Long-Term Effectiveness of Dynamic Speed Monitoring Displays

(DSMD) for Speed Management at Speed Limit Transitions 

Wayne Sandberg, Ted Schoenecker, Kristi Sebastian, and Dan Soler 

Abstract. Speeding continues to be a significant safety issue on today’s roadways. Studies have 

demonstrated that increased compliance with properly established speed limits reduces crash 

incidence and severity. One of the outcomes of Intelligent Transportation System (ITS)

technology is the development of practical tools to enable the traffic engineer to more effectively 

manage speed on their roadway system. The Dynamic Speed Monitoring Display (DSMD) sign 

is one such tool. These signs measure the speed of the approaching vehicles and then feed this

information back to the driver in real time via a dynamic message display. Portable DSMD 

signs (a.k.a. speed trailers) have been shown to be an effective engineering countermeasure for 

short-term speed control. However, experience has shown that as soon as the device is removed, 

speeds soon return to their previous levels. 

This paper reports the results of a long-term evaluation of DSMD signs at speed reduction 

transition zones, which are those locations where the speed limit changes from a higher speed to 

a lower speed. The study was specifically targeted at locations were a rural highway transitions 

into an urbanized area. The study found a statistically significant decrease in overall vehicle 

speed immediately after the installation of the DSMD signs. The average speed reduction across

all of the study sites was seven mph and it was found that these speed reductions were 

maintained over the course of the one year duration of the study. DSMD signs were shown to be 

effective long-term for speed management at speed transitions zones. 

INTRODUCTION 

Drivers who exceed the posted speed limits have become a major concern for transportation

agencies, cities and communities. These drivers, whether intentionally or not, place themselves

and others in danger as well as reduce the overall quality of life for nearby residents and 

neighbors. Recent research suggests that safety can be improved by increased driver 

conformance to the posted speed (1). 

The challenge agencies face is how to improve conformance with the posted speed limit. Many 

speeding drivers are local residents who are comfortable with the area. These motorists, many 

times, unconsciously speed through their own neighborhoods. The static speed limit sign alone, 

while effective in many areas, does not always create the conformance that is desired. 

Generally, the concern related to speed conformance manifests itself at locations where the 

regulatory speed limit changes. These locations, generally involving changes from a higher 

speed (e.g., 50 mph) to a lower speed (e.g., 35 mph), are often related to a change in the 

characteristics of the roadway environment. For example, a two-lane highway may have a speed 

limit of 55 miles per hour. As the same highway enters into a more residential area, the speed

 1
limit may drop to 35 miles per hour. Although the amount of traffic is constant, the presence of

homes, businesses, and pedestrians necessitates the need for a lower travel speed. 

Historically engineers have looked to enforcement tools, either active or passive, as a solution to 

speeding. Active enforcement entails police vehicles patrolling the roadway writing tickets to 

speeding motorists. Passive enforcement relies on the motorists to correct their own driving 

behavior. An example of this is the use of a portable speed trailer placed along a roadway. In 

both cases, observations have show that once the police vehicle is out of sight or the speed trailer 

is removed, vehicle speeds return to their previous levels (2, 3). 

Engineers have had a limited toolbox when it comes to improving speed limit conformance. 

Additionally, ideas that once worked, soon become obsolete or lose their effectiveness. Traffic 

characteristics of roads can change with time and development. Many locations that were once 

outlying low volume rural roads are seeing significant increases in traffic volume and vehicle

speeds as urban areas grow. Conventional tools included the installation of signs and/or 

pavement markings and the use of high visibility sheeting to increase sign conspicuity. Even 

with these efforts, many drivers will still exceed posted speed limits. 

One new tool that addresses speed issues by combining engineering and education is the 

Dynamic Speed Monitoring Display (DSMD) sign (Figure 1). DSMD signs are a practical 

outcome of advances in ITS technology. These traffic control devices are self contained ITS 

systems that measure the speed of an approaching vehicle using a radar embedded in the sign, 

then feeding this information back to the driver in real time via a dynamic message display. The 

DSMD sign encourages the driver to act more safely by adjusting their speed to come into 

compliance with the posted speed limit. The DSMD sign, permanently installed in conjunction 

with a standard static regulatory speed limit sign (MUTCD R2-1), provides information to the 

motorist of the speed at which they should be driving with the static sign and the speed at which 

they are driving with the DSMD sign – a total package of information that is easy for the driver

to comprehend without distraction. 

 2
Figure 1 – Dynamic Speed Monitoring Display (DSMD) Assembly used in this study 

THE STUDY 

Studies have been conducted on the effectiveness of permanently installed DSMD signs in a 

number of applications, particularly for speed management in school zones and urban traffic 

calming (4, 5). The purpose of this paper is to report on the results of a long-term evaluation of 

these devices at speed reduction transition zones, which are those locations where the speed limit 

changes (transitions) from a higher speed to a lower speed. The study was specifically targeted 

at locations were a rural highway transitions into an urbanized area. An important objective of 

this study was to assess the long-term effectiveness of permanently installed DSMD signs. It is 

well documented that DSMD signs are an effective speed management tool, but the majority of 

the studies have only evaluated short term effectiveness – typically over the course of a few days 

to a few months (6, 7). Concerns have been raised that DSMD signs may lose their 

effectiveness over time as drivers become accustomed to seeing them on a regular basis. 

 

STUDY DESIGN

The study was conducted as a Before-and-After with Control site design (8). This format was 

chosen due to the long-term nature of the study. Use of a control (untreated) site chosen 

randomly from the population of possible treatment sites overcomes the drawbacks associated 

with simple Before-and-After studies. A control site provides information on both seasonal and 

long-term variation in traffic. The criteria used to identify the test sites were: 

 3
1) Located on county controlled roads within Washington County or Dakota County, 

Minnesota. 

2) Transition from a rural high speed highway to an urbanized area. 

3) Reduction in posted speed limit of 10 mph or greater at the transition. 

4) Existing history of speed related safety concerns. 

5) No other engineering measures planned at the site for at least 12 months. 

Five locations were chosen from among a number of potential locations meeting the criteria. 

Four locations were designated as experimental sites and one as the control site (Table 1). The

three sites in Washington County (2 experimental, 1 comparison) were speed reductions from 50 

to 30 mph, 55 to 40 mph and 55 to 30 mph (Control) on rural two lane highways as they entered 

urban areas. The Dakota County locations were located along a single stretch of highway where

there were two successive speed transitions. The first transition was from 55 mph to 45 mph 

followed by a second transition 0.7 miles downstream from 45 mph to 35 mph. All the locations

in this study were two lane roads. At each of the experimental locations, the existing R2-1 sign

indicating the reduced speed was replaced with an assembly consisting of a DSMD sign mounted

directly below the speed limit sign (see Figure 1). No changes were made at the Control site. 

Table 1- Study Test Sites 

Location Initial Speed

Limit 

(mph) 

Reduced Speed 

Limit at

Transition 

(mph) 

Average Daily 

Traffic 

 (ADT) 

Date DSMD 

Signs 

Installed 

Experimental Sites 

Hugo (CSAH 8) 

Washington County 

50 30 12,000 Nov 2004

Bailey (CSAH 18) 

Washington County 55 40 4,000 Nov 2004

Hastings #1 (CSAH 46 )

Dakota County

55 45 11,000 May 2005

Hastings #2 (CSAH 46 )

Dakota County

45 35 11,000 May 2005

Control site (untreated) 

Stonebridge (CSAH 5) 

 Washington County

55 30 5,000 -- 

Note: CSAH = County State Aid Highway

Dynamic Speed Monitoring Display Assembly

The DSMD signs used in this study were 3M Driver Feedback Signs operating on AC power. 

These signs conform to the requirements of the MUTCD for changeable message signs that 

display to approaching drivers the speed at which they are traveling (9). The dimensions of the

speed limit sign and the DSMD sign were both 36 inch x 48 inch. This sign size is 

recommended in the MUTCD for use on higher speed rural highways. The signs used in the 

study utilize a NEMA TS4 Hybrid dynamic message display that combines Fluorescent YellowGreen retroreflective pixels with integrated high-output 590 nm InGaAIP LEDs (10). Hybrid 

displays were chosen to maximize sign target value and legibility under all conditions – day, 

 4
night and inclement weather. The frame surrounding the hybrid display as well as the face of the

R2-1 Speed Limit sign was White ASTM Type IX retroreflective sheeting. 

The DSMD used K-band radar embedded within the sign to measure the speed of the 

approaching vehicles. The signs were programmed to display the speed to the motorist in real 

time and to flash until that motorist slowed down to at or below the posted speed limit at the

transition point. The DSMD signs were programmed with minimum and maximum speed

display cut-off limits to discourage reckless drivers attempting to see how fast they could go. 

These signs also have the capability for vehicle speed data collection; however, this feature was 

not used for this study. 

Data Collection

Limited data for analysis is a common problem in field research. Sufficient data must be 

collected in order to allow a thorough analysis of the results of the experiment. Vehicle speed 

and traffic volume data was collected at two positions at each location. The first position, 

denoted the Advance site, was one-third to one-half mile upstream of the speed limit reduction.

The position of the Advance site was chosen such that the DSMD was inconspicuous in the 

distance. The Advance sites also function as comparison sites since speeds at these locations

should not be influenced by the DSMD. The second set of data was collected adjacent to the 

DSMD sign, which is the point where the reduced speed limit officially begins and where the 

driver should now be traveling at the new lower speed. 

The plan called for the signs to be installed at the same time at all of the sites. Data collection 

was then to be conducted at all sites simultaneously at defined intervals over the course of one 

year. These intervals were nominally: 

• Before installation of the DSMD sign 

• One week after 

• Two months after 

• Seven months after 

• One year after 

The original plan was adhered to at the Washington County sites (2 experimental sites and the 

control site) with only a few modifications due to the Minnesota weather. These signs were 

installed in November 2004. Installation of the DSMD assemblies at the test location in Dakota 

County that comprised of two consecutive speed transitions were delayed until May 2005 due to 

difficulty installing power for the signs during the winter. Due to logistical problems One Week 

After data was not collected for the Dakota County locations. 

This study used commercial pneumatic tube traffic data recorders with electronic data collection 

to measure vehicle speed and volume. Vehicle speeds were binned in 1 mph increments at 15-

minute intervals. All measurements were taken mid-week for 48 to 72 consecutive hours 

simultaneously at both the Advance and DSMD sign positions. Simultaneous data collection 

provided a counter balance for day–to–day variability. 

 5
STUDY RESULTS 

In any long-term study, there is natural variation in traffic volume and speed. In order to draw

conclusions on the persistent effectiveness of the DSMD signs, a review should be made to 

check for potential external influences other than the DSMD sign. Table 2 presents the average 

directional daily traffic volume through each of the sites during the measurement periods. The 

corresponding Average Daily Traffic (ADT) is approximately twice the volumes listed in the 

table. With one exception, the data shows the 24-hour average traffic to be relatively stable. The 

majority of the test sites showed only a two to four percent variation in traffic volume over time 

with no distinct trend. However, at the Bailey site, there is a consistent increase in volume over 

the course of the study, which is mainly due to completion of a nearby major construction

project. 

Table 2 - Average Directional Daily (24-hour) Traffic Volume through the Study sites 

Location Before 2 Months 7 months 1 year 

Hugo Advance 6214 5614 6560 5899 

Hugo DSMD 6115 5527 6385 6197 

Bailey Advance 2107 2440 3506 2720 

Bailey DSMD 2193 2450 3526 2788 

Hastings Advance 5343 5342 4914 5507 

Hastings #1 DSMD 5863 5747 --1 5924 

Hastings #2 DSMD 5133 4940 4706 5281 

Stonebridge Advance 2568 --1 2804 --

2

Stonebridge Control 2511 2223 2754 --2

Notes: 1Data lost due to equipment malfunction; 2Data not collected due to installation of a DSMD sign at this site 

The speed data was compiled, reduced and analyzed using both Microsoft ® Office Excel 2003 

and Minitab ® Release 14.13 statistical software. A number of descriptive statistics were 

generated as a function of time and location, including: 

• Average speed 

• 50th (median), 85th and 95th percentile speeds 

• 10-mph Pace 

The 24-hour speed results for the control and study sites are summarized in Tables 3, 4 and 5. 

Statistical analyses were run on the data comparing changes in vehicle speed distributions as a 

function of time period and location. Significance testing included an analysis of Variance, Ztest, t-test and Odds Ratio. All statistical measures showed highly significant associations (alpha 

< 0.01) between the presence of a DSMD sign and speed reductions within the transition zone. 

The study sites with the DSMDs experienced reductions in the 50th, 85th and 95th percentile

speeds averaging 6.3, 6.9 and 7.0 mph, respectively. The 10-mph Pace speeds also decreased at 

all the DSMD locations. These results indicate the DSMD shifted the entire speed distribution at 

 6
the transition zone. At the Advance sites and the Control site, the corresponding speeds were 

either flat or increased slightly over the course of the research. 

The data at the Stonebridge Control site was only collected through 7 months. Due to the need 

to address the existing speed related safety concerns at this location and based on the positive 

results of this study up to that point in time, Washington County installed a DSMD sign 

assembly just prior to the One Year After data collection period. 

Table 3 - Results for the Control (untreated) Site 

Mean 

Speed 

(mph) 

Standard 

Deviatio

n (σ) 

Sample 

size

50th

Percentile 

Speed 

85th

Percentile 

Speed 

95th

Percentile 

Speed 

10 mph 

Pace 

(mph) 

Stonebridge Advance (55 mph) 

Before 52.6 6.6 7881 53 59 62 48-57

1 week 50.6 6.4 7547 51 56 60 46-55 

2 months -- -- -- -- -- -- -- 

7 months 53.5 7.0 8416 54 59 63 51-60 

Stonebridge Control (30 mph) 

Before 40.2 6.8 7739 40 45 49 36-45

1 week 41.7 7.0 7397 42 48 52 36-45 

2 months 39.2 6.7 5712 39 45 49 36-45 

7 months 40.0 6.7 8290 40 45 49 36-45 

 7
Table 4 - Results for the Washington County Study Sites 

Mean 

Speed 

(mph) 

Standard 

Deviatio

n (σ) 

Sample 

size

50th

Percentile 

Speed 

85th

Percentile 

Speed 

95th

Percentile 

Speed 

10 mph 

Pace 

(mph) 

Hugo Advance (50 mph) 

Before 51.8 7.5 18403 52 58 60 46-55

1 week 54.0 7.5 17699 54 60 64 51-60 

2 months 52.3 7.4 16979 53 59 62 46-55 

7 months 52.8 7.9 19203 53 59 63 51-60 

1 year 51.2 7.5 15199 51 57 60 46-55 

Hugo DSMD (30 mph) 

Before 44.2 7.7 18085 44 50 54 41-50

1 week 37.1 8.4 17336 36 44 49 31-40 

2 months 36.1 8.1 16613 35 42 47 31-40 

7 months 37.0 8.5 18678 36 43 49 31-40 

1 year 36.0 6.9 16025 36 43 45 31-40 

Bailey Advance (55 mph) 

Before 50.6 6.4 6201 51 56 59 46-55

1 week 51.0 14.9 6360 55 63 67 51-60 

2 months 51.3 6.9 7254 51 58 61 46-55 

7 months 50.4 7.6 10451 51 57 60 46-55 

1 year 50.1 7.0 5645 50 57 60 46-55 

Bailey DSMD (40 mph) 

Before 50.9 7.2 6305 51 58 63 46-55

1 week 44.6 7.9 6048 44 50 57 41-50 

2 months 42.3 5.4 7253 42 47 50 36-45 

7 months 45.7 6.4 10521 45 51 55 41-50 

1 year 43.3 6.4 5433 43 49 53 36-45 

 8
Table 5 - Results for the Dakota County Study Sites 

Mean 

Speed 

(mph) 

Standard 

Deviatio

n (σ) 

Sample 

size

50th

Percentile 

Speed 

85th

Percentile 

Speed 

95th

Percentile 

Speed 

10 mph 

Pace 

(mph) 

Hastings Advance (55 mph) 

Before 52.5 7.3 9782 53 59 62 46-55

2 months 49.8 7.4 10019 50 55 60 46-55 

7 months 49.6 7.2 8995 49 55 60 46-55 

1 year 50.2 7.9 10181 51 56 60 46-55 

Hastings #1 (45 mph) 

Before 52.1 7.4 10667 52 58 62 46-55

2 months 47.1 6.9 10812 47 52 57 41-50 

7 months -- -- -- -- -- -- -- 

1 year 45.9 7.9 10984 47 52 55 41-50 

Hastings #2 (35 mph) 

Before 39.0 8.6 9250 39 45 50 36-45

2 months 36.0 7.9 9318 36 40 45 31-40 

7 months 36.0 7.9 9318 36 40 45 31-40 

1 year 34.5 6.8 9658 36 40 44 31-41 

DISCUSSION

There are two basic questions that must be answered in order to determine whether a new traffic 

control device will be a useful and reliable addition to the engineer’s speed management toolbox: 

1) In what applications is it effective?

2) Does the device have a persistent effect on driver speed behavior? 

The objectives of this study were to address both of these questions. The results of this study 

were very consistent across all test sites as demonstrated by the data in Tables 3-5. This

discussion will use primarily the results from the Hugo locations in Washington County to 

illustrate the answers to these questions. 

 9
Effectiveness for the Application

Recent studies have shown DSMD signs to be effective for speed control at school zones and

urban traffic calming. This project evaluated their effectiveness at speed transition zones, 

particularly where the DSMD sign is used in combination with the regulatory Speed Limit sign. 

The results of the study show the DSMD sign is an effective tool for reducing speed and 

increasing compliance at speed transition areas. Figure 2 illustrates the change in 85th percentile

speed for the Hugo test site, the Stonebridge control site, and the average speed reduction over 

all the DSMD locations. 

Figure 2 – Change in 85th Percentile Speed as a Function of Time period. 

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

1 week 2 months 7 months 12 months

Time relative to DMSD Installation

Change in

85th %ile Speed

(MPH)

Hugo DSMD

Stonebridge Control

Average All DSMD sites

-7.0 MPH -6.8 MPH

-6.3 MPH

-7.5 MPH

Persistent Effect on Driver Speed Behavior

Data was collected over the course of one full year to assess the long-term effect of DSMD signs

on drivers’ speed. Speed and traffic volume data were collected in advance of the speed limit 

transition area and at the speed transition prior to installing the DSMD signs and at regular

intervals afterwards. Analysis of the data showed both statistically significant and, more

importantly, practically significant reductions in vehicle speeds associated with the use of the 

DSMD assembly. At the Hugo Advance location, the Before 85th percentile speed was 57 mph 

(posted Speed Limit of 50 mph) and the 10-mph Pace of 46-55 mph made up of 65 percent of 

vehicles. Over the course of the study period, the 85th percentile speeds remained relatively

consistent at approximately 57 mph for each of the time frames (Figure 3). 

 10
 

Figure 3 - Changes in 24-hour Speed Distribution at the Hugo Advance site (Speed Limit 

50 mph) 

Advance location at Hugo Study site (posted Speed Limit = 50 

At the location of the existing speed limit sign indicating the new reduced speed limit, the 85th

percentile speed in the Before period was 50 mph (the posted speed limit is 30 mph) with the 10-

mph Pace of 41-50 mph made up of 63 percent of the vehicles. One week after the installation 

of the DSMD sign assembly, there was a six mph decrease in the 85th percentile speed, from 50 

mph down to 44 mph (Figure 4). One year after installation, there was still a seven mph 

reduction in the 85th percentile speeds relative to the Before period. Not only did the 85th

percentile speed decrease and stay down, but all speeds decreased, with the higher speeds (95th

percentile) showing an even larger decrease of up to nine mph over time. Additionally, the 10-

mph Pace dropped by 10 mph from an initial 41-50 mph to 31-40 mph within the first week and 

was still 31-40 mph at one year while maintaining essentially the same percentage of vehicles 

(63% Before versus 64 % After one year). 

30

35

40

45

50

55

60

65

70

Before 1 week 2 months 7 months 1 year

Time -relative to DSMD installation

Traffic Speed, mph

25th

50th (Median)

75th

85th 

95th

Percentile 

 11
Figure 4 - Changes in 24-hour Speed Distribution at the Hugo DSMD Site (Speed Limit 30 

mph) 

DSMD location at Hugo Study site (posted Speed Limit = 30 mph)

The data showed the overall results across all the DSMD sign locations were fairly consistent. 

The study found: 

• Speed reductions of approximately 6-8 mph in the 85th percentile speed. 

• Decrease of 10 mph in the 10 mph Pace 

• Consistent reductions through all time frames including the 24-hour data, AM peak hour, 

and PM peak hour. 

• Consistent shift in the speed distribution to lower speeds. 

CONCLUSIONS 

Speeding is and will continue to be a safety concern for users on all roadways. From an 

Engineering perspective, the toolbox is relatively limited on how to address speeding on 

roadways. In the past, the use of law enforcement officials has been the main tool to “combat” 

speeders. An emerging technology, the Dynamic Speed Monitoring Display (DSMD) sign, now 

provides the Engineer with another tool to utilize. A DSMD sign in combination with a 

regulatory speed sign provides direct and relevant information to the motorist using the roadway. 

This information component provides the driver with immediate feedback on their behavior 

relative to the posted speed. 

The goal of this study was to focus on reducing and managing speeds in transition zones where 

the speed limit changes from a higher speed (e.g. 50 mph) to a lower speed (e.g. 35 mph). The 

20

25

30

35

40

45

50

55

60

Before 1 week 2 months 7 months 1 year

Time - relative to DSMD installation

Traffic Speed, mph

25th

50th (Median)

75th

85th 

95th

Percentile 

 12
results of the study show that DSMD signs at transitions zones have a significant long-term (one 

year or greater) positive effect on driver speed. This study found overall decreases in speed of 

approximate six to eight mph at the transition point. 

In addition to the improved speed conformance, the installation of these signs proved extremely 

popular with drivers, nearby residents and businesses, as well as with elected officials. 

With the installation of the DSMD signs, expect: 

• A reduction in overall speeds 

• Increased conformance with posted speeds 

• Positive public/elected official feedback 

The DSMD sign in combination with a standard regulatory speed limit sign was found to be an 

effective long-term speed management solution at speed limit transitions.

ACKNOWLEDGEMENTS 

This project was a collaborative effort between the Washington County, Dakota County and 

Ramsey County Departments of Transportation. The authors would like to recognize the efforts 

of Jeff Bednar and the professional staff of SRF Consulting Group, Inc. for coordinating and 

conducting the data collection. The authors would also like to recognize the participation of 

David Burns, 3M Traffic Safety Systems, in the design of the experiment and his assistance with 

the statistical analysis of the data. 

AUTHORS

Wayne Sandberg, P.E. 

Deputy Director / Assistant County 

Engineer 

Washington County 

11660 Myeron Road North 

Stillwater, MN 55082 

Phone: 651-430-4339 

Fax: 651-430-4350 

wayne.sandberg@co.washington.mn.us

Ted Schoenecker, P.E. 

Transportation Engineer 

Washington County 

11660 Myeron Road North 

Stillwater, MN 55082 

Phone: 651-430-4319 

Fax: 651-430-4350 

ted.schoenecker@co.washington.mn.us

Kristi Sebastian, P.E., P.T.O.E 

Traffic Engineer 

Dakota County 

14955 Galaxie Avenue, 

Transportation Department 

Apple Valley, MN 55124 

Phone: 952-891-7178 

Fax: 952-891-7127 

kristi.Sebastian@co.dakota.mn.us

Dan Soler, P.E. 

Traffic Engineer 

Ramsey County 

1425 Paul Kirkwold Drive 

Arden Hills, MN 55112 

Phone: 651-266-7114 

Fax: 651-266-7710 

dan.soler@co.ramsey.mn.us

 13
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