limit data sources.

Agency

Conducted

Phone Interview

Provided Documents

Used in Synthesis

Arizona Department of 

Transportation (DOT) Yes N/A

Florida DOT Yes Yes

Georgia DOT Yes Yes

Minnesota DOT Yes Yes

Missouri DOT Yes N/A

Nevada DOT Yes Yes

New Jersey Turnpike Authority Yes Yes

Oregon DOT Yes Yes

Pennsylvania Turnpike Yes N/A

Tennessee DOT Yes Yes

Virginia DOT Yes Yes

Washington State DOT Yes Yes

Wisconsin DOT Yes N/A

N/A = not applicable (no variable speed limit systems). 

The research team requested information in the following categories to gain a comprehensive 

description of each agency’s VSL system: 

•	 Planning and policies.

•	 Design, deployment, and standards.

•	 System operations and control.

•	 Maintenance and lifecycle costs.

•	 Costs and benefits.

•	 Liability issues.

•	 Enforcement issues.
7

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

To perform the interviews, the research team developed a list of questions and classified each 

as key or auxiliary. In consideration of agency staff time, interviews were scheduled to last 30 

minutes. During that time, the key questions were discussed first with the intention of collecting 

information on the auxiliary questions from agency-provided documents. Time permitting, 

some auxiliary questions were discussed during the calls as well. On some occasions, agencies 

provided feedback to the full set of questions and/or emailed supporting documents prior to a 

phone interview. These instances provided the team with an opportunity to become familiar with 

the agency’s VSL system to better tailor the phone discussion. The interview questions were not 

generally provided to an agency, but the interviews were completed by project team members using 

a form and gathering as much information possible during each interview. A summary of each 

interview is included in Appendix B. Agency Interview Summary. 

SYNTHESIS AND ANALYSIS

Both during and following each interview, all State representative responses were compiled 

and recorded in an Excel spreadsheet, which served as the research team’s main database. This 

database allowed the team to organize all responses into categories (e.g., general VSL information, 

setting speed limits, equipment and costs, enforcement, VSL signs, etc.). After all responses were 

appropriately categorized, the information was synthesized and incorporated in the appropriate 

sections of the final report. Additional documents provided by various agencies were also utilized 

to better understand and describe VSL systems located throughout the United States. 

9

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 3. VARIABLE SPEED LIMIT 

STATE OF THE PRACTICE

REPRESENTATIVE VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES

Variable speed limit (VSL) systems have been widely used in many of the States for various 

functional purposes. Table 2 briefly summarizes each focused VSL system that is investigated 

thoroughly through the literature review and agency interview. Note that planned and removed 

systems are not included in this table, although they are discussed throughout the report.1 The 

“Primary Functions” column may include any of the following descriptions: 

•	 Congestion: includes speed/incident management-related issues.

•	 Weather: includes visibility/pavement condition-related issues.

•	 Work zones. 

Note that some systems may include only VSL while others may include additional traffic 

management techniques (e.g. variable message signs (VMS), dynamic shoulder lanes, ramp 

metering, etc.). 

Table 2. Description of the variable speed limit systems considered in the report.

State Location

Length of 

System

(miles) Status Authority

Operation 

Type

Primary 

Functions

Florida I-4 10.5 Active Regulatory Hybrid Congestion

US 27 3 Active Regulatory Automated Congestion

Georgia I-285 36 Active Regulatory Hybrid Congestion, 

Work Zones

Minnesota I-35W 18 Temporarily 

Deactivated Advisory Automated Congestion

I-94 10 Temporarily 

Deactivated Advisory Automated Congestion

Nevada US 395 

Alternate 5 Active Regulatory Automated Weather (wind)

New Jersey NJ Turnpike 148 Active Regulatory Manual Congestion, 

Weather

1 The VSL systems included in this synthesis are a snapshot in time as of January 2016. A more comprehensive listing of all known planned 

and existing VSL systems is available at the Federal Highway Administration’s Active Transportation Demand Management Program 

website at http://www.ops.fhwa.dot.gov/atdm/approaches/adm_table/index.htm. 
10

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

State Location

Length of 

System

(miles) Status Authority

Operation 

Type

Primary 

Functions

Oregon

OR 213 Single 

intersection Active Regulatory Hybrid Congestion

OR 217 7 Active Advisory Automated Congestion, 

Weather

Tennessee I-75 9 Active Regulatory Hybrid Weather 

(fog)

Virginia

I-66 13 Active Advisory Automated Congestion, 

Work Zones

I-95 Express 

Lanes ~10 Active Regulatory Manual Congestion

Washington

I-90 (near 

Snoqualmie 

Pass)

25 Active Regulatory Hybrid Weather

US 2 23 Active Regulatory Hybrid Weather

I-5 8 Active Regulatory Automated Congestion

I-90 (Bellevue 

to Seattle) 10 Active Regulatory Automated Congestion

SR 520 8 Active Regulatory Automated Congestion

VARIABLE SPEED LIMIT SYSTEMS USED FOR CONGESTION-BASED ACTIVE 

TRAFFIC MANAGEMENT

VSL systems used for congestion-based active traffic management (ATM) are sometimes referred 

to as “speed harmonization systems.” The purpose of speed harmonization is to dynamically and 

automatically reduce speed limits in or before areas of congestion, accidents, or special events to 

maintain flow and reduce the risk of collisions due to speed differentials. They are usually used in 

conjunction with other ATM strategies such as queue warning and hard shoulder running. Note that 

the speed limits for VSL systems used for congestion are generally updated every 30 seconds to 

15 minutes. An interval of 1 to 5 minutes was found to be the most common practice. 

A regulatory, hybrid VSL system was installed along I-4 in Florida in order to efficiently manage 

the large volumes of traffic that regularly utilize this corridor. The system is 10.5 miles long, and 

there is currently no plan to change the length. This system was not built to manage traffic based 

on weather conditions, rather the main focus of the VSL system is to improve speed harmonization. 

VMS are used in conjunction with VSL to display relevant information to drivers along the 

roadway. Loop detectors and side-fire radar are used to collect various traffic data. The VSL 

system then uses this data to recommend an appropriate speed limit which can be based on the 

current speeds, volume, capacity, roadway geometry, etc. The operator may then accept or alter the 

system’s recommended speed limit.

Table 2. Description of the variable speed limit systems considered in the report. (Continued)
11

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The VSL system along US 27 in Florida is 3 miles long, regulatory, and automated. Much like 

the VSL system located on I-4 in Florida, the US 27 VSL system uses loop detectors and side-fire 

radar to determine appropriate speed limits; however, no VMS is used along the corridor. The US 

27 system was installed to improve safety by lowering vehicular speeds surrounding a school zone, 

thereby, reducing collisions and associated congestion levels. Note that there is presently no plan to 

alter the length of the VSL system on US 27. 

A VSL system was also installed on I-285 in Georgia, where sensors capture volume and speed 

information to calculate appropriate speed limits based on current traffic conditions. Although the 

36 mile, regulatory VSL system is fully automated, manual override may occasionally be necessary 

to properly handle more complex situations (e.g. work zones, etc.), which is discussed in the 

“Variable Speed Limit Systems Used for Work Zones” section below. Note that weather conditions 

are not included in the VSL algorithm.

The first deployment of VSL in Oregon was for a single intersection along Oregon Route (OR) 213, 

west of downtown Portland. This regulatory, hybrid system is still active due to its success, and it 

aims to regulate traffic and reduce congestion levels at the intersection. Note that this VSL system 

utilizes a single, side-mounted sign. 

The advisory system along OR 217 utilizes current traffic and existing weather conditions to 

automatically calculate and display variable speed limits, warn of queues ahead, and estimate travel 

times. The final displayed speed limit depends on which piece reports the most needed condition 

change (weather vs. congestion). OR 217 is divided into various subzones where radar and dual 

loops are utilized to capture real-time speed data. The displayed speed in each subzone is calculated 

as the lower of these two values: 1) 85th percentile speed, or 2) speed of downstream traffic +5-

10 mi/h (Mitchell, 2016). In addition to the VSL system on OR 217, the Oregon Department of 

Transportation (DOT) has installed active, weather- and speed-based curve warning systems at 

both ends of the corridor. Note that the weather-related algorithm for OR 217 is discussed in the 

“Variable Speed Limit Systems Used for Weather” section below. 

The 13-mile, automated, advisory VSL system along I-66 in Virginia was installed to manage 

the high volumes of traffic and related congestion issues existing along the corridor. Note that 

the system is also capable of regulating traffic surrounding work zones, which is discussed in 

the “Variable Speed Limit Systems Used for Work Zones” section below. The speed limits are 

determined by a smoothing speed algorithm, which establishes the current lowest speeds along 

the roadway and appropriately slows upstream traffic. In addition to the VSL signs, VMS are used 

to display vital information to drivers (e.g., “Congestion ahead,” etc.). Lane availability is also 

displayed along I-66 to designate which lanes are open and which lanes are closed to traffic. There 

is no plan to extend the current VSL system along I-66 mostly because there are already significant 

proposed geometric changes along the roadway. The Virginia DOT does not want to invest in more 

traffic flow technology until those changes are known. 

The VSL system on the I-95 Express Lanes was operational in December 2014. The manual, 

regulatory VSL system is approximately 10 miles long. The purpose of the VSL system along the 

I-95 Express Lanes is to control congestion. The I-95 Express Lanes also include high occupancy 

toll (HOT) lanes and lane management functionality (Earnest, 2015). 
12

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The statewide, 148-mile, regulatory VSL system along the New Jersey Turnpike used to be 

automatic but is currently manual due to the level of sensor maintenance required after repaving. 

The system is used to relieve congestion and also accounts for weather conditions. Note that the 

weather-related portion is discussed in the “Variable Speed Limit Systems Used for Weather” 

section below. In general, the speed limit is manually reduced to 45 mi/h in response to a 

downstream incident, except when poor weather conditions are a factor. VMS are posted next to the 

VSL to explain the reasoning behind the speed alteration. 

The automated, regulatory VSL systems along I-5, I-90 (Bellevue to Seattle), and SR 520 in 

Washington uses the same method to alter speeds. Downstream conditions are assessed, and the 

speed limits are updated every minute based on the results of the traffic evaluations. The posted 

speed limits may vary across lanes and throughout the corridor, although currently the system only 

allows differences between the HOV lane and General Purpose lanes and not between individual 

General Purpose lanes. In addition, VMS are used in conjunction with VSL within all three 

systems. Currently, there is no plan to expand or decrease the length of any of the VSL systems in 

Washington (I-5, I-90 (Bellevue to Seattle), and SR 520). If there are expansion or contraction plans 

in the future, the Washington State DOT will base that decision on engineering judgment rather 

than public opinion. 

VARIABLE SPEED LIMIT SYSTEMS USED FOR WORK ZONES

The VSL system on I-66 in Virginia is used not only to relieve congestion, but also to regulate 

traffic surrounding work zones. Relevant information and warnings related to work zones are 

displayed on VMS message boards along the corridor. In addition, overhead lane-use-control signs 

are used to denote lane availability (a green arrow is displayed when the lane is open to all traffic, 

and a red “X” is displayed when the lane is closed to all traffic), which is especially useful for 

traffic surrounding work zones. 

The VSL system located along I-285 in Georgia also accounts for work zones. Roadway 

construction is typically performed at night when traffic is lighter which consequently results in 

faster speeds. Georgia DOT will manually adjust the VSL when needed to reduce speeds in work 

zone areas. 

Note that a temporary VSL system was installed along the Woodrow Wilson Bridge, but the 

Virginia DOT removed the system once all construction tasks were complete. This system is also 

discussed in the “Deactivated Variable Speed Limit Systems in the United States” section below. 

VARIABLE SPEED LIMIT SYSTEMS USED FOR WEATHER

The Nevada DOT selected US 395 in Reno for the State’s first VSL implementation. The highway 

parallels I-580 and functions as an alternate route when the interstate is closed for high winds. The 

VSL system is approximately 5 miles long, automated, regulatory, and activates based on wind 

speeds. The system has experienced some hardware/software issues related to signing such as blank 

signs and inconsistent posted speed limits. Therefore, the Nevada DOT is considering a temporary 

deactivation of the system in order to improve overall functionality. 
13

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The manual VSL system located on the New Jersey Turnpike accounts for both congestion and 

weather conditions. The weather-related algorithm primarily focuses on visibility. Operator guidelines 

are provided to determine the appropriate speed limit based on the number of visible mile markers 

from a stationary location (e.g., 35 mi/h is used when three mile markers are visible, etc.). 

The OR 217 advisory, automated VSL System not only accounts for congestion levels, but it also 

accounts for current weather conditions. As mentioned previously, the final displayed speed limit 

depends on which piece reports the most needed condition change (weather vs. congestion). The 

weather-related algorithm calculates appropriate speed limits based on data collected from friction 

factor sensors. The weather-responsive system considers many variables (e.g. visibility, grip factor, 

surface condition, etc.) to determine the warning message displayed to drivers. 

The Tennessee DOT installed a regulatory, hybrid, weather-responsive VSL system along I-75 in 

Chattanooga, Tennessee. The system is approximately 9 miles long, and the Tennessee DOT does 

not currently plan to alter the length of the VSL corridor. Speeds are calculated based on current 

visibility due to fog conditions. This system reliably and instantly provides speed reduction to 

drivers along I-75 using environmental sensors which monitor current weather conditions. A single 

speed is set for the entire corridor, and a single display is used for all lanes at a particular location. 

The Virginia DOT is currently designing a regulatory, weather-responsive VSL system to regulate 

traffic along I-77. The proposed system will be 15 miles long, and there is no current strategy to 

alter the length of the planned VSL system along I-77. The system will be located in the Fancy 

Gap Area, which is rural and has low traffic volumes. Speed limits will be determined based on 

available visibility levels captured by very reliable sensors. The majority of the signing will be 

VMS, which will post messages related to speed limits and/or traffic management. 

There are two active, hybrid, and regulatory VSL systems used for weather-related issues in 

Washington State: I-90 (near Snoqualmie Pass) and US 2, which are 25 and 23 miles long, 

respectively. Currently, there is no plan to expand or decrease the length of either weather 

management system in Washington. A look-up table is used for both systems to manually determine 

the appropriate speed, which accounts for current pavement conditions, visibility, weather (i.e. 

rain, snow), and incidents. The VSL system also utilizes reliable sensors to calculate travel times 

based on speed converted from occupancy measurements. Signing is located on the roadside and/

or overhead along I-90 (near Snoqualmie Pass). In contrast, all VSL signing is located on the right 

side of the highway along US 2. 

DEACTIVATED VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES

The Missouri DOT installed a regulatory VSL system on I-270 in St. Louis. Law enforcement 

reported that they were uncertain of current speed limits and consequently reluctant to enforce the 

VSL. In response, the system was changed to advisory, but driver compliance became an issue so 

the system was ultimately deactivated. 

A hybrid VSL system was installed on the Woodrow Wilson Bridge along I-95 to regulate traffic 

during construction operations. However, the system was removed once construction was complete 

and the work zone was no longer necessary. 
14

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Minnesota DOT has temporarily turned off the VSL systems installed on I-35W and I-94 in the 

Minneapolis/St. Paul area. Both systems were advisory and operated automatically, but the lag 

in real-time data was an issue. Speed limits were determined using the average of data sent from 

single loops every 30 seconds. The time to do the math to get the average speed slowed down a 

change in speed limits based on current conditions. If the VSL systems are reactivated, Minnesota 

DOT will most likely decrease the length of the corridors. Due to maintenance issues with the 

signs, Minnesota DOT is considering either replacing them in kind or installing a single overhead 

sign as opposed to lane-by-lane signage. This would reduce the cost of installation as well as 

maintenance and operations costs. 

LESSONS LEARNED 

Agency staff offered insights into important lessons their organizations learned from their 

experience with VSL systems (Table 3). Advice was related to overall design, algorithm, and 

infrastructure. 

Table 3. Lessons learned by State agencies from variable speed limit implementations.

Washington State Department of Transportation (DOT) emphasized the importance of following 

the systems engineering process when designing variable speed limit (VSL) systems; let the 

corridor goals drive the operation needs, let the operation needs drive the system requirements, 

and let the system requirements drive the specifications. In addition, it is vital that all speed 

reductions be warranted and never without reason. 

Florida DOT mentioned a few lessons they have learned from the VSL system along I-4: 1) 

improved signing is necessary for comprehension and compliance, 2) overhead signs are ideal, 

and 3) involving law enforcement officials is key to observe compliance. The Florida DOT also 

stressed the importance of investing in durable signing that will not fade due to sunlight exposure 

and will remain comprehensible to drivers.

The Nevada DOT suggests “starting small” when implementing VSL for the first time. The VSL 

corridor along US 395 in Reno was selected since it is a smaller urban area (as compared to Las 

Vegas, for example), experiences lower traffic volume, has less exposure to public and media 

attention, and the VSL could be incorporated with a larger wind-warning system operated by 

the State DOT. Starting small has helped the agency learn the ins and outs of operating a VSL 

without encountering significant consequences (such as negative media coverage) that could 

impact future implementation.

The Oregon DOT noted that developing a VSL algorithm is incredibly challenging. The hardest 

aspect is generating a system that alters speed in a way that feels natural to drivers. Speed 

recovery from a reduced speed is one of the most difficult situations to code, and multiple 

iterations are necessary to develop a system that can be modeled to closely replicate human 

behavior while also incorporating the impacts of horizontal and vertical curvature, pavement 

conditions, weather, and other factors. The Oregon DOT also mentioned that they are willing to 

share their algorithm with other States upon request.
15

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The Oregon DOT also stated that a successful VSL deployment in any State will require a 

qualified professional who understands the algorithm and who is available on a regular basis 

for the first 6 months to 1 year of deployment for algorithm enhancement. Necessary algorithm 

alterations will largely depend on the characteristics of the surrounding area and types of drivers 

utilizing the VSL. 

The Virginia DOT highlighted one particularly difficult challenge when developing a speed 

setting algorithm: there are competing constraints between the assigned “safe speed” and 

actual driver behavior since many drivers travel much faster than the posted speed limit. When 

calculating a suitable VSL, the goal is to display a speed that is safe for travelers but also will not 

create increased variance.

Depending on how the VSL system is designed to operate, a single overhead sign, as opposed to 

lane-by-lane signage, can reduce installation, maintenance, and operations costs.

To encourage enforcement, discuss citation options with law enforcement. Instead of tying 

citations to a specific speed limit, law enforcement may be able to use other types of citations 

such as driving too fast for conditions.

Include additional information to help motorists understand the reason for the speed change. For 

example, use a changeable message sign to display messages such as SLOW TRAFFIC AHEAD.

Table 3. Lessons learned by State agencies from variable speed limit implementations. (Continued)

17

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 4. FINDINGS

As a component of active traffic management (ATM), variable speed limit (VSL) systems are 

subject to a systems engineering process2 and are generally operated under a set of rules from a 

concept of operations document. In addition to planning the technical components of the system, 

VSL operators must also consider policy implications related to administrative law, case, and law 

enforcement priorities and policies. The system management includes operations, maintenance, 

performance monitoring, and coordination with partners, including law enforcement, private 

roadside assistance services, and external partners who may request VSL implementations for 

special events. There are several considerations related to systems management that have been 

learned from past implementations.

PLANNING AND POLICY

Rationale

States have various reasons for implementing traffic control systems (e.g. managing traffic in 

congested areas and following roadway incidents, altering speeds due to current weather and/

or visibility conditions, controlling traffic surrounding work zones, modifying speeds based on 

pavement conditions, etc.). A few examples of varying rationales are discussed below. 

The Oregon Department of Transportation (DOT) chose to implement VSL along OR 217 due to 

large crash rates on the roadway (more than 230 crashes/year). Half of the crashes occurred in peak 

traffic hours, and rear-end collisions accounted for 70 percent of the crashes (Mitchell, 2016). 

The VSL system along US 27, a two-lane, divided, rural roadway in Florida, was installed to 

control high vehicular speeds surrounding a school zone. The goal of the VSL system was to 

increase safety by better controlling the traffic surrounding the school in both directions. 

The Virginia DOT considered utilizing ATM, including VSL, along I-66 to increase safety, 

decrease congestion, and improve environmental sustainability along the corridor. With these 

overall objectives in mind, various stakeholders brainstormed specific goals for the system, which 

included reducing the quantity and severity of collisions, decreasing travel times, increasing system 

reliability, improving safety surrounding construction zones, enhancing communication tactics to 

provide vital information to drivers, and lowering vehicle emissions and fuel consumption. (Iteris, 

Inc., 2011). 

2 The International Council on Systems Engineering defines systems engineering as an interdisciplinary approach that focuses on defining 

customer needs and required functionality early in the development cycle, documenting requirements, then proceeding with design 

synthesis and system validation while considering the complete problem. For more information on systems engineering and the Federal 

Rule for intelligent transportation system projects, visit http://ops.fhwa.dot.gov/int_its_deployment/sys_eng.htm. 
18

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The Virginia DOT is currently designing an active traffic safety management system (ATSMS) for 

I-77 due to two major incidents that occurred because of heavy fog conditions along the corridor. 

In 2005, 26 people were injured and 1 person was killed when approximately 50 vehicles collided 

due to extreme fog conditions. In 2010, another incident involving 70 vehicles occurred due to 

intense fog, where 16 people were injured and 2 people were killed. In addition, the crash in 2010 

negatively impacted the economy, costing about $8 million. Following these crash events, the Virginia 

DOT decided to implement various traffic control methods to improve safety along the roadway by 

decreasing the magnitude and severity of collisions due to weather conditions (URS, 2012). 

While it has not employed VSL systems, the Arizona DOT is currently in the process of 

designing a VSL system to counteract the State’s problem with dust storms. This issue occurs at 

a specific location in Arizona that is heavily impacted by such storms due to the area’s terrain and 

surrounding land uses. Arizona DOT is hopeful that the VSL system will increase safety along the 

roadway. If successful, Arizona DOT would consider implementing other VSL systems in more 

northern areas of the State which are negatively impacted by snow. Further information regarding 

Arizona’s future VSL plans may be found in Appendix B. 

Nevada DOT installed a VSL system on US-395 to reduce speeds during high wind events. The 

VSL is part of a larger wind-warning system and is tied to two road weather information systems 

(RWIS). One RWIS is located on the northern end of the valley and the other is located on the 

southern end. High wind speeds have a history of blowing over high profile vehicles on I-580. 

Therefore, once the wind is high enough, I-580 is closed and vehicles are redirected onto US-395. 

However, the wind can also affect vehicles on the alternate route, so a VSL system was installed to 

reduce speeds when warranted by conditions. Typical speed limits on US-395 are either 55 mi/h or 

50 mi/h. When one of the RWIS measures a 30 mi/h wind gust, the VSL is activated and all speeds 

are lowered to 45 mi/h. At least 30 minutes must pass without a 30+ mi/h wind gust measurement 

from either RWIS before the speed limits can return to 55 mi/h or 50 mi/h.

Initiation Process

One of the first steps in the planning process for the I-66 ATM system was to identify user needs. In 

order to determine these needs, the Virginia DOT held multiple meetings and forums with various 

stakeholders to gain their input regarding system design and then summarized these conversations 

in a Technical Consensus Memorandum. The identified needs were then transformed into overall 

project goals and objectives that would shape the final design of the system (Iteris, Inc., 2011). 

At the beginning of the planning process for the system along I-77, the Virginia DOT identified 

eighteen specific stakeholders and summarized the roles and responsibilities of each once the 

system is activated. The roles and responsibilities were categorized as (URS, 2012): 

•	 Responsible (those that work with intelligent transportation system (ITS) devices 

themselves).

•	 Accountable (those that can allow or reject operational decisions).

•	 Consulted (those that provide insight to others in the Responsible/Accountable groups).

•	 Informed (those that should always be updated and notified of system functionality).
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SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The Virginia DOT outlined the I-77 project goals based on former traffic incidents and previously 

completed safety studies, which focused on quantifying and categorizing collisions that occurred 

along the I-77 corridor. Stakeholders and countermeasures were discussed in these past studies; 

therefore, the Virginia DOT could apply any relevant findings/conclusions from these studies to the 

I-77 design plan. In addition to defining stakeholders, project goals and objectives were developed, 

and Measures of Effectiveness were outlined based on the project objectives. A list of potential 

countermeasures was also developed based on past research studies along I-77. The final, selected 

countermeasures were determined following various consultations with stakeholders/Virginia DOT 

employees and further study of the I-77 corridor (URS, 2012). 

Overarching Design and Operations Considerations

Currently, the VSL system along the NJ Turnpike is manually operated; however, the system used 

to be automatic. The automatic system relied on copper inductive loops located in the pavement 

to gather current traffic data, such as vehicle speed and volume. The automatic VSL system 

successfully and efficiently managed traffic conditions along the New Jersey Turnpike. However, 

the automatic system was switched to the current manual system due to the level of maintenance 

the copper inductive loops required. Any time the roadway was repaved, the inductive loops were 

damaged and needed to be repaired. Therefore, the system became manual and more reliable 

sensors (from Sensys) were installed along the roadway. The New Jersey Turnpike Authority noted 

that the Sensys sensors are very reliable, but they are spaced farther apart than the inductive loops, 

which creates a slight lag in responsiveness. In addition, sensor reinstallation is still included in 

all paving contracts to ensure proper sensor functionality. The New Jersey Turnpike Authority 

is currently working to restore the system’s former automatic capabilities in order to increase 

throughput and efficiency.

Public Outreach

Georgia’s first VSL system was activated in September 2014 as a speed management strategy on 

I-285. Much of the public’s reaction was negative, with many believing it to be a new mechanism 

for generating cash for the State. To help educate the public, the Georgia DOT adopted the slogan 

of “Slow Down to Get There Faster.” They developed an educational video to explain why the VSL 

system is being implemented, what it is, how it works, and the benefits drivers can expect. The video 

is posted on a dedicated VSL webpage (Georgia Department of Transportation, 2015) along with other 

educational information and materials, such as a fact sheet. In addition to the webpage, the Georgia 

DOT established an email address (VSL@dot.ga.gov) where the public can send their comments. 

Minnesota implemented VSL systems on I-35W and I-94, but both are currently turned off. The 

systems were slow to respond to real-time conditions, which ultimately caused the public to lose 

trust in the speed limits. Consequently, the systems were turned off and the Minnesota DOT is 

reevaluating them to make improvements. The Minnesota DOT received very few comments from 

the public when the VSL were activated. Most questions asked about the meaning of the messages. 

For example, drivers expressed confusion about whether the displayed speed indicated the speeds 

ahead or the recommended speed. Of interest, the Minnesota DOT only received two inquiries after 

the VSL were turned off; both were to ask why the system was off.
20

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Nevada did not use an aggressive public relations campaign before activating the VSL, but a press 

release was issued. This was due in part to the nature of US 395 serving as an alternate route in 

a smaller community. Public reaction to the VSL on US 395 has been mixed with positive and 

negative feedback. The negative responses have primarily been from homeowners because they 

most often see when issues occur with the signing. For example, hardware problems have caused 

the signs to go blank. In response to a request by the Highway Patrol, Nevada DOT installed 

beacons on the VSL signs that flash when the speed changes. Homeowners have complained that 

the beacons are too bright. To address this issue, the DOT has temporarily disabled the beacons but 

plans to try a dimmer in the future as a more permanent solution.

When the VSL system was first implemented along OR 217, the Oregon DOT received some 

feedback indicating driver confusion about speed limit reduction. However, after improving the 

speed algorithm, public feedback has been positive. The Oregon DOT stressed the importance of 

public notification about the purpose of the VSL system and why the speed is being reduced. 

The Washington State DOT reported that the VSL systems along I-90 (near Snoqualmie Pass) 

and along US 2 were well-received by the public. However, the public had reservations regarding 

the VSL systems along I-5, I-90 (Bellevue to Seattle), and SR 520 during the first few months of 

deployment. But, after refining the algorithm and lowering the speed threshold, the public is now in 

favor. The Washington State DOT found that during periods of extreme congestion, such as stopand-go traffic, the public feedback was negative regarding the concept of displaying a system floor 

threshold speed such as 30 mi/h.

The University of Florida evaluated the VSL system along I-4. Part of their evaluation included a 

survey that captured driver’s opinions of the VSL system. Participant responses indicated that many 

drivers will not reduce their speed until other drivers begin to slow down as well. Participants also 

stated that overhead gantries and signing above each lane would be helpful. In addition, survey 

results showed support for side-mounted VSL signing (Elefteriadou, Washburn, Yin, Modi, & 

Letter, 2012). 

Liability

To date, the team has not identified an agency experiencing issues with liability. Agency processes 

do include archiving all speed data which can be provided as documentation or evidence of the 

posted speed limit at any specific time.

Only two VSL systems reviewed noted experience or recommendations for liability issues. Nevada 

DOT recommended that lawyers should be involved early on in future deployments to evaluate 

possible tort liability after the VSL implementation on I-80 (Robinson et al. 2002). With respect to 

the VSL system on I-526 in South Carolina, the system was created due to a court order. A Federal 

judge ruled that the I-526 Cooper River Bridge construction project must include a low visibility 

warning system, which included VSL (Goodwin 2003).
21

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

DESIGN, DEPLOYMENT AND STANDARDS

Infrastructure Requirements 

Agencies report a variety of infrastructure used to operate their VSL systems. These components 

may differ based on the function of the VSL. Table 4 shows the fundamental VSL elements for a 

system operating to manage speeds during congestion, weather, and work zones.

Table 4. Fundamental variable speed limit infrastructure requirements.

Variable Speed Limit Function

Variable Speed Limit Infrastructure Component Congestion Weather Work Zones

Changeable Speed Limit Signs   

Weather/Environmental Sensors 

Traffic Speed/Volume Sensors  

Communications Equipment to Transmit Data   

Signage Type and Placement

The Florida DOT reported approximately 20 VSL signs along the I-4 corridor. At least one sign is 

posted every mile, and some signs are located in medians while others are side-mounted along the 

roadway. I-4 also displays various word messages (VMS) in conjunction with their VSL system. 

Some VSL signs along US 27 are posted in medians while others are side-mounted. No VMS signs 

are used along US 27. 

Georgia’s VSL system includes 176 electronic speed limit signs for an interstate corridor that is 

36 miles long (inclusive of both directions). Signs are mounted on both sides of the highway in 88 

locations and are spaced every 1⁄2-mile to 1 1⁄2-mile.

The Minnesota DOT installed 155 signs on I-35W, which is 18 miles long, and 101 signs on I-94, 

which is 10 miles long. When activated, the State used lane-by-lane overhead signs. Although 

displays were by lane, each showed the same speed at the same location. The signs are full matrix 

color CMS that measure 4 feet tall and 5 feet wide. HOT lane signs displayed a white diamond with 

no speed message. This was enacted to address concerns about displaying different speeds over 

different lanes, but at the same time not wanting to artificially slow down the speed in the HOT 

lane. Note that Minnesota did not use word messages in conjunction with the VSL signs.

Due to maintenance issues with the signs, the Minnesota DOT is considering either replacing them 

or using a single message sign as opposed to lane-by-lane signing. The latter alternative will reduce 

installation, maintenance, and operations costs. In addition, the Minnesota DOT is evaluating 

whether a VSL or a simple word message of SLOW TRAFFIC AHEAD is more successful for 

queue warning in a particular high crash area. It is possible that Minnesota could use the system for 

spot locations rather than implementing it throughout an entire corridor.
22

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The Nevada DOT system used to lower the speed for trucks so that a long, elevated structure over a 

canyon can remain open in high winds. Nevada DOT determined the locations of the 

US 395 VSL signing installations based on locations of intersecting roadways; this permitted 

trucks to turn off the roadway if extreme winds were encountered, but the bridge remained open to 

automobile traffic. Signs use embedded LED and are mounted on the right side of the highway.

In addition to the VSL system, VMS are also installed along the New Jersey Turnpike. All VSL 

signs are posted adjacent to VMS that describe the reason for the speed change, as shown in Figure

1. The following word messages are used to warn drivers of conditions ahead:

•	 ACCIDENT AHEAD BE PREPARED TO STOP.

•	 DEBRIS AHEAD DRIVE WITH CAUTION.

•	 DELAYS AHEAD BE PREPARED TO STOP.

•	 MOWING OPERATION AHEAD (with tractor image).

•	 REDUCE SPEED CONGESTION AHEAD.

Figure 1. Photo. New Jersey Turnpike variable speed limit and variable message signing. 

Note that VMS along the New Jersey Turnpike are also used to notify drivers when the far right 

lane can be used as a shoulder (red “X”) and when it is a travel lane (green arrow) during peak 

travel periods. VSL and VMS are only displayed on message boards that are within 2 miles of 

the traffic issue (e.g., lane closing, construction site, congestion, etc.). The New Jersey Turnpike 

Authority noted that the warning messages and altered speed limits remain relevant to drivers when 

they are not posted too far in advance. 

OR 217 in Oregon has a set of VSL signs for each segment. All signs are displayed overhead above 

each lane with additional VMS for traffic-related messages. The Oregon DOT estimates 40 to 50 

signs on the main line plus 30 to 40 VMS signs that display travel time messages prior to entering 

OR 217. All of the VMS are full matrix and display messages such as CONGESTION AHEAD at a 

certain distance upstream of the congestion or information related to current weather conditions. 

In the Chattanooga, Tennessee area along I-75, there are 10 signs that are right-shoulder mounted 

with embedded white LEDs for an interstate corridor that is 9 miles long (inclusive of both 

directions). There is one display for all lanes and signs are located in relation to interstate entrance 

ramps and Manual on Uniform Traffic Control Devices (MUTCD) guidelines. The Tennessee DOT 

Source: ToXcel
23

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

uses a FOG SPEED LIMIT word message on the changeable speed limit sign, which is mounted on 

the shoulder. In addition, the following word messages are activated on VMS in conjunction with 

the VSL.

•	 FOG AHEAD TUNE TO 1620.

•	 REDUCE SPEED TURN ON LOW BEAMS.

•	 CAUTION FOG AHEAD.

•	 FOG AHEAD SPEED LIMIT 50 MPH.

•	 FOG AHEAD SPEED LIMIT 35 MPH.

Along I-90 (near Snoqualmie Pass) in Washington, VSL sign locations vary depending on the 

roadway geometry at any given point. Some areas have overhead signing while others have sidemounted signs on both sides of the roadway for each direction. US 2 is an undivided highway, and 

all VSL signs are located on the right-hand side of the roadway. All of the VSL signs on I-90 (near 

Snoqualmie Pass) and along US 2 are hybrid cut-out LED speed limit signs. VMS are not utilized 

in either location. 

The signs along I-5, I-90 (Bellevue to Seattle), and SR 520 in Washington are full color and full 

matrix, and the spacing between gantries is approximately 0.5 miles. The speed limits along these 

three routes are displayed using specialized graphics files for each message; those files reside 

locally in the sign controllers and have been created to be identical to the FHWA Standard Highway 

Signs catalog so that the signs use the FHWA Standard Alphabet in their depictions. This method 

of providing the sign displays moves beyond text-based displays from typical changeable message 

sign controller units and into the realm of specialized graphics displays, which could include 

warning signs and MUTCD-approved symbols in the future. These three locations also utilize word 

messages in conjunction with VSL. 

There are 21 overhead gantries along I-66 per direction, and each gantry holds 3 to 5 signs. VMS 

are used along I-66 to display messages to drivers in addition to the VSL (e.g. CONGESTION 

AHEAD, etc.). In addition to the VMS and VSL signing, other devices are also provided along 

I-66, including closed circuit television (CCTV) cameras, ramp metering, lane management 

devices, etc. (Earnest, 2015). 

There will be 44 side-mounted signs along the VSL corridor on I-77 in Virginia. Thirty-six of these 

signs will be full matrix, VMS that can post speed limit messages and traffic management messages 

(e.g. FOG AHEAD, etc.). Eight of the signs will be typical variable speed limit signs where the 

display speed can dynamically change. Additional devices will include CCTV cameras, visibility 

sensors, etc. 

To control everyday traffic along the corridor, I-66 uses signing to indicate lane availability, especially 

within and surrounding work zones. VMS are used to display information regarding work zones, 

and green arrows/red “X” symbols over each lane are used to indicate current lane availability. Work 

zones are not expected to be an issue along I-77 since it is a rural, low-volume roadway. 
24

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Integration with Active Traffic Management and/or Road Weather Information 

Systems

Several VSL systems were either planned as part of a larger ATM system or integrated with existing 

ATM or RWIS as a source of data or as a shared backbone for hardware and/or software systems.

The VSL system along I-66 in Virginia is part of a larger ATM that also includes VMS that can 

display other traffic management messages (e.g., CONGESTION AHEAD). The VSL system on 

the Pennsylvania Turnpike built off an existing ATM system to consolidate operations and reduce 

cost. The system used a series of RWIS stations to determine fog conditions. Nevada DOT’s VSL 

relies on data from two separate RWIS stations. Once an RWIS measures a 30 mi/h wind gust, 

the VSL on US-395 is activated to reduce speeds. The VSL signs do not display normal operating 

speeds until neither RWIS measures a wind gust of 30 mi/h or more for 30 minutes. On the I-215 

VSL in Utah, the DOT cited the lack of integration with the existing ATM as a serious obstacle 

during implementation.

OPERATIONS AND MAINTENANCE

Control Algorithms

VSL system speed control algorithms have been widely studied in both academic papers and 

evaluation reports. Lu et al. (2014) and Ma et al. (2016) presented comprehensive reviews 

of advanced algorithms for VSL systems, particularly as components of ATM. While these 

algorithms have been shown to be effective in simulation studies, they are often too complex to be 

implemented in the field. 

Generally, VSL systems are activated when certain conditions (e.g., volume, occupancy, road 

surface conditions, or weather conditions) are met; corresponding algorithms will generate new 

speed limits. Usually, decisions are supported by real-time sensors that can detect current roadway 

conditions (e.g., traffic, weather, visibility, pavement). The algorithms can differ from project to 

project. In many cases, the 85th percentile speeds of downstream congested traffic are used directly 

or indirectly as new speed limits. In other cases where there is no congestion but severe road 

conditions, such as low visibility, engineers use look-up tables to determine speed limits using predetermined values based on condition thresholds. Some VSL systems are deployed during major 

construction projects to slow upstream vehicles for safety purposes; a single reduced speed limit 

may be set in this case.

Based on their objectives, speed control algorithms can be categorized into two types: 1) speed 

homogenization projects that focus on improving safety, and 2) multi-objective projects that may 

strive for improvement of mobility and/or reduction of environmental impacts in addition to speed 

homogenization. Most systems related to weather, visibility, and work zones fall under the category 

of speed homogenization, while systems that react to current traffic conditions belong to the 

category of multi-objective projects.
25

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Two particular challenges of setting variable speed limits were identified during agency interviews. 

One difficulty was generating speed changes in a way that felt natural to drivers, both at the stage of 

speed reduction or speed recovery from reduced speed. Oregon DOT advised that it takes multiple 

iterations to develop a system that reacts naturally enough to reduce negative feedback and to 

increase compliance. The other challenge was determining how to manage competing interests 

between the assigned “safe speed” and actual driver behavior, since many drivers travel much faster 

than the posted speed limit. When calculating a suitable speed limit, the goal should be to display a 

speed that is safe for travelers but also will not create increased variance. This balance can be very 

difficult to achieve. 

In practice, algorithms differ depending on project objectives, purposes, weather conditions, and 

the surrounding environment. Algorithms for congestion-focused VSL systems are typically more 

complex because they need to consider overall effects on corridor traffic conditions instead of 

simply reducing speed and speed variance for safety. Congestion-focused projects may also be 

weather responsive if adverse weather conditions exist. Key questions related to dynamic speed 

limit setting issues include: 

•	 What are the factors to consider, such as volume threshold, occupancy threshold, surface 

conditions, and 85th percentile speeds? How are they considered? 

•	 Are there any safety issues to slow down traffic if the average speed is considerably higher 

than posted speed? 

•	 How should maximum and minimum posted speed limits be determined? 

•	 What is the period over which speed statistics are calculated? 

•	 When should speed limits be adjusted and by what increment? 

•	 How often can the speed limits be changed? 

Various examples of speed control algorithms and the corresponding approaches to implementation 

issues are described below.

The VSL system along the New Jersey Turnpike was installed in the early 1960s and is still being 

used today. Although the VSL system is currently operated manually, it was automatic in the past. 

The VSL were automatically calculated and posted according to average travel speeds collected 

by copper loop detectors located in the pavement (United States Department of Transportation, 

2002). In order to avoid creating a second area of congestion, the VSL signs upstream of the traffic 

issue were posted as 10 mi/h faster than the speed of the downstream traffic. For example, if the 

downstream traffic was traveling at 25 mi/h, the upstream VSL would be 35 mi/h. Supervisors 

only manually intervened when setting speeds for construction work zones or if travel lanes were 

shut down along the roadway. The algorithm for speed reduction was simple: the speed limit was 

reduced in 5 mi/h increments with a minimum speed limit setting of 30 mi/h. The VSL system not 

only displayed the reduced speed limit, but it also displayed a REDUCE SPEED AHEAD message 

on VMS as well as the rationale behind the speed reduction. When appropriate, the distance from 

the warning sign to the congestion, crash, construction, etc. was also displayed (United States 

Department of Transportation, 2002). 
26

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The current VSL system employs approximately 250 signs (both VSL and VMS) along the entire 

New Jersey Turnpike corridor and is manually operated. The maximum regulatory speed limit 

along the New Jersey Turnpike is 65 mi/h, and there are other areas with a regulatory speed limit 

of 55 mi/h. Most of the time when there is a downstream issue, supervisors manually reduce 

the speed to 45 mi/h (except during poor weather conditions). During poor visibility conditions, 

the speed is determined based on how many mile markers are visible from a stationary location 

along the corridor. If three mile markers are visible, then the speed limit is posted as 35 mi/h. If 

two mile markers are visible, then the speed limit is set as 30 mi/h and operators consider closing 

the roadway. Currently, the posted VSL apply to all lanes, though VSL may vary across lanes in 

the future. 

The speed limits along I-4 in Florida are determined with loop detectors and side-fire radar, which 

detect volume, speed, and occupancy. Weather conditions are visible through CCTV, although the 

VSL system along I-4 was primarily built for speed harmonization due to large dynamic waves 

frequently observed along the roadway rather than to observe weather. The Florida DOT reported 

that the loop detectors provide extremely reliable data. The side-fire radar systems have improved 

over the years, but they are still not fully reliable today. 

The displayed speed limits along I-4 in Florida are regulatory and are based on the 85th percentile 

speed in 5 mi/h increments. When an event occurs that requires a speed alteration, the VSL 

system informs the traffic management staff and then recommends an appropriate speed. The 

speed selection algorithm along I-4 accounts for the design speed of the roadway, which depends 

on roadway curvature, superelevation, sight distance, etc. The staff may then accept or alter the 

recommendation. Once a suitable speed has been accepted, the speed limit is posted according to 

the following rules (FDOT: Traffic Engineering and Operations Office, 2010): 

1. The posted speed limit is reduced and the yellow warning light begins flashing.

2. The new traffic flow is monitored and it is ensured that the new speed limit is appropriate. 

3. If necessary, the speed is reduced in 5 mi/h increments while never dropping the speed 20 

mi/h or more under the normal roadway speed limit.

4. Once the event has cleared, the normally posted speed limit is displayed and the flashing 

yellow light is turned off. 

Because of ongoing construction on the I-4 corridor in Florida, the Florida DOT completely turns 

off the VSL system to accommodate work zones in the vicinity. This obviates the need to move 

signs, maintain electrical and communications to signing systems, and ensure that adequate data 

collection is taking place, particularly in areas where the freeway surveillance systems are disrupted 

by the ground works associated with grading and pavement reconstruction. 

US 27 in Florida also uses loop detectors to determine speed limits. The Florida DOT later 

added side-fire radar systems just to detect current speeds along US 27 and to check compliance 

rates. US 27 does not experience much construction; therefore, it is not necessary for the VSL to 

accommodate work zones.
27

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Georgia uses various sensors placed 1/3 mile apart on I-285. Sensors transmit data every 

20 seconds, including the total volume and average speed. Weather stations do not contribute to 

the VSL system, and the Georgia DOT does not use probe data—primarily because there is a lag 

in that data. VSL speeds are determined based on speeds downstream of the sensors, so the cause 

of a slowdown in traffic is not a determinant. The VSL system is fully automated, but there is the 

option to change the speed manually as well. The VSL is manually changed for situations like 

work zones. In Georgia, construction typically occurs at night when there is less traffic volume. 

However, motorists can drive much faster with less traffic, so the VSL is used to lower speeds in 

active work zones.

The Minnesota DOT set speeds using an algorithm developed by the University of Minnesota – 

Duluth. When activated, the VSL would display a speed 5 mi/h lower than the posted speed (which 

is 55 or 60 mi/h) with a minimum speed of 30 mi/h. The same speed did not have to be set for the 

entire corridor. Instead, when congestion was detected, as many as three sets of lane control signals 

could be activated prior to the congestion. Because the lane use control signals for the active traffic 

management system (ATMS) equipment also functioned as the display modules for the VSL, the 

activation of VSL in advance of the congestion was desirable for the purposes of step-down speed 

harmonization. With lane control structures located every 1⁄2 mile, the VSL could be activated as 

much as 1.5 miles upstream of the congestion. This allowed traffic management staff to reduce 

speeds gradually as the traffic approached congestion. 

The Nevada VSL system is part of a larger wind-warning system. Wind-speed data is tied to two 

RWIS stations; one on the north end and one on the south end of the valley. For the VSL to be 

activated, one of the RWIS stations must record a wind gust of 30 mi/h or more. Once activated, 

the speed limit is lowered to 45 mi/h. It is not raised back to its operating speed (either 50 mi/h or 

55 mi/h depending on the section) until neither RWIS station registers a wind gust > 30 mi/h for 

30 minutes. The system operates automatically, but there is some human interface from the traffic 

management center for confirmation.

The displayed speed limit along OR 217 is determined by in-road, radar-based, downstream sensors 

from Wavetronix that measure 85th percentile speed at a 1 minute interval. The displayed speed is 

calculated as the lower of the following values: 1) 85th percentile speed, or 2) Speed of downstream 

traffic + 5-10 mi/h. If the calculated speed is less than 30 mi/h, then the system displays SLOW 

(Mitchell, 2016). The Oregon DOT has found these sensors extremely reliable. There are at least 

seven to eight different segments throughout the entire VSL corridor, and each segment is evaluated 

separately and assigned an appropriate speed limit. The speed setting algorithm ensures that the 

changes in speed between different segments are no more than 10 mi/h. Although the current VSL 

system is advisory, the VSL algorithm can easily be converted from an advisory to a regulatory 

VSL system. Oregon statutes regarding basic speed establish strict criteria for the installation of 

non-advisory speed limits. In addition, case law has established precedent for drivers overturning 

citations for violating posted maximum speeds, which, in Oregon, are signed with SPEED LIMIT 

signs instead of the SPEED signs found on rural primary highways. 
28

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The OR 217 speed algorithm accounts for both weather conditions and congestion levels. The final 

displayed speed limit depends on which piece reports the most needed condition change (weather 

vs. congestion). There is a friction factor sensor at each speed sensor location which considers 

roadway condition during calibration. When the friction factor reaches a certain level, the displayed 

speed limit is calculated based on the current weather conditions instead of congestion levels. A 

summary of the weather responsive algorithm is shown in Figure 2 (Mitchell, 2016).The necessary 

speed alterations based on weather and/or traffic are automatically calculated and implemented 

within the VSL algorithm itself. The algorithm does not account for roadway curvature since OR 

217 is a freeway-level facility. In addition, the Oregon DOT does not have much experience in 

accommodating work zones in the vicinity of VSL since the last major construction in the area was 

the installation of the VSL system itself. 

Weather Responsive System

Condition 

Code Visibility Grip Factor Surface Condition Classification Condition Speed Weather Message Actual Sign Message

A <Visibility 

Threshold

>= Grip Factor 

Threshold Moist, Wet Maximum Speed - 10 MPH “LOW VISIBILITY LOW VISIBILITY

B <Visibility 

Threshold

< Grip Factor 

Threshold Moist, Wet Minimum Speed

Slippery when wet 

sign + “USE CAUTION” USE CAUTION

C >=Visibility 

Threshold

>= Grip Factor 

Threshold Moist, Wet Maximum Speed None

D >=Visibility 

Threshold

< Grip Factor 

Threshold Moist, Wet Maximum Speed - 20 MPH

Slipper when wet sign 

+ “USE CAUTION” USE CAUTION

E <Visibility 

Threshold

>= Grip Factor 

Threshold

Frosty, Snowy, 

Icy, Slushy

Maximum 

Speed - 10 MPH “LOW VISIBILITY” LOW VISIBILITY

F <Visibility 

Threshold

< Grip Factor 

Threshold

Frosty, Snowy, 

Icy, Slushy

Minimum 

Speed

ICE sign + “USE 

CAUTION” ICE USE CAUTION

G >=Visibility 

Threshold

>= Grip Factor 

Threshold

Frosty, Snowy, 

Icy, Slushy

Maximum 

Speed None

Figure 2. Chart. Weather responsive system for Oregon Route 217.

The Tennessee VSL system on I-75 is weather-responsive with speeds changed based on visibility 

during fog conditions. It functions in a hybrid fashion with speed changes occurring both 

automatically and manually. Speed limits are determined by a conditional visibility algorithm 

due to weather event(s) related to fog, traffic speed, and stopping distances. The same speed is set 

throughout the corridor using the following parameters:

•	 Speed = 70 mi/h when visibility is < 10 miles and > 1,320 ft.

•	 Speed = 50 mi/h when visibility is < 1,320 ft. and > 480 ft.

•	 Speed = 35 mi/h when visibility is < 480 ft. and > 240 ft.

Environmental sensors are used with the I-75 system and are reported by the Tennessee DOT to be 

very reliable.

The VSL system along I-66 in Virginia has dynamic (instead of fixed) speed segments. Dynamic 

segments allow the speed limit to apply to different lengths of the roadway depending on existing 

needs. A smoothing speed algorithm is used to appropriately alter vehicle speed within each 

dynamic section to maintain suitable traffic flow. The smoothing algorithm determines the slowest 

speeds along the corridor, and then it transitions the oncoming traffic into that slower speed. 

(Source: Oregon Department of Transportation)
29

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Note that the Virginia DOT is still improving the current I-66 speed setting algorithm. Weather, 

roadway curvature, sight distance, and pavement type/condition are not included in the speed 

limit calculations for I-66, although some of those variables may be included in future iterations. 

I-66 currently utilizes Wavetronix speed sensors, which have been extremely reliable in providing 

relevant traffic conditions. 

Since the I-66 VSL system in Virginia has only been consistently active for approximately six 

months, the Virginia DOT is still evaluating the effectiveness of the VSL system for reducing 

speeds. The Virginia DOT stated that maybe the most important determinant of the effectiveness is 

if the algorithm is successfully transitioning drivers into both higher and lower speed zones. 

In addition to the VSL system along I-66, the Virginia DOT is currently designing a VSL system 

on I-77, which will primarily be used for visibility purposes. The VSL will be determined based 

on the available visibility with the goal of reducing speed variance. Similar to I-66, I-77 will also 

have dynamic speed segments, but the length of these segments will change depending on visibility 

levels. The algorithm will determine the areas with the worst visibility and then set the appropriate 

speed limits around those areas. Sight distance is included in the speed setting algorithm since it is 

a visibility-based system. Wet conditions, roadway curvature, and pavement type/condition are not 

included in the speed limit calculations for I-77, although some of those variables may be included 

in future iterations. The planned I-77 VSL corridor will use Wavetronix sensors to capture data.

Two of Washington’s VSL systems have similar methods of operation: I-90 (near Snoqualmie 

Pass) and US 2. Both systems are regulatory, operate in rural areas, and display speed based on an 

operator look-up table, which accounts for current pavement conditions, visibility, weather (i.e. 

rain, snow), and incidents, as shown in Table 5. Currently, the operator uses the table to determine 

the appropriate speed and then manually displays it. Roadway curvature, sight distance, and 

pavement type/condition are not considered in the speed setting algorithm. The displayed speed 

limits are not necessarily the same throughout the entire corridor, and there is lane discrimination 

(e.g., high occupancy vehicle (HOV) lanes might have a different speed limit than the general 

purpose lanes). The Washington State DOT reported that both VSL systems have been effective at 

reducing speeds, and speed variation is small. In addition, the Washington State DOT stated that 

their current sensors are very reliable, and they have extensive experience in calculating travel 

times based on speed converted from occupancy measurements.

Table 5. Washington State Department of Transportation speed limit reference.

Traction 

Requirements

Speed 

Limit

Pavement 

Conditions Visibility Weather Blocking Incidents

None 65 Dry or Bare/Wet. Good: Clear > 

0.5 Miles.

Fair To 

Moderate 

Rain.

Incident On 

Shoulder.

Traction Advisory 55

Light Snow, 

Slush, or Ice In 

Places.

Moderate: Fog 

< 0.2 Miles. Hard Rain. Incident On Shoulder.
30

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Traction 

Requirements

Speed 

Limit

Pavement 

Conditions Visibility Weather Blocking Incidents

Tractor Trailer 

Requirement /

Vehicle Over 10,000 

GVW Chains 

Required

45

Comp. Snow/

Ice, Deep Slush, 

Shallow Water.

Poor: Blowing 

Snow < 0.1 

Miles

Heavy Rain 

Or Snowfall.

Lanes Blocked 

Traffic 

Moving.

Chains Required All 

Vehicles Except All 

Wheel Drive

35

Severe Freezing 

Rain, Deep 

Snow, Slush Or 

Standing Water.

Poor: Blowing 

Snow < 0.1 

Miles.

Heavy Rain 

Or Snowfall.

Lanes Blocked 

Traffic 

Stopped 

Ahead.

Emergencies or 

Extreme Conditions 

Only

25

Use this speed for severe conditions as requested by crews on the 

scene. Confirm with supervisor, when available. Poorest possible 

road conditions and human life endangered. Conditions should be 

well documented. Return to higher speed limit as soon as possible.

The displayed speed limits along I-5, I-90 (Bellevue to Seattle), and SR 520 in Washington are 

computed using the same method. All three systems are regulatory and located in urban areas. The 

displayed speeds are determined and adjusted every minute by monitoring downstream conditions: 

1) the 85th percentile speed is calculated, 2) multiple speed values are compared in the corridor, 

3) smoothing/transitional calculations are performed, and 4) the displayed speeds are updated as 

needed. Since the displayed speeds are calculated by using measured downstream conditions, 

there is no need to include wet conditions, roadway curvature, sight distance, nor pavement type/

condition in the speed calculations. The displayed speed limits are not necessarily the same 

throughout the entire corridor, and there is lane discrimination (e.g. HOV lanes might have a 

different speed limit than the general purpose lanes). All three of these VSL systems have been 

effective at reducing speeds. 

Manual versus Automatic Operations

Many VSL systems operate in a hybrid fashion using a combination of automated and manual 

speed changes. There are fewer instances of a system being fully manual or entirely automated, but 

there are examples of each. 

Speed limits on I-495 in Delaware are manually determined by the chief traffic engineer of Delaware 

DOT, the traffic management center manager of the DOT, or by request of the Delaware State Police, 

according to weather and road conditions. Using expert opinion and on-the-ground input limits 

unexpected speed variation due to faulty sensors or poorly calibrated control algorithms.

Presently, the VSL system along the New Jersey Turnpike is manual; however, the system used 

to be automatic. The New Jersey Turnpike Authority is currently working with other entities, 

including IBM and Rutgers University, in order to restore the automatic capabilities of the VSL 

Table 5. Washington State Department of Transportation speed limit reference. (Continued)

Source: Washington State Department of Transportation.
31

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

system. The NJ Turnpike Authority noted that automatic VSL systems allow more throughput; 

therefore, transitioning back to an automatic system is ideal. Alternatively, the Nevada DOT system 

is fully automated and speeds are changed using wind-speed data from RWIS stations. A threshold 

wind gust of 30 mi/h automatically activates the VSL system so that the changeable speed limit 

signs show a reduced speed limit.

When an agency uses a hybrid approach, they typically rely primarily on an algorithm to 

automatically change the speed limit and supplement with a human interface. This may involve 

looking at data or video feeds to confirm the VSL is appropriately set for current conditions or 

overriding the automated speed limit for an extenuating variable. For example, the Georgia DOT 

system on I-285 automatically adjusts the VSL using speed data transmitted from sensors. However, 

agency personnel can override the system to manually change the speed limit during nighttime 

construction to reduce speeds in work zone areas. In addition, the VSL system in Florida along I-4 

recommends a certain speed limit based on field sensor output, and the operator must then approve 

or alter the proposed speed. On I-5 and I-90 in Washington, operators monitoring the system can 

override automatically adjusted VSL if necessary, though this is not desirable for typical operations.

Advisory versus Regulatory Operations (Enforcement)

The success of VSL systems is highly dependent on compliance, and therefore it is essential that 

regulatory systems are consistently enforced. However, in real-world deployments, particularly 

those in the United States, many systems are still advisory or cannot be enforced as intended.

In some cases, State laws prevent VSL systems from being enforced. In Minnesota, the VSL 

systems on I-35W and I-94 were advisory because regulatory systems would have required a 

legal change. Even so, stakeholders in Minnesota shared the same views as many other agencies: 

VSL systems require enforcement to gain driver compliance. If the VSL system not enforced, it is 

suggested that speeds need to match drivers’ expectations of what is sensible. 

OR 217, located in the Portland, Oregon area, is an advisory system due to limited shoulder space 

along the roadway and also due to State law. In order to implement a regulatory VSL system in 

Oregon, a long legal process would be necessary to change State law to accommodate VSL along 

interstates. Currently, State troopers and local police in Oregon “enforce” VSL by using a “basic 

rule” whereby law enforcement officers judge whether drivers are traveling safely and prudently 

rather than examining vehicle speed. Oregon DOT is planning to utilize VSL systems in other areas 

once the State law has been altered to allow VSL installation on interstates, including a 30 mile, 

weather-based VSL system. 

In many other cases, VSL systems were intended to be regulatory, but actual enforcement 

was limited. One major obstacle is the lack of direct access to speed limit information by law 

enforcement. The former Missouri VSL system was located on I-270 in St. Louis. It commenced 

as regulatory, but law enforcement was reluctant to issue citations because they were unsure of the 

current speed limit. Consequently, the system was changed to advisory, but compliance became an 

issue. The system was therefore ultimately deactivated. Missouri has no other VSL systems as of 

May 2016.
32

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The Georgia Highway Patrol as well as law enforcement from 14 local jurisdictions can enforce 

speeds on I-285. Prior to activating the VSL system, the Georgia DOT met with the Highway Patrol 

to explain the system. As many others have experienced, the reaction from law enforcement was 

not positive with concerns centralized on the officers not knowing the current speed that should be 

enforced. To help address this issue, the Georgia DOT provided the Highway Patrol with a direct 

data feed so they can see the signs at all times. Additionally, the Highway Patrol is using a different 

citation tactic to work around needing to know the exact speed. Instead of focusing on speed as the 

offense, law enforcement issues citations for reckless driving or driving too fast for conditions. To 

further support law enforcement, the Georgia DOT archives all of their VSL data and can provide 

information to the Highway Patrol when needed to verify the set speed at a specific time.

The VSL in Nevada is also regulatory. Law enforcement response has not been positive, primarily 

because of the hardware and software problems that have caused issues with the VSL signs. For 

example, a 45 mi/h speed limit may be displayed in one direction, but the signs display a 55 mi/h 

speed limit in the opposite direction. This has caused law enforcement to lose confidence in VSL, 

and Nevada DOT is considering temporarily turning off the system to replace the hardware. Law 

enforcement is not directly notified when the VSL is activated, but they are aware by default 

because they are notified when the larger wind-warning system is closing routes; so the speed limit 

reduction is implied.

Finally, it can be difficult to enforce speed limits in conditions where it is unsafe for law 

enforcement to exceed the posted speed limit. For example, the VSL system on I-77 in Virginia 

is regulatory and enforceable, but speeds are most often decreased due to heavy fog. Heavy fog 

is not only a safety issue for drivers, but it is also a safety problem for law enforcement officials. 

Therefore, enforcement along I-77 is a complex issue that transportation officials in Virginia are 

still working through. 

OUTCOMES

Performance Measurement

Depending on functional requirements and system goals, VSL systems have been evaluated with 

various performance measures or measures of effectiveness (MOEs) as follows:

•	 Traffic efficiency: average speed and travel time at a certain time interval (e.g., 1 minute or 

5 minutes), travel time reliability, traffic throughput, driver journey times, traffic flow 

stability, number of significant shockwaves. 

•	 Safety: general crash rates (categories by crash severity: fatal and injury, property damage 

only, and crash types: rear-end, sideswipe, and others), crash rates during certain seasons 

(e.g., winter crash rate if a VSL is deployed for winter severe weather conditions).

•	 Other: driver subjective ratings, compliance rates, emissions measured by environmental 

sensors.
33

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Agencies either apply these general performance measures directly or adapt them on the basis of 

special requirements and goals of the deployment site. The Virginia DOT defined MOEs for the 

planned VSL system on I-77 to guide the evaluations of system effectiveness. The primary MOEs 

focus on reducing the quantity of various crash types (e.g. fatal, injury, property, weather-related, 

work zone-related, etc.) along the corridor. The crash reduction goals would be met if the number 

of collisions over a five-year period following VSL operation is less than the number of collisions 

in the 5 years just before system implementation. Crash severity reduction goals were also 

developed related to injury and property crashes. The crash severity goals would be accomplished if 

the severity of injury and property crashes decreased every year following system implementation. 

In addition, the speed compliance goal would be achieved if the rate of compliance improves 

for a period following the system introduction when compared to a period just before system 

implementation (URS, 2012).

Compliance

States have observed varying levels of driver compliance with VSL systems. Compliance rates 

depend on multiple factors (e.g., regulatory vs. advisory systems, enforcement strategies, public 

education/outreach, etc.). Also, some speed homogenization projects, such as on I-94 in Minnesota, 

reported high compliance rates, perhaps because drivers are aware of the risks of high speeds in 

bad weather or work zones. Some deployments (e.g., A99 in Munich, Germany, and many others in 

Europe) adopt automated enforcement, which is effective in improving compliance. The feasibility 

(i.e., adoption issues and public support) of automated enforcement in the United States should 

be studied in the future. In addition, European sites typically report higher compliance rate and 

larger benefits. Future research should consider if cultural differences between U.S. and European 

drivers affect system effectiveness. Various State experiences related to VSL compliance rates are 

discussed below.

The manual VSL system along the NJ Turnpike is regulatory; therefore, the speed limit is enforced 

by the State Police to ensure that drivers are abiding by the posted speed limits. In addition, any 

time there is a severe collision along the VSL route, the police will issue any necessary citations 

based on the VSL that was posted at the time of the incident (United States Department of 

Transportation, 2002). 

An outreach program was implemented (e.g. brochures, radio announcements, websites, etc.) for 

the VSL System along I-4 in Florida to educate the public about the purposes of the VSL system; 

however, without proper enforcement, minimal compliance is still observed. In contrast, Florida 

is obtaining high compliance rates along US 27 – a two-lane, divided, rural, high-speed roadway. 

Here, the VSL are regulatory and are being enforced along this roadway by the Florida Highway 

Patrol, which is believed to be causing the higher compliance rates. 

The VSL system was advisory in Minnesota because a regulatory system would have required a 

legal change. Minnesota shares similar views as many other agencies – VSL requires enforcement 

to gain driver compliance. If the VSL is not enforced, the speed needs to match drivers’ 

expectations of what is sensible.
34

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The VSL system on OR 217 is currently advisory due to limited space for enforcement and 

State law. Even though current compliance along OR 217 is not perfect, the VSL system is still 

considered successful. Oregon has observed a substantial reduction in speed differentials, improved 

harmonization, increased roadway capacity, and a reduction in crashes along OR 217. However, 

Oregon would expect to see higher compliance rates with a regulatory system versus their current 

advisory system.

As mentioned previously, the former Missouri VSL system on I-270 in St. Louis was ultimately 

deactivated due to compliance-related issues. As of May 2016, Missouri has no other VSL systems.

System Benefits

With the variety of objectives and implementation approaches across VSL systems, benefits vary 

from deployment to deployment. Table 6 shows results for a number of representative VSL projects 

from the United States and other parts of the world. Not every deployment in Table 6 has been 

evaluated in the literature. European sites have been included in the results to better illustrate how 

system benefits can vary from site to site due to various influencing factors.

Speed homogenization projects usually used simple algorithms in response to real-time traffic, 

road, and other conditions (e.g., weather, work zone, incidents, visibility, etc.). Many of these 

studies reported improvement in traffic safety via before-and-after analyses (with some exceptions 

such as the I-270/I-255 corridor in Missouri, likely because of low compliance rates). Many of 

the multi-objective projects reported that VSL had positive effects on mobility, safety, and the 

environment. There are some discrepancies, although it is difficult to generalize the reasons for 

these discrepancies due to many uncontrolled factors among different sites, such as different 

compliance rates, heterogeneous driver behaviors, and various road geometries. 

Table 6. Variable speed limit system practices and field results.

Location, Time Variable Speed Limit (VSL) 

Summary Evaluation Results

Speed Homogenization Projects

Germany, 1990 • Advisory VSL.

• Only three speed limit options: 

100, 80, or 60 km/h.

• 20-30% reduction in crash rates.

E18 in southern 

Finland, 1990

• A central control unit analyzed the 

data and selected one of three 

speed limits, 120 km/h, 100 km/h, 

or 80 km/h, to display, based on 

driving/road conditions; the 

system is advisory.

• 95% of drivers reported positive 

ratings of its effectiveness.

• Compliance rates were as high as 

76%.

• Significant safety improvements 

attributable to the weather VSL 

implementation: accidents during 

the winter dropped by 13% and 

during the summer by 2%.
35

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Location, Time Variable Speed Limit (VSL) 

Summary Evaluation Results

Attiki Odos 

Toll Motorway, 

Greece, 2004

•	Speed signs are to notify drivers 

when the advisory speed limit 

inside the tunnel is different from 

the other parts of the motorway.

•	The system provides advice to 

motorists approaching the tunnel 

regarding the safe speed limit 

inside the tunnel.

•	Significant reduction in injury 

accidents by 10%.

I-494, Minnesota, 2006•	Reduce the speed of the vehicles 

approaching the work zone.

•	25-35% reduction in maximum 

1-minute average speed and a 7% 

increase in throughput between 6 

and 7 a.m., although no throughput 

increase between 7 and 8 a.m. 

•	Even though the speed limit was 

advisory, motorist compliance was 

significant.

I-270/I-255 

Corridor, 

Missouri, 2010

•	The maximum and minimum 

speed limits on the corridor are 

60 mi/h and 40 mi/h, in 5 mi/h 

increments. 

•	Uses a 5 minute update interval 

(less in case of incidents).

•	The system is advisory.

•	No mobility gains (in terms 

of throughput improvement 

or congestion reduction) were 

observed. 

•	The evaluation did show a 

significant reduction in number and 

severity of crashes by 8%. 

•	Speed limit compliance remained 

surprisingly low, even though the 

signs were mandatory.

Multi-Objective Projects

E4, E22, Sweden, 2003

•	Both advisory and regulatory.

•	Goal: increase throughput, reduce 

shockwave, improve safety.

•	5 to 15 km/h (≈ 3 to 9 mi/h) 

reduction in speeds across the 

study sites, high rates of speed 

compliance (in particular in 

severe weather conditions), fewer 

disturbances in traffic flow, and less 

severe shockwaves.

•	Reduce travel time by 5%.

•	Most effective when they combined 

with additional speed enforcement 

and better information.

Table 6. Variable speed limit system practices and field results. (Continued)
36

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Location, Time Variable Speed Limit (VSL) 

Summary Evaluation Results

MD 100, 

Maryland, 2009

•	Smooth the transition between free 

flow to congested state.

•	Algorithm consider driver response.

•	Increase average speed and 

throughput, shorter travel time.

I-5, I-90, 

Washington, 

2010

•	Include a few preset speed 

thresholds.

•	When thresholds reached, adjust 

VSL in 5 mi/h increment, with a 35 

mi/h lower bound. 

•	Operator can overwrite automatic 

VSL manually.

•	Reduced average speed, reduced 

flow, travel time reliability increased.

A7/E15 south 

of Lyon, France, 

2011

•	Objective: traffic throughput and 

safety improvement.

•	Triggered by pre-set traffic flow 

thresholds (3000 vehicles per hour) 

with maximum speed limit of 110 

km/h (≈ 68 mi/h).

•	Increased average speed by 4-10%, 

reduced the number of bottlenecks 

by 50%, reduced average travel time 

by 30 seconds, no change in lane 

capacity, reduced incidents by 17%. 

•	Low compliance rate.

I-35W, 

Twin Cities, 

Minnesota, 2010

•	VSL displayed 1.5 miles upstream 

gradually reducing the speed of 

incoming traffic. 

•	Using constant deceleration rate to 

decide VSL at the end of queues.

•	Updated every 30 seconds.

•	Reduced travel time, increased 

traffic volume, less deceleration rate.

I-4, Florida, 

2014

•	Objective: to improve traffic flow; 

to reduce rear-end and lane change 

crash risks. 

•	FDOT conducted an engineering 

and traffic investigation that 

identified reasonable and safe 

speeds under different weather and 

traffic conditions; e.g., some section 

in congested period has VSL at 

20-30 mi/h—lowering upstream 

speed limits by 5 mi/h and raising 

downstream speed limits by 5 mi/h.

•	Not available at time of this 

synthesis.

Table 6. Variable speed limit system practices and field results. (Continued)
37

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Location, Time Variable Speed Limit (VSL) 

Summary Evaluation Results

I-66, Virginia, 

2016

•	A component of active traffic 

management, to improve safety and 

operations on I-66 through better 

management of existing roadway 

capacity. 

•	The ATM includes advisory variable 

speed limits, queue warning 

systems, lane use control signs, and 

hard shoulder running.

•	No specific VSL effects were 

analyzed.

•	Active traffic management has 

limited operational and safety 

impacts during the weekday peak 

periods and some impacts during 

the midday and off-peak weekday 

periods (2% to 6% improvement).

New Jersey 

Turnpike1

•	Deployment for both congestion 

and road weather management; 

operated manually; regulatory for all 

travelers.

•	The speed control strategy 

effectively decreases traffic 

speeds in adverse conditions. 

Speed management and traveler 

information dissemination have 

improved safety by reducing the 

frequency and severity of weatherrelated crashes (improvement 

quantity is not available).

1

 Federal Highway Administration, “Best Practices for Road Weather Management,” (n.d.). Available at: http://ops.

fhwa.dot.gov/weather/Publications/Case%20Studies/14.pdf

Life-Cycle Costs

Limited information is available on the cost of VSL systems. Through interviews with operators, 

it was roughly estimated that the cost of deployment a VSL system along a route varies from less 

than $10 million to almost $40 million. This cost is highly dependent on the existence of current 

intelligent transportation system facilities, such as traffic detectors, VMS, and gantries.

It was even difficult for some agencies to estimate the cost of the VSL system(s) in their States. For 

example, many pieces of hardware, devices, and processes had already been implemented prior 

to VSL deployment in Washington; therefore, the actual cost of their VSL system, sensors, and 

maintenance is unclear. In addition, since the Georgia and Nevada systems are fairly new (both 

are less than 2 years old), neither agency has comprehensive data yet on lifecycle costs. Also, the 

Minnesota DOT is considering using one sign for all lanes (instead of lane-by-lane signs) to reduce 

their maintenance and operations costs.

The approximate cost to install the VSL systems on I-35W and I-94 in Minnesota was $16 million 

and $10 million, respectively. These costs do not include the sensors since they already existed, but 

they do include the lane control signals and structures. 

Table 6. Variable speed limit system practices and field results. (Continued)
38

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

The total cost of the I-66 VSL system in Virginia was $39 million. However, this cost estimate was 

unique to I-66 due to additional costs related to communication, cameras, infrastructure, gantry 

construction, etc. The gantries themselves cost approximately $24 million. The total cost of the 

planned I-77 VSL system in Virginia is $9.6 million (Earnest, 2015). This figure includes a fair 

amount of additional upgrades (e.g. power, etc.). 
39

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

CHAPTER 5. CONCLUSION – BENEFITS 

AND CHALLENGES OF VARIABLE 

SPEED LIMIT SYSTEMS

KEY BENEFITS

In most cases, variable speed limit (VSL) deployments are capable of generating desired traffic 

efficiency and safety system benefits. Because VSL systems have varying deployment goals and 

corresponding system design, varying system benefits resulted. Speed homogenization projects 

usually use simple algorithms in response to real-time traffic, road, and other conditions (e.g., 

weather, work zone, incidents, visibility, etc.), and report safety improvements. Multi-objective 

projects, mostly as a part of active traffic management (ATM) systems, report positive effects on 

mobility, safety, and even the environment. System benefits vary from site to site and it is difficult 

to generalize the reasons for these discrepancies due to many uncontrolled factors among different 

sites, such as compliance rate, driver behavior, or road geometry. However, VSL systems generally 

result in the following benefits:

•	 Smoother traffic flow and less delay. As a component of ATM, VSL proactively manages 

speed to improve traffic flow and safety. Generally, some of the benefits of VSL include 

shortened queues, reduced congestion, quicker clearance during incidents, and fewer 

crashes. For example, Oregon has observed several of these benefits with a substantial 

reduction in speed differentials, improved harmonization, increased roadway capacity, and 

a reduction in crashes along Oregon Route (OR) 217. 

•	 Safer speeds in work zones. While agencies have found that performing nighttime 

construction reduces congestion and shortens traffic queues (compared to daytime 

construction), the lower volume also allows for faster speeds creating dangerous conditions 

in work zones. A VSL system allows the speed limit to be reduced so that vehicles approach 

construction areas and pass through work zones at safer speeds.

•	 Ability to tie to road weather information system (RWIS) data to reduce speeds 

during inclement weather. When installing a VSL system for weather, many agencies can 

tie into existing RWIS stations to provide the data needed to determine when the speed 

limit should be reduced. Implementing VSL during adverse weather conditions can 

significantly improve safety and, in some cases, traffic efficiency.
40

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

KEY CHALLENGES

Implementing a VSL system also comes with challenges, including enforcement of speeds that 

change, driver comprehension, and setting thresholds for speed limit changes (e.g., how much 

precipitation triggers a change, how often the signs should be updated, etc.). Although many VSL 

systems have been implemented across the country (and around the world), each site is unique and 

each system has its own characteristics and capabilities. Virginia DOT’s challenges have included 

acquiring staff with vast capabilities, maintaining reliable system-wide communication, developing 

methods of encouraging compliance, and generating public approval through outreach activities 

(Earnest, 2015). Following are some of the other key challenges agencies experience when 

developing and implementing VSL systems.

•	 VSL enforcement. Nearly every agency operating a regulatory VSL system reports 

challenges with speed enforcement. Law enforcement must know whenever the speed limit 

changes to be able to successfully enforce a VSL. In some instances, law enforcement may 

be hesitant to issue citations because they are unsure of the speed limit or fear a lack of 

supporting evidence for citations to be adjudicated. 

•	 Driver compliance. While enforcement of a regulatory system can be challenging, some 

agencies operating advisory VSL systems report a lack of driver compliance. Some 

agencies operate an advisory system due to current State statutes and agency policies. 

Others initiated their systems as regulatory, but changed to advisory after unsuccessful 

enforcement.

•	 Hardware/software failures. Minnesota has experienced a shorter than expected life from 

their changeable speed limit signs while Nevada has seen their signs displaying different 

speeds when the VSL is activated and all displayed speed limits should be the same. 

Nevada has also had issues of their signs going blank and not displaying any speed limit. In 

addition, the New Jersey Turnpike Authority cited the constantly changing technology and 

necessary system upgrades as major challenges when maintaining a VSL system (United 

States Department of Transportation, 2002). 

•	 Lag in data. Depending on the source of the data or the algorithm used to analyze data, 

there can be a delay which results in the signage not displaying the appropriate speed limit 

for conditions. The New Jersey Turnpike Authority stresses the importance of calculating/

posting the appropriate speed and ensuring that all variable messages are displayed/

removed in a timely manner. 

•	 Returning the VSL back to the normal operating speed. One of the most challenging 

aspects operating a VSL is smoothly and efficiently returning the speed limit back to the 

regulatory speed limit following an issue along the roadway. The better the VSL system can 

transition drivers back to the regulatory speed limit, the better the system will be at 

preventing secondary crashes and keeping drivers safe in general. 

•	 Lack of cost/benefit information to support rationale for a VSL system. As highway 

agencies receive less funding, it is imperative to determine if a system’s benefit will equal 

or outweigh the cost. There is limited information on cost/benefit analyses that agencies can 

use to support new implementation or expand existing systems.
41

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

VARIABLE SPEED LIMIT KEY CONSIDERATIONS

This section provides a list of key factors that agencies should consider when implementing a VSL 

system. The report provides a more detailed discussion on the various items, but this list provides a 

summary for developing preliminary concepts for a VSL implementation.

General Considerations

•	 First, develop some overall goals of what the VSL system should accomplish. It is important 

to note that VSL is not appropriate in all situations. Perform an analysis of whether or not 

VSL will be able to meet the overall goals. 

•	 The goals should include the desired situations in which speeds would be reduced 

(congestion, weather, work zone, etc.). The system design and further planning will depend 

on the overall situations in which the VSL system will be used.

Planning

•	 The planning process should include a detailed systems engineering process to clearly 

identify and communicate objectives, requirements, and anticipated costs/benefits are 

crucial to successful implementations.

•	 Based on VSL system goals and appropriate laws, carefully determine whether the system 

should be regulatory or advisory.

Design

•	 The infrastructure requirements will depend on the system purpose. VSL infrastructure 

requirements can include changeable Speed Limit Signs, weather/environmental sensors, 

traffic speed/volume sensors, and communications equipment to transmit data. 

•	 Selection of control algorithms also varies based on system goals. Algorithms can be 

difficult to calibrate so ample time should be spent on fine tuning the algorithms, 

particularly when incorporating real-time decisions based on congestion.

•	 Systems can be set up to automatically display speeds or to provide recommendations for 

traffic management center staff for choosing to accept the recommendations. It is important 

to determine the method used for speed changes to occur.

Legal and Enforcement Considerations

•	 Review State and local statutes and agency policies to ensure that a VSL system is 

enforceable if a regulatory speed limit is desired.

•	 Begin meeting with law enforcement partners early in the process to discuss any concerns 

and processes for enforcing the VSL system, if enforcement is required.

•	 Ensure that law enforcement personnel can safely enforce speed limits with potential safe 

places to stop violators, if enforcement is required.
42

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

Cost Considerations

•	 When calculating the cost, be sure to account for items beyond the initial system cost, such 

as maintenance, operations, staffing, evaluations, and end-of-life replacement.

FUTURE OF VARIABLE SPEED LIMITS

Following the comprehensive literature review and agency interviews, the research team has 

identified the following needs for future developments of VSL systems.

Variable Speed Limit Systems with Connected and Automated Vehicles

Highway technologies on information sharing and vehicle automation have made encouraging 

successes in recent years. Connected vehicle technology allows infrastructure units and vehicles 

to share high-resolution information from not only aggregated traffic, but also individual vehicles 

with other vehicles on the road, roadside infrastructure, and traffic management centers. With 

such connectivity, traffic operators can transmit traffic control information to individual drivers 

through wireless communication and in-vehicle devices. In addition to connectivity, connected 

and autonomous vehicle (CAV) technology enables vehicles to be controlled by precise, fastresponding, error-free computers instead of error-prone, slowly responding human beings. 

CAV can be expected to provide much richer real-time traffic information (e.g., high-resolution 

vehicle trajectories) than traditional traffic sensors. Such information can be used to better 

understand what is happening, and what is to happen, with highway traffic, which is a new 

information basis for real-time traffic control. Studies have found that only a small market 

penetration percentage of CAV can yield very high benefits. Automation provides a new dimension 

for implementing traffic management strategies by directly regulating each individual vehicle’s 

motion with precise, quickly responding computer algorithms. This will make it possible to extend 

traditional aggregated infrastructure-based traffic control to a disaggregated individual-vehiclebased control. This will achieve higher traffic efficiency, better safety and more comfortable 

individual driving (or riding) experience. 

VSL speed-control algorithms should be updated to take advantage of these new technologies. 

Practically speaking, there will be a long period during which human drivers share the right-of-way 

with CAVs. In such mixed traffic scenarios, how to properly understand interactions between CAVs 

and manual vehicles, and how to utilize their interacting behavior and patterns to improve the 

system performance, is a highly relevant implementation issue yet to be addressed.

It is expected that with wide deployment of CAV technologies, traditional VSL systems that use 

gantries and variable message signs will gradually phase out. VSL or speed harmonization based 

on CAV technology, however, will require high market penetration. This is particularly critical for 

safety purposes and all drivers are supposed to be informed of hazardous and dangerous traffic and 
43

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

weather conditions. In the near term, VSL will still play an important role in traffic management 

systems and it will not be replaced completely by CAV technologies unless high market 

penetration, in some cases 100 percent for safety, is achieved and it is estimated that high market 

penetration of CAV technologies cannot be realized until 2030 (Underwood, 2016).

Collecting and Processing Big Data

Traditional VSL techniques only use aggregated traffic data obtained from regular point detectors 

(e.g., loop detectors, traffic cameras). Nowadays, increasingly advanced traffic sensors, such as 

in-vehicle Global Positioning System devices or connected vehicle technologies, provide higher 

resolution data with a wider coverage area: primarily, a more accurate aggregated traffic state 

(e.g., density, speed), and more detailed individual vehicle data (e.g., vehicle trajectories). The 

data generated from connected vehicle technologies (when fully deployed) will be much greater 

in quantity and much more complex in structure than traditional point detector data. How to 

utilize these data in the future VSL or connected-vehicle-based control paradigms is an interesting 

question. Real-time collection, storage, processing and decision-making using emerging big data 

sources is a promising VSL development.

Consideration of Driver Compliance

Driver compliance or driver response is a critical factor for effectiveness of VSL systems. Driver 

compliance rates, however, can vary dramatically across different projects due to information 

communication mechanisms, regulation, education, culture, and many other factors. Traditional 

VSL broadcasts uniform speed limit information via roadside infrastructure, and emerging 

connected vehicle technologies can send customized messages to each individual vehicle. Hayat et 

al. (2015) selected a small number of representative drivers to conduct a field test to evaluate driver 

compliance with different advisory messages, including VSL, lane change advisory, and merge 

control. From a human factors perspective, it is critical to understand how to design ATM signs. 

The recent FHWA ATM signage study (Perez et al., 2010) developed and tested alternative signs for 

variable speed limit (VSL) signs and used the deployments in Minnesota and Washington as inputs 

to sign development. Laboratory and field studies determined both the comprehension of the ATM 

signs as well as their respective legibility distances. Another major issue is how to make drivers 

believe that they would be better off (e.g., save time or reducing crash risks) if they comply with 

VSL messages and travel at a slower speed than they intuitively desire. 

Further, VSL speed control algorithms should explicitly consider potential driver response or 

compliance rate. Considering driver compliance inevitably increases the complexity of such 

algorithms, but real-world deployments of these algorithms should consider the added complexity. 

Without pre-validated evidence of compliance rates, field studies should be conducted before 

implementing and tuning these algorithms in the real world.

45

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

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SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

APPENDIX B. AGENCY INTERVIEW SUMMARY

ARIZONA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY 

Note that Arizona has not implemented a variable speed limit (VSL) system; however, three 

representatives from the Arizona Department of Transportation (DOT) were interviewed to discuss 

their opinions of VSL systems in general as well as the future of their VSL plans and to establish 

specific information regarding VSL systems that would assist the State in the future.

ARIZONA

Interview Date: 

Name: 

Agency: 

Phone Number: 

9/9/2016

Mark Poppe

Arizona DOT

602-359-2277

INTERVIEW QUESTIONS

Based on what you have heard 

from other agencies, what is your 

impression of VSL?

VSL has promise and is capable of reducing the 

number of collisions along roadways. 

Have you considered implementing 

VSL in Arizona?

Yes. We are considering one rural location that 

is prone to dust storms, which is similar to a fog 

problem.

What institutional and policy 

hurdles would you encounter if 

implementing VSL?

This is unknown at this time.

If Arizona implemented VSL, would 

you use it for congestion, work 

zones, weather, or other functions?

We would use it for weather. A safety study was 

completed at the potential site, and it suggested that 

VSL may be a good idea (although this is still being 

assessed). 

What kind of information would 

you want to have in a VSL document 

to help with decisions?

A few topics would be helpful: other States’ 

experiences, public acceptance, information about 

compliance/enforcement, etc. Speed limit compliance 

is always an issue with any type of system. 
52

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

ARIZONA

Interview Date: 

Name: 

Agency: 

Email Address:

Phone Number: 

9/20/2016

Scott Beck

Arizona DOT

sbeck@azdot.gov

602-712-6391

INTERVIEW QUESTIONS

Based on what you have heard 

from other agencies, what is your 

impression of VSL?

The research we have found says that VSL is fairly 

effective on weather-related incidences. We know 

Colorado and Washington have implemented VSL 

systems for snow. We are now in the process of designing 

a VSL system to counteract our current dust problem. We 

have a specific location that is impacted by dust storms. 

As far as benefits from VSL systems to solve routine 

congestion issues, we are not sure if there is significant 

research showing the effectiveness.

Have you considered 

implementing VSL in Arizona?

Yes, in one specific area. We have one place we are 

currently designing a VSL system for, which is our most 

concentrated area because of the terrain and land uses 

surrounding it. There are some old farm fields that are 

no longer maintained surrounding the area, so there 

is layer of loose soil that gets picked up by the wind. 

If the feedback is successful, then we would look into 

implementing other VSL systems for some northern areas 

of the State which struggle with snow-related issues.

What institutional and policy 

hurdles would you encounter if 

implementing VSL?

Our State statutes allow VSL systems, so we don’t have 

any issues from a legislative perspective. We are allowed 

to set speed limits and govern speed limits by time of 

day, vehicle type, weather conditions, etc. The only 

hurdle Arizona has now is how to coordinate with law 

enforcement. Our system is designed to be regulatory, 

so we need a plan for how law enforcement will actually 

enforce the speed limits.

If Arizona implemented VSL, 

would you use it for congestion, 

work zones, weather, or other 

functions?

Weather.
53

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

ARIZONA

What kind of information would 

you want to have in a VSL 

document to help with decisions?

Short Answer: There is no authoritative document related 

to compliance. Arizona would like to see quantitative 

measures on voluntary compliance and whether the 

compliance rates have any safety benefit from those 

locations that have already implemented VSL. 

Long Answer: A lot of States have implemented VSL 

systems, but it doesn’t seem like there is significant 

research on the actual compliance rates. VSL is still fairly 

new – If you looked for VSL systems 5 years ago, there 

weren’t many, but now there are quite a few. There are a 

couple research papers that suggestively infer that VSL 

systems make roadways safer, but it’s based on theory 

and some modeling. Arizona would like to see statistical 

data related to active VSL systems, particularly on the 

safety side. This is especially important when considering 

implementing VSL systems to reduce congestion. 

Arizona’s downtown areas have higher crash rates, so they 

might consider implementing VSL systems to make those 

areas safer; but, just putting up a sign isn’t going to slow 

people down. 

ARIZONA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

11/22/2016

Reza Karimvand 

Arizona DOT

RKarimvand@azdot.gov

602-712-7640

INTERVIEW QUESTIONS

Based on what you have heard from 

other agencies, what is your impression 

of VSL?

VSL is good for rural areas, but it is not good for 

urban areas. In urban areas, we don’t need to add 

gantries to inform the public that they need to 

reduce their speeds; pretty soon we will have V2V, 

V2I, etc. and we will have all that information 

available on the dashboard. However, in rural areas, 

VSL would definitely be helpful.

Have you considered implementing VSL 

in Arizona? We are currently designing a VSL system. 

What institutional and policy hurdles 

would you encounter if implementing 

VSL?

We don’t have any – our policies say we can specify 

the VSL.
54

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

ARIZONA

If Arizona implemented VSL, would 

you use it for congestion, work zones, 

weather, or other functions?

Weather. 

What kind of information would you 

want to have in a VSL document to help 

with decisions?

We already have everything. 

FLORIDA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/22/2016

Alan S. El-Urfali 

Florida DOT

alan.el-urfali@dot.state.fl.us

805-410-5416

GENERAL VSL INFORMATION

What State is the VSL system(s) located 

in? Florida.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire State, 

only a certain area, along certain 

roadways, etc.)?

1) I-4.

2) US 27 – West of Fort Lauderdale (two-lane, 

divided, rural, high speeds).

How long is the VSL system(s) (in miles)? 1) 10.5 miles.

2) 3 miles, both directions.

What is the operation type of the VSL 

system(s) (Manual, Automated, or 

Hybrid))?

1) Mainly automated, but operator input is still 

used.

2) Automated. 

What is the primary function of the VSL 

system?

• Congestion

• Weather

• Work Zones

Both are used for congestion. 

SETTING SPEED LIMITS

Describe how the displayed speed limit is 

determined. If you use an algorithm, can 

you share it with us?

1) The algorithm uses 85th percentile for the speed 

limit with 5 mi/h increments.

How do wet conditions affect speed 

selection (rain intensity of “X” reduces 

the speed by “Y,” etc.)?

1) When we measure, conditions have to be dry. 

FLORIDA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
55

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

FLORIDA

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

1) Design speed is taken into consideration 

(American Association of State Highway and 

Transportation Officials criteria – curves, 

superelevation, sight distance, etc.). We measure 

prevailing speed. 

Has VSL been effective at reducing 

speeds? Is operating speed data available 

in the vicinity of your VSLs?

1) This VSL system has not been very successful, 

but it is still in operation. There was a study 

conducted 3 years ago from University of Florida 

(before/after) that looked at the VSL algorithm and 

made recommendations for improvement, although 

Florida DOT has not implemented the changes yet. 

Overhead signs and more enforcement is needed. 

2) The VSL system is effective now. We did not 

observe much compliance until enforcement was 

added. 

What are the VSL system(s) pros and 

cons with respect to setting speed?

If you sign the VSL system well and have 

enforcement, you will have a successful operation; 

if not, then you have no compliance. The 

infrastructure used for the VSL system must have 

good quality and not fade in the sunlight. 

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

1) Loop detectors, side-fire radar (volume, speed, 

occupancy), closed-circuit television cameras (can 

see weather, but not really using the system for 

weather conditions).

2) Loop detectors, side-fire radar (added later to 

detect speed and check compliance rates). 

How reliable are the sensors that are 

used?

The loop detectors are very reliable. The old sidefire radar systems had huge issues – the best you 

could get with side-fire radar systems is 92%. 

Do you have any design drawings for 

your system layout? These drawings 

can be a typical layout or a site-specific 

layout.

Could get them, if necessary. 

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)? Both are regulatory. 
56

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

FLORIDA

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

1) We did hold some meetings with the Highway 

Patrol to get their input and comments. When 

you have such a large volume of vehicles in the 

peak hours, it is very difficult to pull over one 

or two violators, especially without causing a 

secondary crash. So, the Highway Patrol stayed 

out of it completely. Florida DOT provided 

outreach materials (e.g. brochures, websites, 

announcements, radio ads, etc.), but the outreach 

was unsuccessful. Some of the lessons learned: 

we need better (possibly overhead) signs and 

enforcement partners should be involved to 

observe better compliance rates. 

2) Once the Highway Patrol began issuing tickets, 

everyone complied.

VSL SIGNS

How many VSL signs are associated with 

your system(s)? 1) Approximately 20 – one sign or more per mile. 

Where are the VSL signs located (right/

left shoulder, overhead, median, sidemounted, etc.)?

Both systems have signage on the roadside and on 

medians. 

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

1) Yes.

2) No. 

MISCELLANEOUS

How do you accommodate work zones in 

the vicinity of the VSL system(s)?

1) We completely turn off the system to 

accommodate work zones in the vicinity of 

the VSL system. The entire corridor is being 

reconstructed, so work zones are common. 

2) There isn’t much construction in this area, so 

work zones are not a problem. 

To your knowledge, what is the public 

perception of the VSL system?

1) The University of Florida studied the VSL 

system and discussed outreach/public opinions of 

the system within the final report.

Are you planning to expand or decrease 

the length of the VSL corridor (based on 

the public’s response)?

No for both. 

Do you have any additional information/

comments that we should include in our 

report?

Read through the University of Florida study 

results for additional information. 
57

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

GEORGIA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/23/2016

Mark Demidovich

Georgia DOT

mdemidovich@dot.ga.gov

678-852-0852

GENERAL VSL INFORMATION

What State is the VSL system(s) located in? Georgia.

What route(s) is the VSL system(s) located along 

and how would you describe that location (the 

entire State, only a certain area, along certain 

roadways, etc.)?

I-285.

How long is the VSL system(s) (in miles)? 36 miles total (includes both directions).

What is the operation type of the VSL system(s) 

(Manual, Automated, or Hybrid))? Automated.

What is the primary function of the VSL system?

• Congestion

• Weather

• Work Zones

 Congestion.

SETTING SPEED LIMITS

Describe how the displayed speed limit is 

determined. If you use an algorithm, can you 

share it with us?

The speed limit is determined by sensormeasured speed of traffic downstream. Various 

sensors are used and they are placed every 1/3 

mile. Data is transmitted every 20 seconds and 

includes traffic volume and the average speed. 

Some video detection is used. Cameras are 

installed on 80’ poles, but they are fixed and 

do not pan. Video runs through a processor. 

Georgia DOT does not use probe data because 

there is a lag in that data. The system is 

automated with an option to change the speed 

manually for work zones. Construction is 

typically performed at night when traffic is 

lighter, which makes vehicle speeds faster.

What lanes does your display(s) apply to (one 

display for all lanes, there are displays per lane 

but speeds are identical, HOV lane is a different 

speed, etc.)?

The speed limit is applicable to all lanes. 

GEORGIA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
58

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

GEORGIA

What lanes does your display(s) apply to (one 

display for all lanes, there are displays per lane 

but speeds are identical, HOV lane is a different 

speed, etc.)?

The speed limit is applicable to all lanes.

How do wet conditions affect speed selection 

(rain intensity of “X” reduces the speed by “Y,” 

etc.)?

Weather stations have no input into the VSL 

system. 

Has VSL been effective at reducing speeds? Is 

operating speed data available in the vicinity of 

your VSLs?

Georgia DOT is not seeing much improvement 

when analyzing basic elements (such as average 

speed). A possible reason is that the recession 

ended at the same time the VSL was activated 

so there are more vehicles on the road.

EQUIPMENT AND COSTS

How reliable are the sensors that are used? Georgia DOT reports that the sensors being 

used are reliable.

Do you have a cost/benefit analysis? No.

ENFORCEMENT

How is your VSL system(s) enforced (regulatory, 

advisory, hybrid)? Regulatory. 

What are you experiences with enforcement and 

the judicial process when it comes to VSL?

The Georgia Highway Patrol (GHP) enforces 

the speed limit as well as local jurisdictions. 

(The VSL system crosses 14 different 

jurisdictions.) Georgia DOT met with GHP to 

explain the system and they were not excited. 

Georgia DOT archives data and can give it 

to GHP. Law enforcement does not focus 

on speed, but more on aggressiveness. They 

ticket for recklessness or driving too fast for 

conditions instead of ticketing for speeding. 

Georgia DOT has given GHP a data feed so 

they can see the VSL signs at all times, but not 

sure if it is being used. 

VSL SIGNS

How many VSL signs are associated with your 

system(s)? 176.

Where are the VSL signs located (right/left 

shoulder, overhead, median, side-mounted, etc.)?

Signs are mounted on the right and left 

shoulders; VSL system signs are also posted on 

Georgia DOT's website so people can see the 

current speeds.

MISCELLANEOUS

How do you accommodate work zones in the 

vicinity of the VSL system(s)?

Ability to manually change the speed limit for 

work zones.

To your knowledge, what is the public perception 

of the VSL system?

Negative because the public thinks its purpose 

is for generating funds.
59

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

MINNESOTA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

7/1/16

John McClellan and Brian Kary 

Minnesota DOT (MnDOT)

john.mcclellan@state.mn.us, brian.kary@state.mn.us

651-234-7025

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Minnesota.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

I-35W (18 miles; Burnsville to Minneapolis).

I-94 (10 miles; Minneapolis to St. Paul).

How long is the VSL system(s) (in 

miles)?

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

Both systems are currently deactivated.

What is the operation type of 

the VSL system(s) (Manual, 

Automated, or Hybrid))?

Both systems used automated operations.

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

Congestion. 

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

Both used an algorithm developed by the University of 

Minnesota-Duluth.

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

No; lane-by-lane signage is spaced every 1⁄2 mile along 

the corridor. When congestion is detected, up to three 

sets of lane control signals (1.5 miles) can be activated 

prior to the congestion and speeds are gradually stepped 

down as traffic approaches congestion.

MINNESOTA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
60

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

MINNESOTA

What lanes does your display(s) 

apply to (one display for all lanes, 

there are displays per lane but 

speeds are identical, HOV lane is a 

different speed, etc.)?

The displays are per lane, but the same speed is 

displayed on all at the same location. The highoccupancy toll (HOT) lane displays a white diamond 

with no speed message. This was done to mitigate 

concerns about displaying different speeds over 

different lanes, but MnDOT did not want to artificially 

slow down the HOT lane.

What lanes does your display(s) 

apply to (one display for all lanes, 

there are displays per lane but 

speeds are identical, HOV lane is a 

different speed, etc.)?

The displays are per lane, but the same speed is displayed 

on all at the same location. The high-occupancy toll 

(HOT) lane displays a white diamond with no speed 

message. This was done to mitigate concerns about 

displaying different speeds over different lanes, but 

MnDOT did not want to artificially slow down the 

HOT lane.

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y,” etc.)?

The system could be turned off by operators, but this 

was never done in practice; the system was usually 

left on.

Does roadway curvature 

(horizontal or vertical) and sight 

distance get considered in your 

speed setting algorithm? Did 

you locate your VSL signage in 

its current spot because of sight 

distance or other issues?

No.

Is pavement type/condition 

considered in the speed setting 

algorithm?

No.

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your 

VSLs?

A minor reduction was observed in one location, but 

overall, there has been little observed difference in 

speeds.

What are the VSL system(s) pros 

and cons with respect to setting 

speed?

VSL requires enforcement to gain compliance. If it 

is not enforced, then speeds need to match drivers' 

expectations of what is sensible to them. The system 

was slow at responding to real-time conditions which 

caused the public to lose trust. The system was turned 

off to allow MnDOT time to reevaluate the system and 

make improvements.

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

Loops and microwave.
61

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

MINNESOTA

How reliable are the sensors that 

are used?

Both are reliable. Data was transmitted every 

30 seconds, but that was not fast enough to match 

conditions. Loop detector data was averaged which 

added to the slow response of the system.

Do you have any design drawings 

for your Can you share any cost 

information for your system(s) (cost 

of the system, cost of any sensors 

used, maintenance costs, etc.)?

I-35W: Approximate cost to install was $16M. 

This included the lane control signals and structures. 

The sensors already existed. 

I-94: Approximate cost to install was $10M. This 

included the lane control signals and structures. 

The sensors already existed. 

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)?

Advisory; making the system regulatory would have 

required a legal change. Static speed limit signs are 

55 to 60 mi/h. The VSL would start displaying at 5 mi/h 

below posed speed with a minimum speed limit of 

30 mi/h.

VSL SIGNS

How many VSL signs are associated 

with your system(s)?

I-35W: 155 signs.

I-94: 101 signs.

Where are the VSL signs located 

(right/left shoulder, overhead, 

median, side-mounted, etc.)?

Overhead.

What is the display technology used 

(R2-1 signage, embedded YLED, 

shared CMS, etc.)?

Full matrix color CMS; 4 feet tall by 5 feet wide. 

What were your control specs for the 

actual VSL signs (Bid documents? 

Standards and specs book?)? 

RFP for equipment which was then provided as Statefurnished materials to the installer.

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

No.

MISCELLANEOUS

How do you accommodate work 

zones in the vicinity of the VSL 

system(s)?

No changes.
62

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

MINNESOTA

To your knowledge, what is the 

public perception of the VSL 

system?

MnDOT received very few comments when the VSL 

was first implemented. Most were questions about 

the meaning of the messages. Drivers were not sure 

if the displayed speed reflected speeds ahead or the 

recommended speed. After the system was deactivated, 

MnDOT only had two inquiries about why the system 

was turned off. 

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

MnDOT will most likely decrease the length of the 

corridor. There have been maintenance issues with the 

signs so they are considering either replacing them in 

kind or using a single message sign as opposed to laneby-lane signage to reduce installation, maintenance, and 

operations costs. MnDOT is evaluating whether a VSL 

or a simple message of SLOW TRAFFIC AHEAD is 

better for queue warning in a particular high crash area. 

Depending on the outcome, MnDOT would consider 

using VSL for spot locations rather than a full corridor 

approach.

Do you have any additional 

information/comments that we 

should include in our report?

VSL worked well for recurring congestion, but did not 

work well for non-recurring congestion.

One of the VSL roadways is a 5-lane freeway with good 

sight distance so drivers can see the slowed traffic 1 mile 

ahead. Roadway design is a factor in the effectiveness of 

VSL. If MnDOT were to do it again, they would focus 

VSL in places where sight distance is not good; they 

would use it in spot locations.

Drivers would not decelerate from 70 mi/h to 45 mi/h 

because they would get run over; it was unsafe.

VSL should include additional information to help 

motorists understand the reason for the speed change. For 

example, messages such as SLOW TRAFFIC AHEAD.

MnDOT has not seen a reduction in crashes. 

MnDOT will be using a queue warning system on a 

construction project on I-94 that uses Doppler radar 

sensors and PCMSs every 1⁄2 mile. The system will 

display the actual speed downstream. So the signs will 

display XX MPH SPEED 1⁄2 MILE AHEAD. When 

the speed is <15 mi/h, the sign will display STOPPED 

TRAFFIC AHEAD. This system could be more of a 

replacement for VSL if it is effective. The construction 

project will be 2 years. The queue warning system will 

be activated in August 2016 for 3 months and will then 

be active all next construction season. UMN will do the 

evaluation after summer 2017.
63

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

MINNESOTA

Do you have any additional 

information/comments that we 

should include in our report? 

(Continued)

MnDOT uses ramp metering for roads with closer 

interchanges and VSL for roads where interchanges are 

farther apart. What is the cost/benefit for each?

MnDOT emphasizes that there is a difference between 

speed harmonization and queue warning. They consider 

queue warning in spot locations.

If the system is being used primarily as a queue warning 

system, would a dynamic message sign (DMS) or even 

a static sign with remote flashers be just as effective 

at less cost? Is speed harmonization effective in urban 

areas or is it more suited for exurban areas with long 

distances between ramps? The entire VSL cost needs to 

be compared with DMS and ramp metering strategies. 

Full VSL costs must include operations, maintenance, 

utilities, and end-of-life replacement, all of which are 

more involved than other strategies. MnDOT has found 

ramp metering to be the more cost effective.

MISSOURI

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

5/20/2016

Alex Wassman 

Missouri DOT

alex.wassman@modot.mo.gov

573-526-0121

GENERAL VSL INFORMATION

What State is the VSL system(s) located in? Missouri.

What route(s) is the VSL system(s) located along 

and how would you describe that location (the 

entire State, only a certain area, along certain 

roadways, etc.)?

I-270 in St. Louis.

What is the status of the VSL system(s) (Active, 

Removed, Under Construction, Planned, etc.)? Deactivated.

ENFORCEMENT

How is your VSL system(s) enforced (regulatory, 

advisory, hybrid)?

The system was implemented as regulatory, 

but law enforcement was not sure what the 

speed limit was and reluctant to enforce and 

give citations. The system was consequently 

changed to advisory, but driver compliance 

became an issue so the system was deactivated. 

MISSOURI DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
64

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEVADA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY

NEVADA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/24/2016

Alex Wolfson

Nevada DOT (NDOT)

awolfson@dot.state.nv.us

775-834-8365

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Nevada.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

US-395 in Reno.

How long is the VSL system(s) (in 

miles)? ~4 – 5 miles.

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

Active.

What is the operation type of the 

VSL system(s) (Manual, Automated, 

or Hybrid))?

The system is all automated with no human interface 

from the traffic management center.

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

Weather.

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

There are two different speed limits on US-395 (55 

mi/h and 50 mi/h). When the VSL is activated, all 

speeds are lowered to 45 mi/h. The VSL system is 

tied to 2 RWIS stations. A 30 mi/h wind gust is the 

threshold for activating the VSL. At least 30 minutes 

must pass with no 30+ mi/h wind measured on either 

RWIS before the speed limit can return to 50/55 mi/h. 

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

Yes.
65

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEVADA

What lanes does your display(s) apply 

to (one display for all lanes, there 

are displays per lane but speeds are 

identical, HOV lane is a different 

speed, etc.)?

The VSL applies to all lanes.

How do wet conditions affect speed 

selection (rain intensity of “X” reduces 

the speed by “Y,” etc.)?

N/A.

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

No.

Is pavement type/condition considered 

in the speed setting algorithm? No.

What are the VSL system(s) pros and 

cons with respect to setting speed?

The system has been hit or miss; NDOT has 

experienced hardware problems with the signs.

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

2 road weather information system (RWIS) stations; 

one on the northern end and one on the southern end 

of the valley.

Do you have a cost/benefit analysis? No.

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)? Regulatory.

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

Law enforcement is not positive and their confidence 

is the system is shaky because of the hardware 

problems being experienced. (For example, 45 mi/h is 

displayed in one direction and 55 mi/h is displayed in 

the opposite direction.) Law enforcement is notified 

by phone by the District's Road Operations when 

the larger wind warning system closes routes so 

the speed limit reduction implemented by the VSL 

system is implied.

VSL SIGNS

How many VSL signs are associated 

with your system(s)? 7 using embedded LEDs.

Where are the VSL signs located 

(right/left shoulder, overhead, median, 

side-mounted, etc.)?

Right shoulder mounted using NDOT’s methodology 

for sign placement (typically placed at cross streets 

with US-395).
66

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEVADA

MISCELLANEOUS

To your knowledge, what is the public 

perception of the VSL system?

Some positive and negative reactions. Homeowners 

have been negative because they see the problems 

with the signs (e.g., blank signs). At the request of law 

enforcement, beacons flash when the speed changes. 

These beacons are too bright for homeowners so 

NDOT has unplugged them for now and they will try 

a dimmer.

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

The US-395 system may be deactivated due to the 

hardware problems and law enforcement's lack 

of confidence. Additional VSL systems are being 

planned in urban areas. Specifically, interstates in 

Las Vegas.

Do you have any additional 

information/comments that we should 

include in our report?

No liability issues to report. They were not aggressive 

with a public relations campaign initially. VSL was 

implemented on US-395 because it is a low-volume 

road and does not attract a lot of attention. A good 

lesson is to start small when implementing VSL.

NEW JERSEY

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/27/2016

Henry “Chip” Eibel

New Jersey Turnpike Authority

eibel@turnpike.state.nj.us

732-442-8600 ext. 2901 

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? New Jersey.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

Statewide. 

How long is the VSL system(s) (in 

miles)? ~148 miles.

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

Active.

NEW JERSEY DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
67

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEW JERSEY

What is the operation type of the 

VSL system(s) (Manual, Automated, 

or Hybrid))?

Manual (used to be automatic). 

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

Congestion, Weather.

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

We used to use California’s algorithm back when it 

was still automated. Currently, there is no automation 

to the VSL right now, although we are working with 

IBM and Rutgers to come up with a plan to automate 

the system. The supervisors drop the speed according 

to a current downstream issue. Most of the time, 45 

mi/h is posted, unless there are foggy conditions or 

bad operations. The system used to work by posting 10 

mi/h more than the downstream traffic. For example, if 

the speed was 25 mi/h downstream, we would post 35 

mi/h because we didn’t want to create a second pocket 

of congestion due the change in speed. 

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

There are different sections, but right now, the 

maximum speed is 65 mi/h. There are areas that 

are 55 mi/h. The supervisors manually populate the 

VSL, and just post 45 mi/h, unless there are poor 

conditions (snow, fog, etc.). 

What lanes does your display(s) 

apply to (one display for all lanes, 

there are displays per lane but 

speeds are identical, HOV lane is a 

different speed, etc.)?

All lanes are same, but we are looking to have different 

speeds across lanes in the future. We do have a shoulder 

we use during high peak periods with variable message 

signs (VMS) and green arrows/red X’s. 

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y,” etc.)?

There are guidelines regarding fog, heavy rains, 

heavy snow, etc. For visibility issues, troopers call 

in with how many mile markers they can see. If they 

can see 3 mile markers, then the speed is set to 35 

mi/h. If they can only see 2 mile markers, then the 

speed is set to 30 mi/h, and then they contemplate 

closing the road. 

Is pavement type/condition 

considered in the speed setting 

algorithm?

Pavement conditions are account for. If the road is 

being treated, then we will drop the speed. 
68

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEW JERSEY

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your 

VSLs?

It used to be very effective when it was automated. 

Our VSL signs are adjacent to VMS, so the VMS 

message emphasizes the reason for the speed 

reduction. For example, the VMS would say 

“Accident Ahead, Be Prepared to Stop.” There is 

always a VMS adjacent to the VSL that gives a 

reason for the speed. 

What are the VSL system(s) pros 

and cons with respect to setting 

speed?

We have had the VSL system for over 40 years, 

maybe almost 50. It is enforceable (not advisory), and 

the State police do issue tickets accordingly. Courts 

have also held to the standard. Automatic systems 

would allow more throughput, so that's the next 

step. The old system was automatic, but we had to 

switch to manual due to maintenance on the copper 

inductive loops every time we repaved. Since the 

traffic volume has increased so much now, it makes 

it really hard to carve out time for maintenance. Now 

we have difference sensors up and down the roadway 

that are very reliable, but they are spaced farther 

apart than the conductive loops we used to use, so 

there is a little bit of lag in the system. 

EQUIPMENT AND COSTS

How reliable are the sensors that are 

used?

We get exact traffic counts now - no problems at all. 

Sensys is the name of the new sensors they use now. 

Caltrans uses them and they are also used on Golden 

Gate Bridge. 

Can you share any cost information 

for your system(s) (cost of the 

system, cost of any sensors used, 

maintenance costs, etc.)?

I would have to look into it. Sensor reinstallation 

is always included in our paving contracts. So 

now when we repave, we have funding to replace 

the sensors in that repaving section to ensure 

functionality.

Do you have any design drawings 

for your system layout? These 

drawings can be a typical layout or 

a site-specific layout.

http://www.state.nj.us/turnpike/standard-drawings.

html.

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)? Regulatory. 
69

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

NEW JERSEY

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

We have experiences with law enforcement all 

the time. Anytime there is a bad crash, especially 

secondary crashes, troopers will come back to us and 

ask what the VSL said at that time of the incident. 

Then, the troopers will issue a summons based on the 

speed that was posted. 

VSL SIGNS

How many VSL signs are associated 

with your system(s)? ~250, including VMS and VSL signs.

Where are the VSL signs located 

(right/left shoulder, overhead, 

median, side-mounted, etc.)?

The Turnpike has overhead VSL signs over the right 

lane and full VMS signs as well. The speed limit and 

variable messages are all posted in the same sign. 

Do you use any associated word 

messages (changeable message 

signs) in conjunction with VSL 

(“congestion ahead,” “slow ahead,” 

etc.)?

Yes:

• ACCIDENT AHEAD BE PREPARED TO STOP.

• DEBRIS AHEAD DRIVE WITH CAUTION.

• DELAYS AHEAD BE PREPARED TO STOP.

• MOWING OPERATION AHEAD.

• REDUCE SPEED CONGESTION AHEAD. 

MISCELLANEOUS

To your knowledge, what is the 

public perception of the VSL 

system?

The tricky part is you have to post the appropriate 

speed. There is nothing worse than saying there is 

a downstream problem, and then the problem isn't 

there. This is why we want to make our system 

automated again. The main difficulty is returning the 

speed back to normal after the problem has cleared. 

The better you get at this, the better it is for drivers, 

and you can hopefully prevent secondary collisions. 

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

No. Our entire network has VSL signs on it now. 

Do you have any additional 

information/comments that we 

should include in our report?

We only post VSL and VMS if it’s within 2 miles 

of the lane closing due to construction, congestion, 

debris, etc. If we post signs further away, it’s too 

far away and drivers tend to forget by the time they 

get there. For example, if there is a lane closing 

from mile marker 10 to mile marker 8, the VSL 

and VMS at mile marker 12 may say "Road Work 

Ahead, 2 miles." Then at mile 8, the VMS would 

say "roadwork continues" b/c you are now within the 

work zone. 
70

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

OREGON

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/24/2016

Mike Kimlinger

Oregon DOT

Michael.J.Kimlinger@odot.state.or.us

503-986-3557 

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Oregon.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

1) Oregon Route (OR) 217 – Adjacent to I-5 in 

Portland area.

2) OR 213 – West of downtown Portland; single 

location sign at a single intersection, recreational 

traffic issues, 1st deployment of VSL in Oregon. 

How long is the VSL system(s) (in 

miles)?

1) 7 miles.

2) 1 intersection.

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

Both are active.

What is the operation type of the 

VSL system(s) (Manual, Automated, 

or Hybrid))?

1) Fully automated.

2) Semi-hybrid. 

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

1) Congestion, weather.

2) Congestion. 

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

1) In-road, downstream sensors are used that measure 

85th percentile speeds at a 1 minute interval. We pair 

up the segments of the highway so that the decrease 

from one speed to the next is no more than 10 mi/h 

between segments. The current algorithm can be used 

for advisory and regulatory systems. 

2) Single sign. 

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

1) No. There are 7-8 segments through the whole 

corridor, and each segment is evaluated separately. 

OREGON DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
71

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

OREGON

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y,” etc.)?

1) There is a friction factor sensor at each sensor 

location. When the friction factor gets down to 

certain level, the weather piece of the algorithm takes 

over from the congestion piece of the algorithm. It 

just depends on which one reports the most needed 

condition change. Everything is automated. 

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

1) No. OR 217 is a freeway-level facility with a 

regulatory speed of 55 mi/h, but vehicles can drive 

much faster than that (curvature piece is not an issue). 

We have 5 other systems in line to be installed in the 

next couple years, and none of them need to account 

for roadway curvature. 

Is pavement type/condition 

considered in the speed setting 

algorithm?

1) Surface conditions are all weather related, so the 

pavement type/condition is included in the friction 

factor portion. The condition of the roadway is 

considered when calibrating the friction factor. 

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your 

VSLs?

Yes for both systems. 

What are the VSL system(s) pros and 

cons with respect to setting speed?

1) It is very difficult for any algorithm to react like 

a driver. Recovery from a reduced speed is very 

difficult, and it is hard to build a VSL algorithm to 

react like a human would. It takes a lot of tweaking to 

make it more naturalistic. Most of the public feedback 

has been along these lines, although we have not 

received much public feedback. 

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

1) Radar-based sensors from Wavetronix. 

How reliable are the sensors that are 

used? 1) Very reliable.

Can you share any cost information 

for your system(s) (cost of the system, 

cost of any sensors used, maintenance 

costs, etc.)?

1) They could send us a PowerPoint presentation 

containing cost information. 

Do you have any design drawings for 

your system layout? These drawings 

can be a typical layout or a sitespecific layout.

1) They could send us a set of plans, if necessary.
72

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

OREGON

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)?

1) Advisory.

2) Regulatory. 

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

1) One of the reasons we went to an advisory system 

on OR 217 was that there was not a lot of extra 

shoulder and the State troopers and local police 

were worried about being able to do any kind of 

enforcement since enforcement can cause more 

congestion. Currently, the system is enforced by “basic 

rule” rather than examining vehicle speed (Is the 

individual driving safely and prudently?). The same 

reasoning is being used for the other areas where VSL 

systems will be installed. We are planning to install 

a 30-mile, weather-based, regulatory VSL system 

because we hope that regulatory will mean better 

compliance. But first, we have to change State law to 

install a VSL system on the interstate. Currently, the 

State law discusses what the speed limit should be 

on each interstate, so our ability to change the speed 

requires us to go through a legal adoption process, 

which is very long and drawn out. In order to install 

the future regulatory system, we must have the laws 

in place. There is some compliance on OR 217, a 

substantial reduction in speed differentials, better 

harmonizing of speeds, increase in capacity, and a 

reduction in crashes. Since safety has improved, the 

system is considered effective. 

VSL SIGNS

How many VSL signs are associated 

with your system(s)?

1) There are 7 or 8 segments and a set of signs for 

each segment. There is a sign over each lane at each 

location plus VMS sign that further discusses current 

conditions. There are about 40-50 signs on main line 

plus “travel time” messages at every intersection 

as you enter the OR 217 corridor, which accounts 

for another 30-40 VMS signs. All the signs are full 

matrix. We could change the system from advisory to 

regulatory, if needed, at any time.

2) Just one sign.

Where are the VSL signs located 

(right/left shoulder, overhead, 

median, side-mounted, etc.)?

1) Overhead.

2) Side-mounted.
73

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

OREGON

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

1) Yes – “Congestion Ahead” signage is used and an 

approximate distance is given. Weather conditions 

are also included in messages (e.g. wet, icy, etc.). 

MISCELLANEOUS

How do you accommodate work 

zones in the vicinity of the VSL 

system(s)?

1) We do not have much experience with work zones 

on OR 217 yet. The last major construction in the area 

was the installation of the VSL system itself. This is 

an experience still to be learned.

To your knowledge, what is the public 

perception of the VSL system?

1) We have received bits and pieces of feedback. 

The feedback seems to be fairly positive, other than 

early in system deployment (the algorithm has been 

tweaked since). We have not received any bad press 

or public comments since the last set of tweaking.

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

1) OR 217 is an isolated corridor, so the VSL system 

will not be extended. It is a heavily instrumented 

corridor – weather-based, speed-based active curve 

warning systems at either end also exist. 

Do you have any additional 

information/comments that we should 

include in our report?

1) In order to achieve low bad press, public notification 

is important, including why and when the system will 

be installed, functioning, etc. Deployment of a VSL 

system anywhere requires someone who understands 

the system and can tweak the algorithm on a regular 

basis for the first 6 months to a year. The algorithm 

will need to be adjusted based on its location and the 

kinds of drivers that utilize the roadway. It is easier to 

switch a system from advisory to regulatory, but it is 

harder to do the reverse. 

TENNESSEE

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

5/13/2016

Donald Gedge

Tennessee DOT

donald.gedge@tn.gov

615-253-0041 

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Tennessee.

TENNESSEE DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
74

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

TENNESSEE

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

I-75 in Chattanooga. 

How long is the VSL system(s) (in 

miles)?

Total of 9.0 mi .

NB = 3.4.

SB = 5.6.

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

Active.

What is the operation type of the VSL 

system(s) (Manual, Automated, or 

Hybrid))?

Hybrid. 

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

Weather.

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

Determined by conditional visibility algorithm due 

to weather event(s) related to fog, traffic speed, and 

stopping distances. 

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

Yes:

70 mi/h: visibility <10 mi / >1320 ft.

50 mi/h: visibility <1320 ft. / >480 ft.

35 mi/h: visibility <480 ft. / > 240 ft.

What lanes does your display(s) apply 

to (one display for all lanes, there 

are displays per lane but speeds are 

identical, HOV lane is a different 

speed, etc.)?

One display is used for all lanes.

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y,” etc.)?

N/A. 

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

Yes; sight distance relative to fog conditions.

Signs are located in relation to entrance ramps and 

MUTCD guidelines. 
75

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

TENNESSEE

Is pavement type/condition considered 

in the speed setting algorithm? N/A. 

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your VSLs?

Yes. 

Speed data is available from the Roadway Traffic 

Monitoring System.

What are the VSL system(s) pros and 

cons with respect to setting speed?

Pros include real-time, instant speed reduction and 

functional reliability.

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

Environmental sensors that are monitored 24/7; 

Preventive maintenance and calibration are 

performed quarterly.

How reliable are the sensors that are 

used? The sensors are very reliable. 

Can you share any cost information 

for your system(s) (cost of the system, 

cost of any sensors used, maintenance 

costs, etc.)?

The VSL system was installed as part of the Fog 

Warning System, so the costs are in the original 

construction contracts and past and current 

preventative maintenance contracts. 

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)? Regulatory. 

VSL SIGNS

How many VSL signs are associated 

with your system(s)? 10.

Where are the VSL signs located 

(right/left shoulder, overhead, median, 

side-mounted, etc.)?

Right shoulder mounted.

What is the display technology used 

(R2-1 signage, embedded YLED, 

shared CMS, etc.)?

Embedded white LEDs.

What were your control specs for the 

actual VSL signs (Bid documents? 

Standards and specs book?)? 

Manufacturer specification sheets.

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

Yes.
76

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

TENNESSEE

MISCELLANEOUS

How do you accommodate work zones 

in the vicinity of the VSL system(s)?

The VSL system is used to enhance work zone safety 

and driver awareness of work zones.

To your knowledge, what is the public 

perception of the VSL system? It has been positive.

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

No, not in this area.

VIRGINIA

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

6/22/2016

Mike Fontaine

Virginia DOT

Michael.Fontaine@VDOT.Virginia.gov

434-293-1980 

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Virginia.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

1) I-66 – US 50 to I-495.

2) I-77 – Fancy Gap Area.

3) I-95 Express Lanes. 

How long is the VSL system(s) (in 

miles)?

1) 12.5 – 13 miles.

2) 15 miles.

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

1) Active (VSL was turned on, then back off after a 

week of operation. It took 3 months of turning the 

algorithm to turn it back on again, which was in the 

middle of January). 

2) Planned (end of summer 2016).

3) Active. 

What is the operation type of the VSL 

system(s) (Manual, Automated, or 

Hybrid))?

1) Automated.

2) Hybrid.

3) Manual. 

VIRGINIA DEPARTMENT OF TRANSPORTATION INTERVIEW SUMMARY
77

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

VIRGINIA

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

1) Congestion, Work Zones.

2) Weather. 

3) Congestion.

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

1) The algorithm is still being iteratively tweaked.

2) The posted speed primarily depends on the available 

visibility. We do not want a lot of speed variance, 

so we are trying to “split the difference” between 

vehicle’s current speeds and the desired speeds. 

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

Dynamic segments are calculated based on current 

traffic conditions. We look at the slowest speed and then 

slow down the oncoming traffic into that slower speed.

2) We use a smoothing and trooping algorithm. 

Dynamic segments exist on this corridor, much like on 

I-66. We determine where the worst visibility conditions 

exist, and the speed is set based on the worst case. 

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y,” etc.)?

1) Rain is not used at all.

2) Rain is not used at all. We want to pursue this 

in the future, but it is not currently included in the 

algorithm. 

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

1) Design speeds are much higher than the posted 

speed limits. Roadway curvature is not included in 

the algorithm because the roadway is designed for 

70 mi/h or more.

2) Sight distance is included in the algorithm since it 

is a weather-based system. 

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your VSLs?

1) We are still evaluating the effectiveness of the 

system. It is a congestion-based system, so vehicles 

can only go but so fast when there is congestion on the 

roadway. A more important question for evaluating 

system effectiveness may be “Are we more smoothly 

transitioning vehicles into different speeds?”

 2) The system has not been turned on yet. 
78

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

VIRGINIA

What are the VSL system(s) pros and 

cons with respect to setting speed?

1) Compliance is always a question, especially 

without automated speed enforcement (Europe has 

this). We have to rely on traditional enforcement and 

make-do with what we have.

2) We have a challenge due to competing constraints: 

there is a “safe speed” and then there is actual driver 

behavior. Sometimes drivers are traveling 20 mi/h 

over the safe speed. When it comes time to set 

the speed limit, we don’t want to create increased 

variance, but we also want drivers to travel at a safe 

speed. This balance can be very difficult. 

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

Wavetronix speed sensors are used on I-66 and I-77.

How reliable are the sensors that are 

used? The sensors are extremely reliable on I-66 and I-77.

Can you share any cost information 

for your system(s) (cost of the system, 

cost of any sensors used, maintenance 

costs, etc.)?

1) The system was turned on in the middle of September 

2015. The total cost was $39 million. This cost included 

a lot of additional costs for commutations, cameras, 

infrastructure, gantries, etc. The gantries themselves 

were probably about $24 million. 

2) This system is going to cost about $7.5 million, which 

includes a fair amount of additional upgrades (power, etc.). 

Do you have any design drawings for 

your system layout? These drawings 

can be a typical layout or a sitespecific layout.

1) I-66 has a huge plan set, but it would be hard to track 

down and hard to interpret. 

2) We could see if we can share. 

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)?

1) Advisory.

2) Regulatory. 

3) Regulatory. 
79

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

VIRGINIA

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

1) The system is advisory, so drivers cannot be cited 

for speeding. The system was implemented more for 

speed harmonization-related goals. We have different 

driver attitudes in the United States than in Europe. 

In the United States, drivers don’t slow down unless 

they see a reason. 

2) Enforcement is very challenging. This system 

drops the speed when there is fog, and we can’t have 

an officer on the side of the roadway when there is 

heavy fog because it is a safety concern. We are still 

working through how to enforce the VSL system 

without endangering the officers. It is possible that 

enforcement would occur after-the-fact. 

VSL SIGNS

How many VSL signs are associated 

with your system(s)?

1) 21 gantries in each direction and 3-5 signs per gantry.

2) 44 signs.

Where are the VSL signs located 

(right/left shoulder, overhead, median, 

side-mounted, etc.)?

1) Overhead.

2) Side-mounted. 

What is the display technology used 

(R2-1 signage, embedded YLED, 

shared CMS, etc.)?

2) Typical speed limit signs are used, expect that the 

posted speed can be changed on the sign. There will 

be 8 of these VSL signs. There will also be 36 full 

matrix DMS signs where the speed limit and various 

messages may be posted. 

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

1) Yes. For example, “Congestion ahead.”

2) Yes. For example, “Fog ahead.”

MISCELLANEOUS

How do you accommodate work zones 

in the vicinity of the VSL system(s)?

1) Overhead lane use control signs are used (can 

either post VSL or green arrow/red "X" indicating 

lane availability).

2) VSL can be used by operators to reduce the speed 

limit; however, this is a rural, very low volume area, 

so work zones don’t really cause traffic problems.

To your knowledge, what is the public 

perception of the VSL system?

1) The challenge in educating drivers with repeated 

explanations for speed limits and other outreach 

activities. VDOT employees in the northern region 

could provide additional insight. 

2) We won’t know until the system is activated.
80

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

VIRGINIA

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

1) There are currently no plans to extend the current 

system. There may be significant geometric changes 

along I-66, so nobody wants to do anything until we 

know exactly what the roadway will look like. 

2) There are no plans to extend the VSL system. 

Do you have any additional 

information/comments that we should 

include in our report?

Note that the Concept of Operations document I sent 

you for I-66 was never updated.

WASHINGTON STATE

Interview Date: 

Name: 

Title:

Agency:

Email Address: 

Phone Number: 

6/3/2016

Vinh Q. Dang

Freeway Operation Engineer

Washington State DOT

dangv@wsdot.wa.gov

206-410-0003 

GENERAL VSL INFORMATION

What State is the VSL system(s) 

located in? Washington.

What route(s) is the VSL system(s) 

located along and how would you 

describe that location (the entire 

State, only a certain area, along 

certain roadways, etc.)?

There are 2 functional groups:

Group 1: Weather / road environmental condition 

responsive:

• US-2 Vicinity Steven pass.

• I-90 Vicinity Snoqualmie pass.

Group 2: Congestion / Q-Warn / Speed Transition as 

part of the ATM corridors.

• I-5 Tukwila to Seattle.

• I-90 Bellevue to Seattle.

• SR 520 Bellevue to Seattle.

How long is the VSL system(s) (in 

miles)?

The VSL system lengths are: 

• US-2 Vicinity Steven pass (23 mi over the pass).

• I-90 Vicinity Snoqualmie pass (25 mi over the pass).

• I-5 as part of ATM corridor (8 mi).

• I-90 as part of ATM corridor (10 mi).

• SR 520 as part of ATM corridor (8 mi).

What is the status of the VSL 

system(s) (Active, Removed, Under 

Construction, Planned, etc.)?

All systems are active. 

WASHINGTON STATE DEPARTMENT OF TRANSPORTATION 

INTERVIEW SUMMARY
81

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

WASHINGTON STATE

What is the operation type of the VSL 

system(s) (Manual, Automated, or 

Hybrid))?

Group 1: Both systems are hybrid, semi-automatic.

Group 2: For the ATM corridors, all 3 systems are 

fully automatic. 

What is the primary function of the 

VSL system?

• Congestion

• Weather

• Work Zones

1) Weather.

2) Congestion. 

SETTING SPEED LIMITS

Describe how the displayed speed 

limit is determined. If you use an 

algorithm, can you share it with us?

Group 1: The display speed is determined from a lookup table. Currently the operator looks-up the table and 

manually displays the speed.

Group 2: The displayed speeds is determined and 

adjusted every minute by monitoring downstream 

conditions, calculating the 85th percentile speed, 

comparing multiple speed values in the corridor, 

performing smoothing/ transitioning calculations, and 

displaying speed updates on one or more gantries as 

needed.

Are the same speeds set throughout 

the corridor? Is there a minimum 

distance?

No. See above. 

What lanes does your display(s) apply 

to (one display for all lanes, there 

are displays per lane but speeds are 

identical, HOV lane is a different 

speed, etc.)?

All GP lanes at the same station have the same speed. 

An HOV lane at a station might have different speed 

from the GP lanes. 

How do wet conditions affect speed 

selection (rain intensity of “X” 

reduces the speed by “Y”, etc.)?

Group 1 has wet pavement conditions as part of the 

look-up table. Speeds are displayed accordingly.

Group 2 calculates speed based on actual measured 

downstream condition, hence no need for wet 

condition adjustment. 

Does roadway curvature (horizontal 

or vertical) and sight distance get 

considered in your speed setting 

algorithm? Did you locate your VSL 

signage in its current spot because of 

sight distance or other issues?

No to both. 

Is pavement type/condition considered 

in the speed setting algorithm? No. 
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SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

WASHINGTON STATE

Has VSL been effective at reducing 

speeds? Is operating speed data 

available in the vicinity of your VSLs?

Yes to both.

What are the VSL system(s) pros and 

cons with respect to setting speed?

Group 1 is in rural setting. The spacing between signs 

are farther apart. Most of the time, the speed are set 

for longer segment of the corridor (if not for the entire 

length). The speed variation tends to be small.

Group 2 is in urban setting and responsive to 

downstream congestion. Spacing between gantries is 

approximately 1⁄2 mi apart. Variation is tighter at 5 mi 

increment.

EQUIPMENT AND COSTS

What sensors are used to determine 

speed limits (speed indicators, 

environmental sensors, etc.)?

The measured occupancy is converted to speed for 

calculations. At locations where we have speed trap, 

the measured speed is used.

How reliable are the sensors that are 

used?

Very reliable. We have extensive experience in 

calculating travel times based on speed converted 

from occupancy measures.

Can you share any cost information 

for your system(s) (cost of the system, 

cost of any sensors used, maintenance 

costs, etc.)?

The cost is not very clear because many hardware, 

devices, and processes have already been in place before 

VSL deployment.

Do you have any design drawings for 

your system layout? These drawings 

can be a typical layout or a sitespecific layout.

Yes.

ENFORCEMENT

How is your VSL system(s) enforced 

(regulatory, advisory, hybrid)? Regulatory. 

What are you experiences with 

enforcement and the judicial process 

when it comes to VSL?

We have not been challenged yet.

VSL SIGNS

How many VSL signs are associated 

with your system(s)? Few hundred for both Groups 1 and 2.

Where are the VSL signs located 

(right/left shoulder, overhead, median, 

side-mounted, etc.)?

Group 1: On US 2, undivided highway, VSL signs are 

on the right. On I-90, mixed. Some locations have over 

head, some have signs on both side of one direction. 

Varied by location’s geometrics. 
83

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

WASHINGTON STATE

What is the display technology used 

(R2-1 signage, embedded YLED, 

shared CMS, etc.)?

Group 1: Hybrid cut-out LED speed.

Group 2: Full color, full matrix. Speed limits are 

graphics resided locally in the sign controller. 

What were your control specs for the 

actual VSL signs (Bid documents? 

Standards and specs book?)? 

Design, bids, build.

Do you use any associated word 

messages (changeable message signs) 

in conjunction with VSL (“congestion 

ahead,” “slow ahead,” etc.)?

Group 1: No.

Group 2: Yes.

MISCELLANEOUS

How do you accommodate work zones 

in the vicinity of the VSL system(s)? Display the reduced speed if needed.

To your knowledge, what is the public 

perception of the VSL system?

Group 1: Well received.

Group 2: Initial reservation during the first few 

months of deployment. Well received by now after 

tweaking of algorithm and lower threshold.

Are you planning to expand or 

decrease the length of the VSL 

corridor (based on the public’s 

response)?

No adjustment to the existing system limits. If there 

are any, it will be based on engineering judgment 

rather than public opinion process. 

Do you have any additional 

information/comments that we should 

include in our report?

Always follow the system engineering process. Let 

the corridor goals drive the operation needs. Let the 

operation needs drive the system requirements. Let the 

system requirements drive the specifications. Do not 

deploy VSL just because it’s “cool.”

Credibility is critical. Following a display of a reduced 

speed limit should be a real condition warranting a 

reduction of speed.
84

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

WISCONSIN

Interview Date: 

Name: 

Agency:

Email Address: 

Phone Number: 

7/7/2016

Paul Keltner

Wisconsin DOT (WisDOT)

paul.keltner@dot.wi.gov

414-225-3727 

INTERVIEW QUESTIONS

Based on what you have heard 

from other agencies, what is your 

impression of VSL?

Some agencies are backing away from VSL because 

of unexpected consequences.

Buy-in from law enforcement (after VSL is 

implemented) is an issue.

The expected benefits have not proved out.

Have you considered implementing 

VSL in Wisconsin?

WisDOT has looked at the VSL systems in 

Minnesota and Seattle.

Wisconsin wants to increase capacity; they want 

speed harmonization and less crashes to increase 

throughput.

What institutional and policy hurdles 

would you encounter if implementing 

VSL?

To implement VSL in Wisconsin there would need to 

be modifications to State statutes.

VSL would also require a change in the process for 

setting speeds.

If Wisconsin implemented VSL, 

would you use it for congestion, work 

zones, weather, or other functions?

WisDOT would use VSL for ATM, managing 

congestion, and during winter weather events. 

WISCONSIN DEPARTMENT OF TRANSPORTATION 

INTERVIEW SUMMARY
85

SYNTHESIS OF VARIABLE SPEED LIMIT SIGNS

WISCONSIN

What kind of information would you 

want to have in a VSL document to 

help with decisions?

Can you ever get compliance with an advisory 

system?

Cost/benefit information. They can estimate costs, 

but want to know what other agencies’ actual costs 

have been. It is also hard to explain to the public the 

benefits based on the cost.

Information on safety benefits. What have other 

States seen in terms of safety?

The synthesis report should include best practices 

on collaboration, cooperation, communication, and 

outreach.

Information on signage; specifically spacing (how far 

apart to install the signs).

What threshold do other agencies use for activating 

weather-responsive VSL systems? Do they activate 

for mist, flurries, or 1’ of snow?

How are messages activated?

What rate of change do agencies use? Do agencies 

step down speeds by 5 mi/h or something different?

Do agencies use lane-by-lane signage with different 

speeds or the same speed for all lanes?

Have any public surveys been done? Wisconsin will 

have a queue warning system this summer which will 

operate somewhat like an advisory VSL. They will 

have a survey as part of this project.


U.S. Department of Transportation

Federal Highway Administration 

Office of Operations 

1200 New Jersey Avenue, SE 

Washington, DC 20590 

Toll-Free “Help Line” 866-367-7487

www.ops.fhwa.dot.gov

May 2017

FHWA-HOP-17-003