Identifying and Assessing Key WeatherRelated Parameters and Their Impacts

on Traffic Operations Using Simulation

PUBLICATION NO. FHWA-HRT-04-131 SEPTEMBER 2004

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD 

Adverse weather conditions can have a dramatic impact on the quality of traffic flow. Traffic 

analysts need adequate traffic analysis tools to design better for the impacts of adverse weather. 

One available type of analysis tools is microscopic traffic simulation, which allows analysts to 

model and evaluate complex roadway geometries, traffic control devices, and Intelligent 

Transportation Systems (ITS). 

The objectives of this effort, as captured in the report, Identifying and Assessing Key WeatherRelated Parameters and Their Impacts on Traffic Operations Using Simulation, are to identify 

how adverse weather affects traffic operations, to assess the sensitivity of weather-related traffic 

parameters in a microscopic traffic simulation package (CORSIM), and to develop guidelines for 

using the CORSIM simulation model to account for the affects of adverse weather. 

The intended audiences for this report are transportation professionals who use traffic analysis 

tools, in particular microscopic traffic simulation, to plan, evaluate, or design roadway or traffic 

control improvements and are interested in incorporating the impacts of adverse weather into 

their analysis. 

 Toni Wilbur 

 Director, Office of Operations Research and 

 Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard, 

specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes 

to ensure continuous quality improvement. 
Technical Report Documentation Page 

1. Report No. 

FHWA-HRT-04-131 

2. Government Accession No. 

 

3. Recipient’s Catalog No. 

 

5. Report Date 

September 2004 

4. Title and Subtitle 

Identifying and Assessing Key Weather-Related Parameters and 

Their Impacts on Traffic Operations Using Simulation 6. Performing Organization Code 

 

7. Author(s) 

Li Zhang, Peter Holm, and James Colyar 

8. Performing Organization Report No. 

10. Work Unit No. 

 

9. Performing Organization Name and Address 

ITT Industries Inc. 

Systems Division 

P.O. Box 15012 

Colorado Springs, CO 80935-15012 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Final Report 

September 2002–December 2003 

12. Sponsoring Agency Name and Address 

Office of Operations Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

14. Sponsoring Agency Code 

 

15. Supplementary Notes 

The Contracting Officer’s Technical Representative was John Halkias. 

16. Abstract 

The objectives of this study were to identify how weather events affect traffic operations, to assess the sensitivity of 

weather-related traffic parameters in the CORridor SIMulation (CORSIM) traffic microsimulation model, and to 

develop guidelines for using the CORSIM model to account for the affects of adverse weather conditions on traffic 

operations. 

This report summarizes the methodologies, findings, and conclusions for each of these study objectives. A high-level 

conclusion from this project is that CORSIM can be used adequately to model the affects of weather events on traffic 

operations. This conclusion is based on the fact that a majority of the generic weather-related parameters identified 

are currently available in CORSIM, and that the key weather-related parameters are adequately sensitive in 

producing model outputs inline with that expected from adverse weather. 

 

17. Key Words 

traffic simulation, traffic operations, CORSIM, adverse 

weather, weather responsive traffic management, 

highway capacity 

18. Distribution Statement 

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

19. Security Classif. (of this report) 

Unclassified

20. Security Classif. (of this page) 

Unclassified

21. No of 

Pages 

77 

22. Price 

 

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized 
ii 

SI* (MODERN METRIC) CONVERSION FACTORS 

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 

in2 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 1000 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) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

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/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM 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 in2 

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) o

C Celsius 1.8C+32 Fahrenheit oF 

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 is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)
iii 

Table of Contents 

Executive Summary...................................................................................................................... 1 

1. Introduction.............................................................................................................................. 5 

Background............................................................................................................................... 5 

Study Objective......................................................................................................................... 5 

Study Approach ........................................................................................................................ 6 

Report Outline........................................................................................................................... 8 

2. General Relationship Between Weather Events and Traffic Operations........................... 9 

Definition of Weather Event................................................................................................... 10 

Relationship Between Roadway Environment and Weather Event........................................ 10 

Relationship Between Roadway Environment and Traffic Parameters.................................. 10 

3. Literature Review .................................................................................................................. 13 

Free-Flow Speed ..................................................................................................................... 13 

Startup Lost Time ................................................................................................................... 14 

Saturation Headway................................................................................................................ 14 

Traffic Demand....................................................................................................................... 15 

4. Identifying Simulation Parameters Affected by Weather Events ..................................... 17 

Road Geometry Parameters .................................................................................................... 17 

Traffic Control and Management Parameters......................................................................... 18 

Vehicle Performance Parameters............................................................................................ 18 

Traffic Demand Parameters .................................................................................................... 19

Driver Behavior Parameters.................................................................................................... 19 

5. CORSIM Sensitivity Analysis............................................................................................... 21 

FRESIM Analysis Methodology ............................................................................................ 21 

NETSIM Analysis Methodology ............................................................................................ 27 

Data Processing Procedure ..................................................................................................... 34 

FRESIM Sensitivity Analysis Results .................................................................................... 39 

Sensitivity of Car Following Parameters............................................................................. 41 

Sensitivity of Lane Changing Parameters............................................................................ 43 

Sensitivity of Free-Flow Speed Parameters......................................................................... 45 

NETSIM Sensitivity Analysis Results.................................................................................... 45 
iv 

Sensitivity of Car Following Parameters............................................................................. 46 

Sensitivity of Lane Changing Parameters............................................................................ 46 

Sensitivity of Free-Flow Speed Parameters......................................................................... 49 

Sensitivity of Discharge Headway Parameters.................................................................... 49 

Sensitivity of Startup Delay Parameters.............................................................................. 50 

Sensitivity of Turning Speed Parameters............................................................................. 51 

Summary of Sensitivity Analysis............................................................................................ 51 

6. Guidelines for Modeling Weather Events in CORSIM...................................................... 55 

Step 1—Scope Project ............................................................................................................ 57 

Step 2—Data Collection ......................................................................................................... 57 

Step 3—Base Model Development......................................................................................... 58 

Step 5—Model Calibration..................................................................................................... 61

7. Conclusions............................................................................................................................. 63 

Relationship Between Weather Events and Traffic Operations ............................................. 63 

Microsimulation Parameters Affected by Weather Events..................................................... 63 

CORSIM Sensitivity Analysis ................................................................................................ 64 

Guidelines for Modeling Weather Events in CORSIM .......................................................... 64 

Future Research Needs ........................................................................................................... 65 

References.................................................................................................................................... 67

Bibliography .................................................................................................................................69 
v 

List of Figures 

Figure 1. Study Approach............................................................................................................... 7 

Figure 2. Relationship Between Weather Events and Traffic Operations...................................... 9 

Figure 3. NETSIM Suburban Intersection Network..................................................................... 28 

Figure 4. NETSIM Urban Intersection Network. ......................................................................... 28 

Figure 5. Analysis Area Information for Sample Sensitivity Test................................................ 36 

Figure 6. Vehicle-Kilometers Traveled Graph for Sample Sensitivity Test................................. 37 

Figure 7. Average Speed Graph for Sample Sensitivity Test. ...................................................... 37 

Figure 8. Average Delay Graph for Sample Sensitivity Test. ...................................................... 38 

Figure 9. Average Density Graph for Sample Sensitivity Test. ................................................... 39 

Figure 10. Sensitivity of Pitt Car Following Constant on Freeway System Network— 

Example of “Expected” Sensitivity Group. .......................................................................... 40 

Figure 11. Sensitivity of Maximum Emergency Deceleration on Freeway System Network—

Example of “Inconsistent” Sensitivity Group....................................................................... 41 

Figure 12. Sensitivity of Jerk Value on Freeway System Network—Example of “No Effect” 

Sensitivity Group. ................................................................................................................. 41

Figure 13. Microsimulation Model Development and Application Process..................................56 
vi 

List of Tables 

Table 1. Impacts of Weather Events on Roadway Environment.................................................. 10 

Table 2. Range of Generic Traffic Simulation Parameters........................................................... 12 

Table 3. Road Geometry Traffic Parameters Impacted by Weather Events................................. 17 

Table 4. Traffic Control and Management Parameters Impacted by Weather Events. ................ 18 

Table 5. Vehicle Performance Traffic Parameters Impacted by Weather Events. ....................... 19 

Table 6. Traffic Demand Traffic Parameters Impacted by Weather Events................................. 19 

Table 7. Driver Behavior Traffic Parameters Impacted by Weather Events. ............................... 20 

Table 8. FRESIM Sensitivity Analysis Networks. ....................................................................... 22 

Table 9. Congestion Levels for FRESIM Sensitivity Analysis. ................................................... 22 

Table 10. FRESIM MOEs for Sensitivity Analysis...................................................................... 23 

Table 11. FRESIM MOE Collection Areas. ................................................................................. 24 

Table 12. Car Following FRESIM Parameters Included in Sensitivity Analysis......................... 25 

Table 13. Lane Changing FRESIM Parameters Included in Sensitivity Analysis. ...................... 26 

Table 14. Free-Flow Speed FRESIM Parameters Included in Sensitivity Analysis..................... 27 

Table 15. NETSIM Sensitivity Analysis Networks...................................................................... 27 

Table 16. Congestion Levels for NETSIM Sensitivity Analysis.................................................. 29 

Table 17. NETSIM MOEs for Sensitivity Analysis. .................................................................... 30 

Table 18. NETSIM MOE Collection Areas.................................................................................. 31 

Table 19. Car Following NETSIM Parameter Included in Sensitivity Analysis.......................... 31 

Table 20. Lane Changing NETSIM Parameters Included in Sensitivity Analysis....................... 32 

Table 21. Free-Flow Speed NETSIM Parameters Included in Sensitivity Analysis. ................... 33 

Table 22. Discharge Headway NETSIM Parameters Included in Sensitivity Analysis. .............. 33 

Table 23. Startup Lost Time NETSIM Parameters Included in Sensitivity Analysis. ................. 34 

Table 24. Turning Speed NETSIM Parameters Included in Sensitivity Analysis........................ 34 

Table 25. General Information for Sample Sensitivity Test......................................................... 36 

Table 26. Vehicle-Kilometers Traveled Table for Sample Sensitivity Test................................. 36 

Table 27. Average Speed Table for Sample Sensitivity Test. ...................................................... 37 

Table 28. Average Delay Table for Sample Sensitivity Test........................................................ 38 

Table 29. Average Density Table for Sample Sensitivity Test..................................................... 38 

Table 30. CORSIM Parameter Sensitivity Groups....................................................................... 40 

Table 31. General Sensitivity of FRESIM Car Following Parameters......................................... 42 
vii 

Table 32. General Sensitivity of FRESIM Lane Changing Parameters........................................ 44 

Table 33. General Sensitivity of FRESIM Free-Flow Speed Parameters..................................... 45 

Table 34. General Sensitivity of NETSIM Car Following Parameters. ....................................... 46 

Table 35. General Sensitivity of NETSIM Lane Changing Parameters. ...................................... 47 

Table 36. General Sensitivity of NETSIM Free-Flow Speed Parameters. ................................... 49 

Table 37. General Sensitivity of NETSIM Discharge Headway Parameters. .............................. 50 

Table 38. General Sensitivity of NETSIM Startup Delay Parameters.......................................... 50 

Table 39. General Sensitivity of NETSIM Turning Speed Parameters........................................ 51 

Table 40. Traffic Parameters with No Effect on MOEs. .............................................................. 52 

Table 41. Traffic Parameters with Expected and Medium-to-High Effect on MOEs. ................. 53 

Table 42. CORSIM Parameters Impacted by Weather Events..................................................... 59 

1 

Executive Summary 

Adverse weather conditions can have a dramatic impact on the operations and quality of traffic 

flow. With the advent of advanced traffic management systems (ATMS), there is an opportunity 

to develop traffic management strategies that seek to minimize negative weather-related impacts 

on traffic operations. Although simulation models are used widely in evaluating various traffic 

management strategies, applying them to evaluate ATMS strategies under adverse weather 

conditions needs to be explored. 

The objectives of this study were to identify how weather events impact traffic operations, assess 

the sensitivity of weather-related traffic parameters in the CORridor SIMulation (CORSIM) 

traffic microsimulation model, and develop guidelines for using the CORSIM model to account 

for the impacts of adverse weather conditions on traffic operations. 

This final report summarizes the methodologies, findings, and conclusions for each of these 

study objectives. A high-level conclusion from this project is that CORSIM can be used 

adequately to model the impacts of weather events on traffic operations. This conclusion is 

based on the fact that a majority of the generic weather-related parameters identified are 

currently available in CORSIM, and that the key weather-related parameters are adequately 

sensitive in producing model outputs inline with that expected from adverse weather. 

This report is organized into seven major sections. A summary of each section is provided 

below. 

Section 1—Introduction 

This section presents the background and motivation for completing this project. It also 

highlights the objectives of the study and work tasks for each phase of the study. 

Section 2—General Relationship Between Weather Events and Traffic Operations 

Conceptually, it is easy to understand that a major weather event, such as a snowstorm, will lead 

to lower average speeds and higher delays. However, it is important to know what this 

relationship is, or in other words, what causes a weather event to degrade traffic operations. 

This section shows that a weather event impacts traffic operations through a chain reaction: a 

weather event causes a change in the roadway environment (e.g., reduced visibility and 

pavement friction), which causes a reduction in traffic parameters (e.g., lower free-flow speeds 

and capacities), thereby creating a degradation in traffic flow (e.g., higher delays and lower 

average speeds). 

The qualitative impacts of weather events are seen easily through this relationship, but the 

quantitative impacts have been historically difficult to measure for a number of reasons. For 

example, there are many “shades” of the severity of a weather event, and the impacts are 

different regionally (i.e., a snowstorm in Florida will have more impact than the same storm in 

Minnesota) and by time of year (i.e., a snowstorm at the beginning of winter will likely have 

more impact than the same storm near the end of winter after drivers have acclimated to the 

adverse weather). 
2 

Section 3—Literature Review 

This section summarizes past research regarding the impact of weather events on traffic 

parameters, or inputs to a traffic model. Past research has shown a quantitative link between 

various weather events and reduced free-flow speeds, saturation (discharge) headway, startup 

lost time, and traffic demand. 

Section 4—Identifying Simulation Parameters Affected by Weather Events 

This section identifies the range of simulation parameters likely impacted by weather events. 

First, researchers developed a list of generic microsimulation parameters that are included in 

most simulation models. Then, parameters that potentially are impacted by weather events 

through a change in the roadway environment were determined based on the literature review 

and engineering judgment (e.g., adverse weather generally causes more conservative driver 

behavior, which means car following behavior is likely impacted by adverse weather). 

Section 5—CORSIM Sensitivity Analysis 

The purpose of the sensitivity study was to identify the most sensitive weather-related 

parameters in CORSIM. Each test parameter was modeled on various geometric networks and 

congestion (volume) levels using the default value and then changing the value to represent 

incrementally more conservative driver behavior, as would occur under adverse weather. The 

measures of effectiveness (MOE) produced by the default value then were compared to the 

MOEs produced with the changed parameter values to determine the level of sensitivity the 

parameter has on the MOEs. 

Due to the large number of roadway networks, congestion levels, and parameters tested, 

approximately 45,000 individual CORSIM runs were completed. As a result, a largely 

automated process of creating the CORSIM input files and summarizing the output files was 

created specifically for this project. 

One interesting result of the sensitivity analysis was that a number of parameters tested (19 total) 

had little or no impact on the MOEs. The majority of these were lane changing parameters. This 

finding does not mean they have no sensitivity whatsoever, but that they showed no sensitivity to 

the aggregate-level MOEs used for this study. 

A number of weather-related parameters had an expected effect on the MOEs and were 

categorized as either having a medium or high effect on the MOEs (relative to the other 

parameters). These parameters are important because they represent the key weather-related 

parameters that should be altered when trying to model weather events in CORSIM. These 

parameters included the car following sensitivity multiplier and mean free-flow speed for 

freeway facilities , and time to react to sudden deceleration of lead vehicle, mean free-flow 

speed, mean discharge headway, and mean startup delay for arterial streets. 

Section 6—Guidelines for Modeling Weather Events in CORSIM 

This section provides practical guidelines for modeling weather events in CORSIM. The 

guidelines are based on Traffic Analysis Toolbox Volume III: Guidelines for Applying Traffic 

Microsimulation Modeling Software, a Federal Highway Administration (FHWA) guidance 

document on the proper development and application of microsimulation models.(1) The 

guidance in this section builds on the more general microsimulation guidance by providing 

additional considerations when modeling weather events in CORSIM. For example, the type, 
3 

severity, extent, and time period of the weather event being modeled should be agreed on before 

coding the model. 

This section also details specific CORSIM parameters to consider changing when modeling 

various weather events. Finally, this section describes an alternate method of calibrating a 

microscopic simulation model when field data collection during adverse weather is not possible. 

Section 7—Conclusions 

This section summarizes the findings and results of each phase of the study and also highlights 

four areas of future research: empirical data collection to improve base understanding of impact 

of weather events on traffic operations, CORSIM enhancements for modeling adverse weather 

events, further study of CORSIM parameters which showed no or little sensitivity, and realworld case study of modeling weather events using CORSIM.

5

1. Introduction 

Background 

Adverse weather conditions can have a dramatic impact on the operations and quality of traffic 

flow. For example, icy pavement conditions can affect the acceleration and deceleration 

capabilities of vehicles. Reduced visibility can cause drivers to alter their desired speed, how 

they change lanes, and how they follow other vehicles. Major weather events can cause drivers 

to modify their travel patterns, such as taking a different route to a destination, leaving for a 

destination at a different time than normal, or canceling a trip altogether. 

With the advent of ATMS, there is an opportunity to develop traffic management strategies that 

attempt to minimize the negative weather-related impacts on traffic operations. For instance, a 

weather event that reduces the average operating speed on an arterial can be mitigated by quickly 

implementing traffic signal plans that account for the lower speeds while still maintaining 

progression through a network. However, to develop and implement strategies that minimize the 

effects of adverse weather conditions, a more complete knowledge of how weather events affect 

traffic operations and how to assess the weather-related effects for a given scenario is needed. 

Currently, the relationship between weather events and traffic operations is moderately 

understood, but only at a macroscopic analysis level, such as the methodologies presented in the 

Highway Capacity Manual (HCM).1(2) Using an HCM-style analysis is, in fact, one way to 

model weather impacts to develop weather-responsive traffic management strategies. However, a 

more detailed and potentially more accurate method is to use a microscopic traffic simulation 

model. A microscopic simulation tool can model individual vehicles on a roadway network, 

typically on a second-by-second basis or less. Simulation models have the benefit of being able 

to model complex roadway geometries, traffic control devices, and vehicle configurations that 

are beyond the limitations of a macroscopic HCM-style analysis. 

However, modeling microscopic driver behavior is difficult under ideal weather conditions, let 

alone under adverse weather conditions. Little research has been conducted on how weather 

events impact microscopic driver behavior logic, such as lane changing and vehicle following, 

both of which are crucial to the accuracy of a microscopic traffic simulation model. In addition, a 

vast number of user-input parameters within simulation models can be altered. Knowing which 

key parameters within a microsimulation model should be changed under various weather 

conditions would aid greatly in developing weather-responsive traffic management strategies. 

Study Objective 

The objectives of this study are to identify how weather events impact traffic operations, assess 

the sensitivity of weather-related traffic parameters in the CORSIM traffic simulation model, and 

develop guidelines for using the CORSIM model to account for the impacts of adverse weather 

conditions on traffic operations. More specifically, this study is tasked to do the following: 

 

1

 HCM methodologies do not specifically address the impacts of weather events on highway capacity and quality of 

service: however, the parameters in the HCM could be user-adjusted to reflect the impacts of weather events. 
6

• Research the relationship between weather events and traffic operations. 

• Identify which types of simulation parameters could be affected by weather events. 

• Conduct a sensitivity analysis on selected CORSIM simulation parameters to identify 

the key weather-related parameters that most affect traffic operations. 

• Develop basic guidelines on how weather events can be modeled using CORSIM. 

This study does not recommend specific values (e.g., free-flow speed of 70 kilometers/hour 

(km/h)) to be used for each parameter under various weather conditions. Rather, it focuses on 

identifying the general sensitivity of a parameter to traffic operation MOEs (i.e., average speed). 

This information then may be used to develop guidelines on how CORSIM can be used to model 

weather events. 

Study Approach 

This study, which began in September 2002, was conducted on a task order basis with a total of 

five tasks. Figure 1 shows a flowchart of the task breakdown and workflow. As shown in this 

figure, the tasks were completed in consecutive order, because the output from one task was 

required for the next task. This report provides the results for each of these tasks. 
7

Figure 1. Study Approach. 

Develop Guidance on 

Using CORSIM to Model 

Weather Events 

Identify Simulation Parameters 

Possibly Affected by Weather 

Events 

Develop Automated Procedure 

for Generating and 

Summarizing CORSIM Runs 

Task E 

Final Report 

Task D 

Sensitivity Analysis 

Task B 

Weather Impacts 

Task C 

Identification of Parameters 

Task A 

Project Management

Incorporate All Deliverables and FHWA 

Comments into Final Report 

Investigate General Relationship 

between Weather Events and 

Traffic Operations 

Review Past Research on Weather 

Impacts on Traffic Operations 

Conduct Literature 

Review 

Develop Work Plan, 

Deliverables, Schedule, 

and Staffing 

Determine Which CORSIM 

Parameters to Include in 

Sensitivity Test 

Code Test Networks and 

Volume Scenarios 

Execute Sensitivity Test Runs 

and Summarize Results 
8

Report Outline 

This report represents the final task (Task E in figure 1) for the “Identifying and Assessing Key 

Weather-Related Parameters and Their Impacts on Traffic Operations Using Simulation” project. 

This report is separated into the following sections: 

• Section 1 discusses the objective and approach of the project, including a background 

discussion on the need for the study. 

• Section 2 discusses the general relationship between weather events and traffic 

operations, including a discussion of how a change in weather leads to a change in the 

quality of traffic flow. 

• Section 3 discusses the results of a literature search on field studies of the effects of 

adverse weather on traffic operations parameters. 

• Section 4 identifies which simulation parameters are potentially sensitive to weather 

events. 

• Section 5 describes the study methodology and results of the CORSIM sensitivity 

study of the key weather-related parameters identified in section 4. 

• Section 6 develops guidelines for modeling weather events using CORSIM. 

• Section 7 summarizes the findings and conclusions of the report and identifies future 

research needs. 

• Section 8 lists the report references. 
9

2. General Relationship Between Weather Events and Traffic Operations 

Conceptually, it is easy to understand that a major weather event, such as a snowstorm, will lead 

to lower average speeds and higher delays. However, it is important to understand what this 

relationship is, or in other words, what causes a weather event to degrade traffic operations. 

Figure 2 shows the general relationship between weather events and the resulting impact on 

traffic operations. This relationship is similar to that shown by Pisano and Goodwin, with the 

exception that the definition of “traffic operations” has been divided into two subparts: traffic 

parameters (or characteristics) and quality of traffic flow.(3) Traffic parameters are quantitative 

values that typically are used as inputs to a traffic analysis model. These parameters account for 

how drivers and their vehicles interact and respond to the roadway network, including the 

response to other vehicles, traffic control devices, roadway geometry, weather, and other 

environmental conditions. The quality of traffic flow is the output from a traffic analysis model 

and is calculated using MOEs. MOEs measure the overall performance of the transportation 

system, which is directly related to how well drivers and their vehicles respond to the 

surrounding factors (traffic parameters). Common MOEs include average speed, average 

density, average delay per vehicle, and number of stops. 

Figure 2. Relationship Between Weather Events and Traffic Operations. 

This distinction between the input and output in traffic operations is important because traffic 

analysts need to know, for a certain weather event, which traffic parameters to change and how 

much to change them when inputting these parameters into a traffic analysis model. These 

changes will produce a new quality of traffic flow reflecting the impacts of the weather event. 

Weather Event Occurs 

Weather Event Impacts 

Roadway Environment 

Change in Traffic Parameters 

(Input to Traffic Model) 

Change in Quality of Traffic Flow 

(Output from Traffic Model)

“Traffic 

Operations” 

Example

- Major snowstorm

- Reduced pavement friction 

- Reduced driver visibility 

- Snow bank blocks shoulder and 

right lane 

- Lower free-flow speeds 

- More cautious lane changing and 

car following behavior 

- Reduced number of lanes 

- Reduced capacity 

- Lower average speed 

- Higher delays and stops 
10

Definition of Weather Event 

Weather events are any meteorological occurrence that causes weather conditions to degrade 

from the “ideal” weather condition. The ideal weather condition is defined as having the 

following conditions: 

• No precipitation. 

• Dry roadway. 

• Good visibility (greater than 0.4 km ). 

• Winds less than 16 km/h.(4) 

Weather events can change quickly in severity and in coverage area. These changes over time 

and space present a challenge in modeling weather events in a traffic analysis model. The ranges 

of possible weather events that are addressed in this study include rain, snow, sleet, hail, 

flooding, fog, ice, sun glare, lightning, dust, wind, and extreme temperatures. 

Relationship Between Roadway Environment and Weather Event 

Weather events cause a change in the “roadway environment,” a term used by Pisano and 

Goodwin, meaning a physical change in the roadway or roadway devices, or a change on the 

immediate environment surrounding the roadway (including the driver), and vehicle changes.(3)

Each weather event impacts the roadway environment differently. Table 1 shows the connection 

between weather events and the roadway environment. As shown in the table, various weather 

events, such as fog, dust, rain, snow, sleet, hail, and sun glare, can reduce driver visibility. 

Table 1. Impacts of Weather Events on Roadway Environment. 

Weather Events Impact on Roadway Environment 

Fog, dust, rain, snow, sleet, hail, sun glare Reduced driver visibility 

Ice, rain, snow, sleet, hail, flooding Blocked lanes or covered signs and pavement markings 

Reduced pavement friction (note that reducing pavement 

friction leads to a reduction in vehicle maneuverability) 

Wind Reduced vehicle maneuverability and stability 

Extreme temperatures, lightning, wind Failed traffic control devices and communications 

Relationship Between Roadway Environment and Traffic Parameters 

As the roadway environment changes, resulting changes in traffic parameters will occur. For 

example, a reduction in driver visibility will logically cause most drivers to drive more 

cautiously, to some degree. This changed driver behavior is reflected in simulation traffic 

parameters, such as lower free-flow speeds and more cautious lane changing and car following 

parameters. Traffic parameters represent values that a traffic engineer can control in a simulation 

model. The ability to modify these parameters in a simulation model provides the means for 
11

simulating the impacts of adverse weather conditions. The challenge with microscopic 

simulation models like CORSIM is that they require many more input traffic parameters than a 

macroscopic HCM-style model due to the complex modeling of driver behavior on an individual 

vehicle basis. 

Before tracing which traffic parameters are impacted by a change in the roadway environment, it 

is important to understand the full range of parameters available in a microscopic simulation 

model. Table 2 displays a generic list of possible traffic parameters in a microscopic simulation 

model. The parameters are considered generic because they are not specific to any one model, 

and the majority of them are included in most simulation models currently available. However, 

each model uses slightly different terminology to define these parameters. Therefore, the 

parameters listed in table 2 may only be a subset of the actual simulation models parameters. For 

example, there are more than 20 parameters in CORSIM that are used to model lane changing 

behavior. 

Tracing which traffic parameters are likely affected by weather events (through a change in the 

roadway environment) was performed based on a review of table 2, the literature review (section 

3), and engineering judgment. The results of this analysis are presented in section 4 after the 

literature review section. 
12

Table 2. Range of Generic Traffic Simulation Parameters. 

Category Parameters 

Road geometry Pavement condition (wet, dry, etc.) 

Number of lanes 

Lane width 

Lane taper length 

Segment link length 

Shoulder type/width 

Grade 

Horizontal and vertical curvature 

Super-elevation 

Traffic control and 

management 

Traffic signal 

- Controller type 

- Green splits, clearance intervals 

- Progression settings 

- Actuated settings (detectors, vehicle extension time, etc.) 

Ramp meter 

Regulatory signs (Stop, Yield, Speed Limit, etc.) 

Warning signs (Lane Ends, Merge Ahead, etc.) 

Traveler information signs (Variable Message Signs, route guidance signs, 

etc.) 

Surveillance detectors (type and location) 

Lane use by movement (turn only, through only, shared through-turn) 

Lane use by vehicle type (HOV, transit only, no trucks, etc.) 

On-street parking 

Driver behavior Car following 

Lane changing 

Free-flow speed 

Discharge headway 

Startup lost time 

Queue separation/spacing 

Gap acceptance at intersections 

Turning speed 

Rubbernecking (response to incidents) 

Response to yellow interval 

Illegal maneuvers 

Events/scenarios Incidents/blockages (severity, duration) 

Incident management (response, emergency vehicle dispatch, etc.) 

Work zones 

Vehicle performance Vehicle type distribution (% trucks, buses, etc.) 

Acceleration/deceleration capability (stopping distance) 

Turning radius 

Vehicle length 

Simulation run control Length of simulation run 

Selected output MOEs (reports, animation files, etc.) 

Resolution of simulation results (temporal and spatial resolution) 

Traffic demand Vehicle demand (including changes over time), expressed as: 

- Entry demands and turning percentages 

- Origin-destination demands 

Route choice 

Multimodal operations Transit operations (routes, stops, headways, dwell times, etc.) 

Bicycle operations (volumes, free-flow speeds, shared/exclusive paths, etc.) 

Pedestrian operations (volumes, walking speeds, priority rules, sidewalk 

characteristics, etc.) 
13

3. Literature Review 

Past research on the simulation of traffic operations under adverse weather conditions can be 

organized into two main groups: those focusing on the link between weather events and traffic 

parameters (i.e., heavy rain reduces free-flow speeds by 30 percent), and those focusing on the 

link between weather events and the quality of traffic flow (i.e., heavy rain increases delays by 

40 percent). This review focuses on the former, because knowing the impact of weather events 

on traffic parameters is the key to using microsimulation to model weather events. 

Very little research focusing on the roadway environment impacts shown in table 1 were found. 

This lack of information probably is due to the difficulty in understanding why motorists respond 

to a weather event (i.e., is a reduction in free-flow speed really due to a reduction in pavement 

friction or reduction in visibility?) The literature review yielded information on the impacts of 

weather events on the following traffic parameters: free-flow speed, startup lost time, saturation 

headway, and traffic demand. 

Free-Flow Speed 

A number of studies have shown that adverse weather events reduce the mean free-flow speed, 

which is defined as the desired speed of drivers in low volume conditions and in the absence of 

traffic control devices.(2) The amount of reduction in free-flow speed is directly related to the 

severity of the weather event. Kyte et al. studied the free-flow speed on a rural freeway during 

wet and snow-covered pavement, high wind (greater than 24 km/h), and low visibility conditions 

(less than 0.28 km).(4) They found the free-flow speed reduced by approximately: 

• 10 km/h (8 percent) during wet pavement. 

• 16 km/h (13 percent) during snow-covered pavement. 

• 17 km/h (14 percent) during high wind. 

• 18 km/h (15 percent) during low visibility. 

• 35 to 45 km/h (30 to 38 percent) during a combination of snow-covered pavement, 

low visibility, and high wind. 

May showed that the free-flow speed on freeways was reduced by approximately:(5)

• 8 percent under light rain or snow. 

• 17 percent under heavy rain. 

• Up to 40 percent under heavy snow. 

Based on a study of two-lane rural highways, Lamm, Choueiri, and Mailaender found that 

drivers do not adjust their speeds very much under light rain or wet pavement, but they do reduce 

speeds when visibility becomes obstructed, such as during a heavy rain.(6) 

On a sample of freeways in Canada, Ibrahim, and Hall also found that free-flow speed is 

noticeably decreased during heavy rain and snow; heavy snow (up to 50 km/h reduction) has a 

much greater effect than heavy rain (up to 10 km/h reduction).(7)
14

Perrin, Martin, and Hansen measured free-flow speed reductions at two signalized intersections 

on an arterial in Salt Lake City, UT of:(8)

• 10 percent on wet pavement. 

• 25 percent on wet and slushy pavement. 

• 30 percent on pavement with slushy wheel paths. 

Other studies have shown a reduction in average speed on arterials.(9,10) Average speed, a typical 

MOE used by traffic engineers, is a different value than free-flow speed; average speed accounts 

for the effects of signal timing and other effects related to the interaction with other vehicles. 

Startup Lost Time 

Startup lost time is defined as the additional time consumed by the first few vehicles in a queue 

at a signalized intersection beyond the saturation headway.(2) This additional time is due to the 

time to react to the start of the green phase and for the vehicle to accelerate from a stopped 

position. Under ideal conditions, the HCM recommends using 2.0 seconds for startup lost time. 

Maki measured an increase in startup lost time of 50 percent, from 2.0 seconds during normal 

conditions to 3.0 seconds under adverse weather conditions, which was defined as being a storm 

with accumulation of 7.6 centimeters (cm) or more of snow, on a signalized expressway in the 

Minneapolis/St. Paul, MN area.(9)

Perrin, Martin, and Hansen measured a startup lost time increase of approximately 25 percent, 

from 2.0 to 2.5 seconds, under severe snow-related conditions.(8) However, only a small 

difference, from 2.0 to 2.1 seconds, was measured during rain-related conditions. 

Saturation Headway 

Saturation headway, or discharge headway, is defined as the average headway between vehicles 

occurring after the fourth vehicle in a signalized intersection queue and continuing until the last 

vehicle in the initial queue clears the intersection.(2) Saturation headway (expressed in units of 

seconds/vehicle (s/veh)) is the inverse of saturation flow rate (veh/s or veh/h). For example, a 10 

percent increase in saturation headway equates to a 10 percent decrease in saturation flow rate. 

The HCM recommends an ideal discharge headway of 1.9 seconds (equates to a saturation flow 

rate of 1900 passenger cars/h/lane). This value then is reduced based on adjustments for lane 

width, heavy vehicles, grade, adjacent parking, bus blockage, area type, lane utilization, right and 

left turns, pedestrians, and bicyclists. 

Perrin, Martin, and Hansen measured an average reduction in saturation flow rate of between 6 

and 20 percent, increasing with weather severity (snow packed on the street surface being the 

highest severity).(8)

Maki found a saturation flow rate reduction of approximately 10 percent, from 1800 to 1600 

veh/h/lane under adverse weather conditions as defined above.(9)

Botha and Kruse measured the effect of residual ice and snow on a signalized arterial in 

Fairbanks, AK. Saturation flow rates were found to be approximately 20 percent lower than the 

ideal HCM-recommended conditions.(11)
15

Traffic Demand 

Maki measured a reduction in traffic volumes of 15 to 30 percent during adverse weather 

conditions when compared to ideal weather conditions.(9) The reduction in traffic volumes was 

attributed to various reasons, including shifting work arrivals and departures, and avoidance of 

discretionary trips. Traffic demand changes depend strongly on the severity of the weather 

conditions and the driver’s comfort in adverse weather conditions. For example, drivers in 

Chicago, IL will react differently to a snowstorm than will drivers in Miami, FL.
 
 17

4. Identifying Simulation Parameters Affected by Weather Events 

The literature review documented a number of traffic parameters that were found to be impacted 

by weather events. However, there are numerous other microsimulation parameters that have not 

been measured empirically to behave differently during adverse weather. It is important to 

identify these parameters and include them in the sensitivity study. 

Tables 3 through 7 show the traffic simulation parameters that likely are impacted by weather 

events (through a change in the roadway environment). The selection of these parameters was 

based on the range of simulation parameters identified in table 1, the literature review, and the 

use of engineering judgment based on the concept that driver behavior becomes more 

conservative during adverse weather conditions. Unfortunately, there is currently no empirical 

research supporting this concept. Therefore, the table only lists the range of potential, not 

proven, simulation parameters that may be used to model adverse weather conditions in a 

simulation model. These simulation parameters may be used as a guide for traffic analysts when 

considering which parameters to adjust when modeling adverse weather. 

The remainder of this section discusses how parameters in each major category (road geometry, 

traffic control and management, vehicle performance, traffic demand, and driver behavior) may 

be impacted by weather events. 

Road Geometry Parameters 

Table 3 displays road geometry parameters likely impacted by weather events though a change in 

the roadway environment. If available in a simulation model, the pavement condition parameter 

should be modified during a weather event, causing a reduction in pavement friction. The traffic 

analyst should be aware, however, how the pavement condition parameter affects other 

parameters. For example, changing the pavement condition parameter in FRESIM (the freeway 

model within CORSIM) causes an automatic reduction in free-flow speed for a link in a 

horizontal curve. Also, a weather event causing a lane or shoulder blockage would alter the 

number and width of available lanes, length of tapers associated with lane adds and drops, and 

shoulder width. 

Table 3. Road Geometry Traffic Parameters Impacted by Weather Events. 

Weather Events 

Fog, Dust, 

Rain, Snow, 

Sleet, Hail, 

Sun Glare 

Ice, Rain, 

Snow, Sleet, 

Hail, 

Flooding 

Wind, Ice, Rain, 

Snow, Sleet, 

Hail, Flooding 

Ice, Rain, Snow, 

Sleet, Hail, 

Flooding 

Extreme 

Temperatures, 

Lightning, Wind 

Roadway Environment Impact 

Generic 

Traffic Simulation 

Parameter 

Reduced 

Visibility 

Reduced 

Pavement 

Friction 

Reduced 

Vehicle 

Maneuverability

/Stability 

Blocked Lanes/ 

Covered Signs 

and Pavement 

Markings 

Failed Traffic 

Control Devices 

and 

Communications 

Pavement condition X 

Number of lanes X 

Lane width X 

Lane taper length X 

Shoulder width X 
 18

Traffic Control and Management Parameters 

Table 4 displays traffic control and management parameters likely impacted by weather events 

though a change in the roadway environment. A reduction in visibility would make it difficult for 

drivers to see traffic signals or signs. Thus, the parameters related to sight or reaction distance to 

the traffic signals and signs would need to be altered. Also, a weather event that caused a sign 

blockage would require altering the parameters related to the visibility of, and compliance with, 

traffic signs. Finally, a weather event causing a power failure and loss of communications 

between traffic devices or to a traffic management center would require altering the traffic signal 

settings (i.e., change to emergency flash operation), or removing the functionality of detector 

devices, including those used at traffic signals, ramp meters, or systemwide surveillance. 

Table 4. Traffic Control and Management Parameters Impacted by Weather Events. 

Weather Events 

Fog, Dust, 

Rain, Snow, 

Sleet, Hail, 

Sun Glare 

Ice, Rain, 

Snow, Sleet, 

Hail, 

Flooding 

Wind, Ice, Rain, 

Snow, Sleet, 

Hail, Flooding 

Ice, Rain, Snow, 

Sleet, Hail, 

Flooding 

Extreme 

Temperatures, 

Lightning, Wind 

Roadway Environment Impact 

Generic 

Traffic Simulation 

Parameter 

Reduced 

Visibility 

Reduced 

Pavement 

Friction 

Reduced 

Vehicle 

Maneuverability

/Stability 

Blocked Lanes/ 

Covered Signs 

and Pavement 

Markings 

Failed Traffic 

Control Devices 

and 

Communications 

Traffic signal X X 

Ramp meter X X 

Regulatory signs X X 

Warning signs X X 

Traveler information 

signs X X X 

Surveillance 

detectors X 

On-street parking X 

Vehicle Performance Parameters 

Table 5 displays vehicle performance parameters likely impacted by weather events though a 

change in the roadway environment. A reduction in pavement friction could affect the 

acceleration and deceleration capabilities of vehicles. These parameters relate to the 

performance of the vehicle only, and not necessarily the behavior of the drivers. The 

acceleration and deceleration capability of vehicles typically are used in the car following and 

lane changing logic of a simulation model; therefore, changing these parameters likely will alter 

the way vehicles follow each other and change lanes in a model. 
 19

Table 5. Vehicle Performance Traffic Parameters Impacted by Weather Events. 

Weather Events 

Fog, Dust, 

Rain, Snow, 

Sleet, Hail, 

Sun Glare 

Ice, Rain, 

Snow, Sleet, 

Hail, 

Flooding 

Wind, Ice, Rain, 

Snow, Sleet, 

Hail, Flooding 

Ice, Rain, Snow, 

Sleet, Hail, 

Flooding 

Extreme 

Temperatures, 

Lightning, Wind 

Roadway Environment Impact 

Generic 

Traffic Simulation 

Parameter 

Reduced 

Visibility 

Reduced 

Pavement 

Friction 

Reduced 

Vehicle 

Maneuverability

/Stability 

Blocked Lanes/ 

Covered Signs 

and Pavement 

Markings 

Failed Traffic 

Control Devices 

and 

Communications 

Acceleration/ 

deceleration 

capability 

 X X 

Turning radius X X 

Traffic Demand Parameters 

Table 6 displays traffic demand parameters likely impacted by weather events through a change 

in the roadway environment. Any weather event causing one or more major roadway 

environment impacts could cause a change in vehicle demand and route choice. For example, a 

major snowstorm over an entire city could cause vehicle demand to be reduced on all links, 

whereas an isolated storm affecting only a small number of roads could result in no change in 

overall traffic demand but different route choices, because drivers would avoid the impacted 

roads. Many simulation models allow the input of traffic demands as origin-destination pairs 

with a traffic assignment procedure (which determines the preferred route for motorists in 

traveling between their origin and destination) built into the model. For these models, changing 

the appropriate parameters to reflect the conditions of the snowstorm on the isolated roads would 

allow the traffic assignment algorithm to predict automatically the change in route choice 

associated with the snowstorm. 

Table 6. Traffic Demand Traffic Parameters Impacted by Weather Events. 

Weather Events 

Fog, Dust, 

Rain, Snow, 

Sleet, Hail, 

Sun Glare 

Ice, Rain, 

Snow, Sleet, 

Hail, 

Flooding 

Wind, Ice, Rain, 

Snow, Sleet, 

Hail, Flooding 

Ice, Rain, Snow, 

Sleet, Hail, 

Flooding 

Extreme 

Temperatures, 

Lightning, Wind 

Roadway Environment Impact 

Generic 

Traffic Simulation 

Parameter 

Reduced 

Visibility 

Reduced 

Pavement 

Friction 

Reduced 

Vehicle 

Maneuverability

/Stability 

Blocked Lanes/ 

Covered Signs 

and Pavement 

Markings 

Failed Traffic 

Control Devices 

and 

Communications 

Vehicle demand X X X X X 

Route choice X X X X X 

Driver Behavior Parameters 

Table 7 displays driver behavior parameters likely impacted by weather events though a change 

in the roadway environment. Many driver behavior parameters are impacted by weather events 

causing visibility, pavement friction, or vehicle maneuverability reductions. Car following and 
 20

lane changing behavior likely will be more cautious during weather events, with the degree of 

caution dependent on the severity of the weather event. Free-flow speed, startup lost time, and 

discharge headway all have been documented to degrade during weather events. In addition, 

intersection-related parameters such as gap acceptance, turning speed, and responses to the 

yellow interval likely are impacted by weather events. 

Table 7. Driver Behavior Traffic Parameters Impacted by Weather Events. 

Weather Events 

Fog, Dust, 

Rain, Snow, 

Sleet, Hail, 

Sun Glare 

Ice, Rain, 

Snow, Sleet, 

Hail, 

Flooding 

Wind, Ice, Rain, 

Snow, Sleet, 

Hail, Flooding 

Ice, Rain, Snow, 

Sleet, Hail, 

Flooding 

Extreme 

Temperatures, 

Lightning, Wind 

Roadway Environment Impact 

Generic 

Traffic Simulation 

Parameter 

Reduced 

Visibility 

Reduced 

Pavement 

Friction 

Reduced 

Vehicle 

Maneuverability

/Stability 

Blocked Lanes/ 

Covered Signs 

and Pavement 

Markings 

Failed Traffic 

Control Devices 

and 

Communications 

Car following X X X X 

Lane changing X X X X 

Free-flow speed X X X X 

Discharge headway X X X X 

Startup lost time X X X X 

Intersection gap 

acceptance X X X X 

Turning speed X X X X 

Response to yellow 

interval X X X X 
 21

5. CORSIM Sensitivity Analysis 

The purpose of the sensitivity analysis was to determine which weather-related traffic parameters 

have the greatest impact on the quality of traffic flow. It was necessary to identify the most 

sensitive weather-related parameters to develop the guidelines for using CORSIM in modeling 

adverse weather conditions. 

The sensitivity study showed how these parameters impacted the quality of traffic flow. Various 

geometric configurations and congestion levels were tested to get a complete assessment of the 

overall sensitivity of a parameter. 

The sensitivity study started with a baseline case created using the default values for the 

parameters. The sensitivity study focused on changing one parameter value at a time, 

regenerating the MOEs, and comparing the new MOEs to the baseline case. 

This method was found to be limiting, but within the scope of this project. A potentially more 

detailed and realistic sensitivity test would be to change multiple parameter values at once, to 

model specific weather events. This method was not within the scope of the project and would 

result in exponentially increased data processing and analysis efforts. It should be considered for 

future sensitivity testing efforts. 

The sensitivity study was divided into two major groups: sensitivity of parameters on freeway 

facilities using Freeway Simulation (FRESIM), which is the simulator within CORSIM that 

models all freeway facilities; and sensitivity of parameters on arterial streets using Network 

Simulation (NETSIM), which is the simulator within CORSIM that models all arterial and local 

streets. 

FRESIM Analysis Methodology 

A number of different geometric scenarios, or networks, were developed to test the sensitivity of 

the parameters under various roadway configurations using the FRESIM model in CORSIM. For 

example, a parameter may not show any sensitivity on a basic freeway segment, but show high 

sensitivity on a short weaving area. The networks developed for the FRESIM sensitivity analysis 

are shown in table 8. 

All networks were assumed to have ideal conditions as defined in the HCM, including 3.65-m 

travel lanes, level grade, no horizontal curves, and no heavy trucks.(2) All freeway segments 

were assumed to have a free-flow speed of 113 km/h, while all on- and off-ramps were assumed 

to have a free-flow speed of 72 km/h. Also, an analysis period of one hour was used for all 

simulation runs. 
 22

Table 8. FRESIM Sensitivity Analysis Networks. 

Network Name Description 

One-lane basic segment One-lane freeway with no on- or off-ramps, 1.6 km long. 

Two-lane basic segment Same as the one-lane basic segment, except with two freeway lanes. 

Three-lane basic 

segment 

Same as the one-lane basic segment, except with three freeway lanes. 

Two-lane merge area Two-lane freeway with a single on-ramp, with a ramp volume of 500 veh/h 

and 230 m acceleration lane. 

Three-lane merge area Same as the two-lane merge area, except with three freeway lanes. 

Two-lane diverge area Two-lane freeway with a single off-ramp, with an exiting ramp volume of 

between 300 and 750 veh/h (fixed at 15% of freeway volume) and 230 m 

deceleration lane. 

Three-lane diverge area Same as the two-lane diverge area, except with three freeway lanes. 

Two-lane weave area Two-lane freeway with an on-ramp and off-ramp separated by 300 m, onramp volume of 500 veh/h, off-ramp volume of between 375 and 825 veh/h 

(fixed at 15% of freeway volume), and single auxiliary lane connecting the 

on- and off-ramps. 

Three-lane weave area Same as the two-lane weave area, except with three freeway lanes. 

System 5.15-m, three-lane freeway system including two merge areas (each with 150-

m acceleration lanes), one diverge area (with a 150-m deceleration lane), and 

one weave area (with a 300-m auxiliary lane). 

For each roadway network, the sensitivity of four different congestion levels was tested by 

incrementally increasing the entering volume (or traffic demand) on the freeway. The four 

congestion levels tested are shown in table 9. 

Table 9. Congestion Levels for FRESIM Sensitivity Analysis. 

Congestion Level Description 

Low 1000 veh/h/lane, equivalent to a volume/capacity (V/C) ratio 

of 0.42. 

Medium 1500 veh/h/lane, equivalent to a V/C ratio of 0.63. 

High 2000 veh/h/lane, equivalent to a V/C ratio of 0.83. 

Very high 2400 veh/h/lane, equivalent to a V/C ratio of 1.0. 

The HCM estimates the capacity of a basic freeway segment with a free-flow speed of 113 km/h 

to be 2400 veh/h/lane assuming ideal conditions.(2) In FRESIM, the upper bound of capacity can 

be limited by using the minimum separation for generation of vehicles parameter. For the 

sensitivity tests, this value was fixed at 1.5 seconds (default is 1.6 seconds), which equates to a 

maximum entering volume of 2400 veh/h/lane. The capacity can be limited by the driver 

behavior logic in some cases; this behavior was seen in the sensitivity study as discussed below. 

Testing at V/C ratios above 1.0 was not conducted for the freeway sensitivity tests. With values 

above 1.0, it was impossible to create a congested state on a basic freeway segment without 

creating a downstream bottleneck. Because simple basic and merge/diverge networks were used 
 23

in this study, any demand volume over capacity would still operate at capacity, while creating 

congested conditions further upstream. Future research into the sensitivity of freeway 

parameters under overcapacity conditions should be considered, based on the results shown in 

this study. 

The MOEs used to quantify the effects of parameter changes on the quality of traffic flow are 

shown in table 10. 

Table 10. FRESIM MOEs for Sensitivity Analysis. 

Measure of 

Effectiveness 

Description 

Throughput (veh/h/lane) Measures the volume of vehicles traveling through a uniform segment. By 

gradually increasing the entering demand volume, the capacity of the segment 

was estimated by noting at what point the actual volume no longer matched 

the entering demand volume. This MOE was used for the basic, merge, 

diverge, and weave networks. However, it was not used for the system 

network because there are different segment types within the system, and each 

segment type has a different capacity. 

Vehicle-kilometers of 

travel (veh-km/h) 

Measures the number of vehicles traveling through a segment or multiple 

segments while taking into account the length of the segments. This MOE, 

which often is used for freeway system analyses, was only used for the system 

network as a surrogate to throughput, as it indirectly measures the capacity of 

the system while also accounting for the length of the network. 

Average speed (km/h) Measures the average space mean speed over the entire freeway network. This 

MOE was used on all test networks. 

Average density 

(veh/km/lane) 

Measures the average density over the entire freeway network. This MOE was 

used on all test networks. 

Average delay (sec/veh) Measures the difference in actual travel time and desired travel time (based on 

the free-flow speed). This MOE was used on all test networks. 

The MOEs listed in table 10 were summarized only within the portion of the network that 

captured the extent of the congestion and experienced the most change in MOEs from one 

congestion level to the next. Table 11 lists the MOE collection area for each network. 
 24

Table 11. FRESIM MOE Collection Areas. 

Network Name MOE Collection Area 

One-lane basic segment Entire 1.6-km length of the freeway segment. 

Two-lane basic segment Same as one-lane basic segment. 

Three-lane basic segment Same as one-lane basic segment. 

Two-lane merge area The length of freeway (including the acceleration lane) beginning with the on-ramp 

gore area and extending downstream 460 m. This distance was used because the 

HCM states that 460 m is generally the area of influence at merge and diverge areas; 

this was found to be fairly accurate based on visual inspection of the CORSIM 

animation for the merge area networks. 

Three-lane merge area Same as two-lane merge area. 

Three-lane diverge area The 1220 m length of freeway starting at the off-ramp gore area and extending 

upstream 1220 m. The area of influence was increased to 1220 m because vehicles on 

the freeway began changing lanes to get in the proper lane 760 m upstream of the 

actual diverge area itself (based on the 760 m default off-ramp reaction point 

parameter), which created congestion between 760 and approximately 1220 m 

upstream of the diverge area. 

Three-lane diverge area Same as two-lane diverge area. 

Two-lane weave area The freeway lanes and auxiliary lane within the 300-m weave area. 

Three-lane weave area Same as two-lane weave area. 

System The entire 5.1 km/h freeway segment, including the auxiliary lanes associated with 

the on- and off-ramps, but not the ramps themselves. 

The FRESIM sensitivity study focused on the car following, lane changing, and free-flow speed 

parameters, because the other driver behavior parameters shown in table 7 apply to intersections 

on surface streets. Also, the majority of the other parameters listed in tables 3 through 7 have 

major impacts on the quality of traffic flow, so they already are known to be very sensitive 

parameters. For example, reducing the number of lanes from three to two due to a lane blockage, 

changing a signal control to emergency flashing due to a power outage, or reducing the traffic 

demand by 20 percent due to a major snowstorm all have major impacts on the quality of traffic 

flow. Such events are easily discernable as having a major effect on traffic flow, but the more 

subtle changes in car following and lane changing behavior are not quite as obvious, and are 

therefore the focus of this sensitivity study. 

Tables 12 through 14 display the FRESIM parameters included in the sensitivity analysis. Each 

parameter was tested at the default value, along with four other values representing incrementally 

more conservative driver behavior, as would be the case with increasingly severe weather 

conditions. As a result, the sensitivity tests were one-sided in that they only tested values to one 

side of the default value. However, a few parameters were tested on both sides, because it was 

not clear which side represented the more conservative driver behavior (e.g., anticipatory lane 

change distance). 

The car following parameters were tested on the basic segment (one-lane, two-lane, and threelane), and system networks. The basic segment networks were used to isolate the sensitivity of 
 25

the car following parameters (without the MOEs being influenced by factors associated with 

merging or diverging), while the system network was used to show the sensitivity within the 

context of a real-world network consisting of merges, diverges, and weave areas. Table 12 

shows car following FRESIM parameters included in the sensitivity analysis. 

Table 12. Car Following FRESIM Parameters Included in Sensitivity Analysis. 

FRESIM 

Parameter 

Definition 

Car following 

sensitivity factor 

This factor is the primary user input in calculating the desired time (in 

seconds) headway between a leader-follower vehicle pair. A higher value 

means more space between vehicles. A different value is specified for each 

driver type (default = 1.25 to 0.35 s based on driver type, mean = 0.80 s). 

Car following 

sensitivity multiplier 

A link-specific multiplier of the car-following sensitivity factor (default = 

100%). The multiplier is applied to all driver types and therefore changes the 

overall mean of the sensitivity factor. This value can be used to calibrate car 

following behavior on a link-by-link basis. 

Pitt car following 

constant 

The minimum distance between the rear of the lead vehicle and front of the 

following vehicle, regardless of vehicle speed (default = 3.05 m). 

Lag acceleration/ 

deceleration time 

The time delay (due to perception/reaction time) for motorists when starting to 

accelerate or decelerate (default = 0.3 s). 

Maximum nonemergency 

deceleration 

The maximum deceleration on level grade and dry pavement in nonemergency conditions (i.e., normal lane changing and car following behavior). 

Reflects driving habits and not capability of the vehicle (default = 2.44 m/s2

). 

Jerk value The maximum change in acceleration between consecutive intervals (default = 

2.13 m/s3

). A higher value results in more aggressive driver behavior. 

The lane changing parameters were tested on all of the networks described in table 8, with the 

exception of the basic one-lane segment network, because lane changes are not possible on a 

one-lane segment. All other networks test various types of lane change environments, including 

mandatory lane changes at on- and off-ramps, discretionary lane changes on a basic freeway 

segment, and anticipatory lane changes upstream of merge areas. Refer to the TSIS Version 5.1 

User’s Guide(12) and Halati, Lieu, and Walker(13) for a detailed description of the FRESIM 

vehicle movement logic. The lane changing FRESIM parameters included in the sensitivity 

analysis are displayed in table 13. 
 26

Table 13. Lane Changing FRESIM Parameters Included in Sensitivity Analysis. 

FRESIM Parameter Definition 

Time to complete lane 

change 

The time to complete a lane change maneuver (default = 2.0 s). Increasing 

this value results in more extended, smooth lane changes. 

Advantage threshold 

for discretionary lane 

change 

Used to calculate the relative advantage in making a discretionary lane 

change (default = 0.4). The advantage (measured in speed and volume) in 

changing lanes must be greater than the condition in the current lane by a 

factor of 0.4. Increasing this value decreases the number of lane changes. 

Discretionary lane 

change multiplier 

A multiplier used in calculating the desire for discretionary lane changes 

(default = 0.5). Increasing this value increases the desire for discretionary 

lane changes. 

Gap acceptance 

parameter 

Used to determine the acceptable gap for mandatory lane changes (default = 

3). A higher value represents less aggressive driver behavior and fewer lane 

changes. 

Percent cooperative 

drivers 

Percentage of drivers desiring to yield the right-of-way to vehicles 

attempting to merge ahead of them (default = 20%). 

Maximum nonemergency 

deceleration 

The maximum deceleration on level grade and dry pavement in nonemergency conditions (i.e., normal lane changing and car following 

behavior). Reflects driving habits and not capability of the vehicle (default = 

2.44 m/s2

). 

Maximum emergency 

deceleration 

The maximum deceleration on level grade and dry pavement in emergency 

conditions (i.e., sudden stop to prevent a collision). Reflects driving habits 

and not capability of the vehicle (default = 4.57 m/s2

). 

Leader’s maximum 

deceleration as 

perceived by follower 

The maximum deceleration of the lead vehicle in an adjacent lane as 

perceived by a potential lane-changing vehicle, which is used to determine 

whether a gap in the adjacent lane is acceptable (default = 4.57 m/s2

). 

Anticipatory lane 

change speed 

Vehicles upstream of a merge area will change lanes to avoid the area if the 

speed of the acceleration lane falls below this threshold (default = 2/3 freeflow speed). 

Anticipatory lane 

change distance 

Vehicles upstream of a merge area begin to react (in terms of a potential 

lane change to avoid congestion related to the merge area) this distance in 

advance of the acceleration lane (default = 458 m). 

The two free-flow speed parameters were tested on all of the test networks. Table 14 shows the 

free-flow speed FRESIM parameters included in the sensitivity analysis. 
 27

Table 14. Free-Flow Speed FRESIM Parameters Included in Sensitivity Analysis. 

FRESIM Parameter Definition 

Mean free-flow speed The desired mean speed of vehicles in the absence of any impedance due to 

other vehicles or traffic control devices (link specific). 

Free-flow speed 

multiplier 

A percentage multiplier for each driver type of the mean free-flow speed. A 

more aggressive driver type receives a higher multiplier to represent a 

higher free-flow speed. Together, the multipliers provide a distribution of 

free-flow speed by driver type (default = 88–112% based on driver type). 

NETSIM Analysis Methodology 

A number of different geometric scenarios, or networks, were developed to test the sensitivity of 

the parameters under various roadway configurations using the NETSIM model in CORSIM. 

The networks developed for the NETSIM sensitivity analysis are shown in table 15. Figures 3 

and 4 show the two intersection networks as viewed in the Traffic Editor (TRAFED), the input 

editor interface for Traffic Software Integrated System (TSIS). 

Table 15. NETSIM Sensitivity Analysis Networks. 

Network Name Description 

One-lane basic 

segment 

One-lane arterial segment (no intersections or driveways) of 1.6 km in length and 

free-flow speed of 72 km/h. 

Two-lane basic 

segment 

Same as the one-lane basic segment, except with two arterial lanes. 

Three-lane basic 

segment 

Same as the one-lane basic segment, except with three arterial lanes. 

Single suburban 

intersection 

Five-lane arterial with free-flow speed of 72 km/h intersecting a three-lane collector 

street with free-flow speed of 48 km/h. The intersection is controlled by a fully 

actuated traffic signal with protected left-turn phasing, 76-m left-turn bays on all 

approaches, and a maximum cycle length of 120 s (if all phases max out). This 

intersection is typical of those found on major arterials in a suburban setting. Figure 3 

displays the suburban intersection in TRAFED). 

Single urban 

intersection 

Three-lane collector intersecting a two-lane collector, both with free-flow speeds of 

48 km/h. The intersection is controlled by a pretimed traffic signal with two phases 

(one for each roadway with permitted left-turn phasing), 46-m left-turn bays on all 

approaches, and a fixed cycle length of 80 s. This intersection is typical of those found 

in urban or downtown settings. Figure 3 displays the urban intersection in TRAFED). 

System 3.2-m arterial corridor with four traffic signals at 610-m spacing. The arterial has a 

free-flow speed of 72 km/h with two through lanes in each direction and 76-m leftturn bays at the traffic signals, and the intersecting minor streets are one lane in each 

direction with 76-m left- and right-turn bays at the intersections. The traffic signals 

are controlled by a semiactuated, coordinated plan with a 120-s cycle. 
 28

Figure 3. NETSIM Suburban Intersection Network. 

Figure 4. NETSIM Urban Intersection Network. 

All networks were assumed to have ideal conditions as defined in the HCM, which includes 

3.66-m travel lanes, level grade, no horizontal curves, and no heavy trucks.(2) An analysis period 

of 1 hour was used for all simulation runs. 

For each test network, the sensitivity to four different congestion levels was tested by 

incrementally increasing the entering volume (or traffic demand) on the entry links. The four 

congestion levels tested on the basic one-, two-, and three-lane networks are listed in table 16. 

Single Suburban Intersection 

Single Urban Intersection 
 29

Table 16. Congestion Levels for NETSIM Sensitivity Analysis. 

Congestion Level Description 

Low 800 veh/h/lane, equivalent to a V/C ratio of 0.4. 

Medium 1200 veh/h/lane, equivalent to a V/C ratio of 0.6. 

High 1600 veh/h/lane, equivalent to a V/C ratio of 0.8. 

Very high 2000 veh/h/lane, equivalent to a V/C ratio of 1.0. 

The HCM does not provide guidance on the segment capacity of arterial streets, mainly because 

the capacity on arterials is determined by traffic signals and not the segment characteristics 

between traffic signals. However, it is clear that the segment capacity of arterials is generally 

lower than on freeways due to the lower free-flow speeds and increased friction effects 

(driveway access, on-street parking, narrow lanes, turning vehicles, etc.). Thus, a capacity of 

2000 veh/h/lane was assumed for the basic arterial test networks based on a free-flow speed of 

72 km/h. Even though this is just an estimate, the purpose of this study is to test relative 

sensitivity of different parameters and not to determine the absolute value of capacity or other 

MOEs. 

As mentioned previously in the FRESIM analysis methodology, the minimum separation for 

generation of vehicles parameter can be used to limit the upper bound of capacity. However, this 

parameter is not available in NETSIM. As a result, segment volumes as high as 2700 veh/h/lane 

on a one-lane arterial and 2600 veh/h/lane on a two-lane arterial, assuming a free-flow speed of 

72 km/h, were achieved in the current version of NETSIM. These values are higher than the 

capacity of typical freeways and are not realistic for an arterial segment. Based on this, it is 

recommended that the minimum separation for generation of vehicles parameter be available in 

NETSIM with an appropriate default value reflecting a realistic capacity of arterials, such as 1.8 

s (equivalent to 2000 veh/h/lane). 

For the single suburban intersection, single urban intersection, and system networks, the entering 

demand volume on all approaches was incrementally increased to achieve V/C ratios of 

approximately 0.6, 0.8, 1.0, and 1.1. The highest volume scenario was limited to a V/C ratio of 

1.1 because ratios higher than 1.1 resulted in queue spillback beyond the limits of the network; 

therefore, the MOEs would not reflect the extent of the congestion. 

Table 17 displays the MOEs used to measure the quality of traffic flow under the various 

network and volume scenarios for the NETSIM cases. 
 30

Table 17. NETSIM MOEs for Sensitivity Analysis. 

The MOEs listed in table 17 were only summarized within the portion of the network that 

captured the extent of the congestion and experienced the most change in MOEs from one 

congestion level to the next. Table 18 lists the MOE collection area for each network. 

The simulation parameters chosen for the arterial sensitivity testing included the car following, 

lane changing, free-flow speed, discharge headway, startup lost time, and turning speed 

parameters within NETSIM. As mentioned previously in the FRESIM analysis methodology, 

the majority of other parameters identified in section 4 as being impacted by weather events 

already are known to have a major impacts on the quality of traffic flow; therefore they were not 

included in this sensitivity analysis. 

Measure of 

Effectiveness 

Description 

Throughput 

(veh/h/lane) 

Measures the volume of vehicles traveling through a uniform segment or 

intersection. By gradually increasing the entering demand volume, the capacity of 

the segment or intersection can be estimated by noting at what point the actual 

volume no longer matches the entering demand volume. This MOE was used on 

the all networks except the system network. 

Vehicle-kilometers 

of travel (veh-km/h) 

Measures the number of vehicles traveling through a segment or multiple 

segments while accounting for the length of the segments. This MOE, which is 

often used for system analyses, was used for the system network only as a 

surrogate to throughput, as it indirectly measures the capacity of the system when 

incrementally increasing the entering demand volume, until the vehicle-miles of 

travel no longer increases at a commensurate rate. 

Average speed 

(km/h) 

Measures the average space mean speed over the entire network. This MOE was 

used on the basic and system networks. However, it was not used on the single 

intersection networks, because stopped delay was deemed a more appropriate 

MOE at an intersection level. 

Stopped delay 

(s/veh) 

Measures the time spent stopped due to the effects of a traffic signal. This MOE 

was used on the single suburban and urban intersection networks because it 

measures the quality of service given by a traffic signal. Control delay was not 

used here because it is a function of the free-flow speed, and free-flow speed is a 

parameter in the sensitivity analysis. Thus, control delay would not give a 

consistent comparison when testing the free-flow speed. 

Average delay 

(s/veh) 

Measures the difference in actual travel time and desired travel time. This MOE, 

used on all test networks, takes into account delays due to traffic control devices 

and to the interaction with adjacent vehicles. 

Number of lane 

changes (lane 

changes/h) 

Measures the total number of lane changes made on the network. This MOE, used 

on all test networks, is not a direct measure of the quality of traffic flow, but it 

was included because it is a helpful measure in understanding why the other 

MOEs did or did not change significantly and how the parameters affect lane 

changing behavior. 
 31

Table 18. NETSIM MOE Collection Areas. 

Network Name MOE Collection Area 

One-lane basic segment The entire 1.6-km length of the segment. 

Two-lane basic segment Same as the one-lane basic segment. 

Three-lane basic segment Same as the one-lane basic segment. 

Single suburban intersection Averaged (weighted based on the approach volume) over all 

intersection approaches. 

Single urban intersection Averaged (weighted based on the approach volume) over all 

intersection approaches. 

System Averaged over the major street links only. 

Tables 19 through 24 display the NETSIM parameters included in the arterial sensitivity 

analysis. Each parameter was tested at the default value, along with four other values 

representing incrementally more conservative driver behavior, as would be the case with 

increasingly severe weather conditions. As a result, the sensitivity tests were one-sided in that 

they only test values to one side of the default value. However, a few parameters were tested on 

both sides because it was not clear which side represented the more conservative driver behavior. 

NETSIM only has one car following parameter, compared to six in FRESIM. In NETSIM, the 

impacts of traffic control devices and lane changing maneuvers to prepare for downstream 

turning movements often control the vehicle movement logic. Thus, a detailed car following 

logic in NETSIM is not as critical to modeling realistic traffic flow as it is in FRESIM. On the 

other hand, the lane changing logic in NETSIM is quite detailed, reflected in the fact that there 

are 15 NETSIM lane changing parameters. Refer to the TSIS Version 5.1 User’s Guide(12) and 

Halati, Lieu, and Walker(13) for a detailed description of the vehicle movement logic in NETSIM. 

Table 19 shows the car following NETSIM parameter included in the sensitivity analysis. 

Table 19. Car Following NETSIM Parameter Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Time to react to sudden 

deceleration of lead vehicle 

The amount of time for a driver to begin decelerating after the leader 

begins a sudden deceleration due to perception/reaction time (default = 

1.0 s). 

The lane changing parameters were tested on all the test networks, except the basic one-lane and 

urban intersection networks (one through lane in each direction) because lane changes are not 

possible on one-lane roadways. Table 20 shows the lane changing NETSIM parameters included 

in the sensitivity analysis. 
 32

Table 20. Lane Changing NETSIM Parameters Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Driver type factor This factor is used to calculate a driver’s “intolerable” speed—the speed below 

which a driver begins looking for a lane change (default = 25). A higher value 

means drivers will have a higher intolerable speed and thus will look for lane 

changes more often. 

Urgency threshold A driver’s desire to change lanes becomes more urgent as he or she gets closer to 

the object requiring a lane change (lane drop or slow-moving leader). After a 

driver’s urgency factor (based on driver aggressiveness and distance to lanechange object) exceeds this factor, then the acceptable deceleration for changing 

lanes begins decreasing (default = 0.2). 

Minimum deceleration for a 

lane change 

A driver’s acceptable deceleration (or risk) for lane changes varies depending on 

his or her urgency. This value defines the acceptable deceleration when a 

driver’s urgency for changing lanes is very low. Decreasing this value decreases 

the amount of risk a driver is willing to take and thus decreases the number of 

lane changes (default = 1.52 m/s2

). 

Difference in minimum 

/maximum deceleration for 

mandatory lane changes 

A driver’s acceptable deceleration (or risk) for mandatory lane changes can vary 

depending on his or her urgency. This factor, measuring the difference in the 

minimum and maximum acceptable decelerations, defines how much the 

acceptable deceleration can vary. The default value (3.05 m/s2

) means the 

minimum and maximum acceptable acceleration can vary as much as 

3.05 m/s2

. 

Difference in minimum/ 

maximum deceleration for 

discretionary lane changes 

A driver’s acceptable deceleration (or risk) for discretionary lane changes can 

vary depending on his or her urgency. This factor, measuring the difference in 

the minimum and maximum acceptable decelerations, defines how much the 

acceptable deceleration can vary. The default value (1.52 m/s2

) means the 

minimum and maximum acceptable acceleration can vary as much as 1.52 m/s2

. 

Safety factor This factor represents the amount of caution by a lane changer (default = 0.8). 

For example, if the minimum acceptable deceleration is 3.05 m/s2

, then the 

acceptable deceleration with the safety factor is 10 x 0.8 = 2.44 m/s2

. 

Headway at which all 

vehicles attempt lane 

change 

The headway below which all drivers will attempt a lane change (default 

= 2.0 s). Increasing this value results in drivers attempting fewer lane changes. 

Headway at which no 

vehicles attempt lane 

change 

The headway above which no drivers will attempt a lane change (default 

= 5.0 s). Increasing this value results in drivers attempting more lane changes. 

Time to react to sudden 

deceleration of lead vehicle 

This factor is used to calculate a driver’s “intolerable” speed—the speed below 

which a driver begins looking for a lane change (default = 25). A higher value 

means drivers will have a higher intolerable speed and therefore will look for 

lane changes more often. 

Duration of a lane change The time to complete a lane change maneuver (default = 3.0 s). This is also the 

minimum time after a lane change is initiated that another lane change can begin. 

Increasing this value results in more extended, smoother lane changes. 

Percent drivers who 

cooperate with lane changer 

The percentage of drivers who will slow down to allow a lane change in front of 

them (default = 50%). Increasing this value results in more lane change 

opportunities. 

Distance over which drivers 

perform lane change 

The mean distance for a driver to contemplate and complete a lane change 

(default = 91.4 m). Higher values result in drivers seeking lane changes over a 

longer distance and therefore make a smoother lane change. 
 33

Table 20. Lane Changing NETSIM Parameters Included in Sensitivity Analysis 

(continued). 

NETSIM Parameter Definition 

Distribution of distance 

to attempt a lane change 

A percentage multiplier for each driver type of the mean lane change 

distance (default ranges from 125–75%). 

Deceleration of lead 

vehicle 

The maximum deceleration rate of a lead vehicle (default = 3.66 m/s2). A 

higher value results in fewer acceptable gaps (because followers will require 

larger gaps) and fewer lane changes. 

Deceleration of 

following vehicle 

The maximum deceleration rate of a following vehicle (default = 3.66 m/s2). 

A higher value means followers will accept smaller gaps and therefore make 

more lane changes. 

The free-flow speed parameters were tested on all the test networks. The two NETSIM free-flow 

speed parameters are the same as those in FRESIM; however, the default multipliers are slightly 

different in each model. Table 21 shows the free-flow speed NETSIM parameters included in 

the sensitivity analysis. 

Table 21. Free-Flow Speed NETSIM Parameters Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Mean free-flow speed The desired mean speed of vehicles in the absence of any impedance 

due to other vehicles or traffic control devices (link specific). 

Free-flow speed multiplier A percentage multiplier for each driver type of the mean free-flow 

speed. A more aggressive driver type receives a higher multiplier to 

represent a higher free-flow speed. Together, the multipliers provide a 

distribution of free-flow speed by driver type (default = 75–127 

percent based on driver type). 

The queue discharge headway, startup lost time, and turning speed parameters are only 

applicable at intersections and thus were tested on all the networks except for the basic segment 

networks. Tables 22 through 24 show the discharge headway, startup lost time, and turning 

speed NETSIM parameters included in the sensitivity analysis, respectively. 

Table 22. Discharge Headway NETSIM Parameters Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Mean discharge headway The mean headway between vehicles discharging from a standing 

queue (mean = 1.8 s). 

Discharge headway multiplier A percentage multiplier for each driver type of the mean discharge 

headway (default ranges from 170–50%). 
 34

Table 23. Startup Lost Time NETSIM Parameters Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Mean startup delay The mean delay (due to perception/reaction time) of the first vehicle in 

a queue due to a traffic signal (default = 2.0 s). 

Startup lost time multiplier A percentage multiplier for each driver type of the mean startup delay 

(ranges from 218–23%). 

Table 24. Turning Speed NETSIM Parameters Included in Sensitivity Analysis. 

NETSIM Parameter Definition 

Maximum allowable left-turn 

speed 

The speed at which vehicles making a left turn will travel through the 

turn if unimpeded by other vehicles (default = 6.71 m/s). 

Maximum allowable right-turn 

speed 

The speed at which vehicles making a right turn will travel through the 

turn if unimpeded by other vehicles (default = 3.96 m/s). 

Data Processing Procedure 

Overall, approximately 45,000 individual CORSIM simulation runs were processed for the 

sensitivity analysis: 25,000 in FRESIM and 20,000 in NETSIM. The need for the large number 

of runs becomes clear when considering the following for the FRESIM runs: 

• Parameters—Eighteen total FRESIM parameters were tested (see tables 3 through 

7). 

• Parameter values—Each parameter was tested using the default value and four 

additional values representing incrementally more conservative driver behavior, as 

would be expected under adverse weather. 

• Networks—Each parameter was tested on an average of seven FRESIM networks 

(car following parameters tested on 4 networks, lane changing on 9 networks, and 

free-flow speed on 10 networks) (see table 8). 

• Congestion level—Each FRESIM network was tested at four different congestion 

levels (see table 9). 

• Simulation runs—Ten simulation runs were performed for each scenario to take into 

account the stochastic variations of the simulation model. 

Therefore, the number of total FRESIM runs equals approximately 25,000 (18x5x7x4x10). 

Due to the large number of simulation runs, the process of creating the CORSIM input files and 

summarizing the output files was largely automated. The data processing was completed 

through four steps, described below. 
 35

Step 1—Create the CORSIM input files. 

A customized script (in both Microsoft® Visual Basic® and C++®) was created that 

automatically generated new TRF files by taking a base input file and changing one or 

more parameters at a time. As a result, thousands of input files could be created with a 

single “Do Loop” command, changing the value of one or more parameters multiple 

times. A spreadsheet was created with all the desired network-congestion level-parameter 

value combinations, which was read by the script to create the input files. 

Step 2—Run CORSIM 10 times for each input file, and create an output file summarizing 

the relevant MOEs from the 10 runs. 

The multirun function available in TSIS 5.1 (the simulation environment that includes 

CORSIM) was used to run CORSIM 10 times for each input file. The “Output Processor” 

function available in TSIS was also used to create an output file in Microsoft® Excel®

format summarizing the mean and standard deviations of the MOEs for the 10 runs. The 

random number seeds were changed for each of the 10 runs. 

Step 3—Copy all relevant MOE data from the output files into a single database. 

Customized Microsoft® Visual Basic® macros were created that copied the relevant MOE 

data from the thousands of output files into two databases, one each for the FRESIM and 

NETSIM runs. The macros also calculated t-values to test the statistical significance of 

the results (at a 95 percent confidence interval). 

Step 4—Create a one-page summary of MOEs for each parameter-network combination. 

One-page summaries for each parameter-network combination (e.g., sensitivity of the car 

following factor on basic two-lane freeway) were created using customized Microsoft®

Visual Basic® macros that read the values from the database created in step 3.

The end product of the data processing is a one-page summary for each parameter-network 

combination (e.g., sensitivity of mean free-flow speed on basic one-lane freeway network). 

These one-page summaries provided a useful tool for evaluating the sensitivity of each 

parameter. Tables 25 through 29 and figures 5 through 9 display a sample of the information 

available on a single one-page summary (formatting requirements of the report precluded the 

original format of the one-page summary from being displayed). The results from these figures 

and tables represent the results of testing the car following sensitivity multiplier on the FRESIM 

system network, which, as will be discussed later, is one of the most sensitive parameters tested. 
 36

Table 25. General Information for Sample Sensitivity Test. 

Component Description 

Parameter name Car following sensitivity multiplier 

Parameter type Car following 

Test network System 

Model FRESIM 

Input level Link 

TRAFED location Double-click a link -> “General” tab 

Record type/field(s): Record type 20/Field 17 

Default value 100 

Sensitivity range 100, 125, 150, 175, 200 

Definition This value adjusts the car following sensitivity factor by a 

multiplier for all vehicles on a given link. The car following 

sensitivity factor is the primary variable in determining the 

desired headway of a vehicle following another vehicle. 

Figure 5. Analysis Area Information for Sample Sensitivity Test. 

Table 26. Vehicle-Kilometers Traveled Table for Sample Sensitivity Test. 

VEHICLE-KILOMETERS OF TRAVEL (veh-km/hr) 

Car Following Sensitivity Multiplier 

Entering 

Volume 

(veh/h/ln) 

100 * 125 150 175 200 

1000 17728 17733 (0%) 17731 (0%) 17724 (0%) 17724 (0%) 

1500 24787 24770 (0%) 24733 (0%) 24740 (0%) 23930 (-3%) 

2000 31102 29363 (-6%) 27945 (-10%) 26066 (-16%) 23939 (-23%) 

2500 31058 29309 (-6%) 27898 (-10%) 26036 (-16%) 24004 (-23%) 

Notes: * = Default value. 

 (XX%) = Percent difference from default value. 

Bold value = Value is statistically different from default value (95th percentile confidence level). 

Freeway Lanes: 3

Analysis Area Length: 5.1 km

Freeway Free-Flow Speed: 113 km/h

Analysis Area
 37

 

Figure 6. Vehicle-Kilometers Traveled Graph for Sample Sensitivity Test. 

Table 27. Average Speed Table for Sample Sensitivity Test. 

AVERAGE SPEED (km/h) 

Car Following Sensitivity Multiplier 

Entering 

Volume 

(veh/h/ln) 

100 * 125 150 175 200 

1000 107.0 107.0 (-1%) 106.0 (-1%) 105.0 (-2%) 104.0 (-3%) 

1500 105.0 103.0 (-2%) 100.0 (-5%) 92.1 (-12%) 61.4 (-42%) 

2000 81.0 66.3 (-18%) 62.2 (-23%) 58.1 (-28%) 53.2 (-34%) 

2500 72.7 66.6 (-8%) 62.7 (-14%) 57.6 (-21%) 54.0 (-26%) 

Notes: * = Default value. 

 (XX%) = Percent difference from default value. 

 Bold value = Value is statistically different from default value (95th percentile confidence level). 

Figure 7. Average Speed Graph for Sample Sensitivity Test. 

VEHICLE-KILOMETERS OF TRAVEL

0

10000

20000

30000

40000

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Vehicle-Kilometers 

of Travel (vehkm/hr)

100 125 150 175 200

AVERAGE SPEED

0

20

40

60

80

100

120

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Speed

(km/h)

100 125 150 175 200
 38

 Table 28. Average Delay Table for Sample Sensitivity Test. 

AVERAGE DELAY (s/veh) 

Car Following Sensitivity Multiplier 

Entering 

Volume 

(veh/h/ln) 

100 * 125 150 175 200 

1000 8.1 9.1 (12%) 10.3 (27%) 12.1 (49%) 14.2 (74%) 

1500 12.0 14.8 (24%) 20.3 (70%) 36.4 (204%) 137. (1040%) 

2000 63.8 114. (78%) 133. (108%) 153. (140%) 182. (186%) 

2500 89.5 113. (26%) 130. (46%) 157. (75%) 178. (98%) 

Notes: * = Default value. 

 (XX%) = Percent difference from default value. 

 Bold value = Value is statistically different from default value (95th percentile confidence level). 

Figure 8. Average Delay Graph for Sample Sensitivity Test. 

Table 29. Average Density Table for Sample Sensitivity Test. 

AVERAGE DENSITY (veh/km/ln) 

Car Following Sensitivity Multiplier 

Entering 

Volume 

(veh/h/ln) 

100 * 125 150 175 200 

1000 10.9 10.9 (1%) 11.0 (1%) 11.1 (2%) 11.3 (4%) 

1500 15.5 15.8 (2%) 16.3 (5%) 17.9 (15%) 26.5 (71%) 

2000 26.2 32.6 (24%) 32.4 (24%) 31.4 (20%) 31.4 (20%) 

2500 33.0 33.2 (1%) 32.1 (-3%) 31.7 (-4%) 31.0 (-6%) 

Notes: * = Default value. 

 (XX%) = Percent difference from default value. 

 Bold value = Value is statistically different from default value (95th percentile confidence level). 

AVERAGE DELAY

0

50

100

150

200

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Delay 

(s/veh)

100 125 150 175 200
 39

Figure 9. Average Density Graph for Sample Sensitivity Test. 

The summary graphs shown above use the entering volumes as the X-axis, which could be 

different than the actual volumes in the at-capacity scenarios. Thus, the graphs do not match 

traditional speed-volume-density graphs typically found in the HCM or other traffic flow theory 

applications. Demand volume was used in this study because the objective was to understand the 

sensitivity of the parameters under different uniform scenarios, so it was important to keep the 

X-axis constant to see how the MOEs varied under each parameter value. Using the actual 

volume in the X-axis would make it more difficult to directly compare the results of each 

parameter value, especially when the parameter values experience a slightly different actual 

volume for the at-capacity scenarios. 

FRESIM Sensitivity Analysis Results 

The sensitivity test results for each parameter-network scenario were examined and a number of 

general trends from the sensitivity study were observed, including: 

• Most of the parameters showed no sensitivity at the lower congestion levels (entering 

volumes of 1000 and 1500 veh/hr/lane). In only a few instances did the most extreme 

sensitivity value produce a statistically significant difference (at a 95 percent 

confidence interval) from the default value. 

• An entering volume of 2000 veh/hr/lane (approximate V/C ratio of 0.83) experienced 

more sensitivity within the parameters than that shown with 2400 veh/hr/lane 

(approximate V/C ratio of 1.0). This trend was likely caused because the at-capacity 

condition allowed less variability in driver behavior due to more closely spaced 

vehicles and less maneuverability. 

• Average delay was the most sensitive MOE. Average speed and average density 

were equally sensitive, and less sensitive than average delay, while throughput and 

vehicle-miles of travel were the least sensitive. 

• Overall, the parameters became more sensitive as the network type became more 

complex. Thus, the system network generally experienced more sensitivity than the 

AVERAGE DENSITY

0

5

10

15

20

25

30

35

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Densit

y

(veh/km/ln)

100 125 150 175 200
 40

basic three-lane network, which in turn experienced more sensitivity than the basic 

one-lane network. 

As stated earlier, the majority of the sensitivity tests were designed as one-sided tests, meaning 

the parameter values were varied on one side of the default value to represent more cautious 

driver behavior, as would be expected during adverse weather. Based on this one-sided 

methodology, it was expected that the parameters would experience a general degradation in 

MOEs (i.e., average speed decreasing and average delay increasing) when changing the 

parameter values to represent more conservative driver behavior. However, this was not always 

the case. In fact, the parameters were divided into three “sensitivity groups” based on their 

general effect on the MOEs, as shown in table 30. 

Table 30. CORSIM Parameter Sensitivity Groups. 

Sensitivity Group Description 

Expected Parameter values that consistently produced degradation in MOEs. 

Inconsistent Parameter values that showed no consistent trend between more conservative 

driver behavior and MOEs. 

No effect Parameter values that had virtually no effect on the MOEs in any of the networkcongestion level scenarios. 

Figures 10 through 12 show the sensitivity of average speed on the freeway system network for 

three parameters representing each sensitivity group. As shown in these figures, the Pitt car 

following constant follows a consistent and expected trend, the maximum emergency 

deceleration does not follow a consistent trend, and the jerk value shows no sensitivity at all. 

Figure 10. Sensitivity of Pitt Car Following Constant on Freeway 

System Network—Example of “Expected” Sensitivity Group. 

PITT CAR FOLLOWING CONSTANT (Default = 10)

0

20

40

60

80

100

120

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Speed (km/h)

10 12 15 17 20
 41

Figure 11. Sensitivity of Maximum Emergency Deceleration on Freeway 

System Network—Example of “Inconsistent” Sensitivity Group. 

Figure 12. Sensitivity of Jerk Value on Freeway System 

Network—Example of “No Effect” Sensitivity Group. 

Tables 31 through 33 summarize the general sensitivity of each freeway parameter tested, based 

on the sensitivity group and general level of sensitivity. Low, medium, or high sensitivity levels 

are based on an evaluation of the overall sensitivity of the parameter values in each networkcongestion level scenario. These are based on relative differences between the parameters and 

not an absolute sensitivity level. 

Sensitivity of Car Following Parameters 

Table 31 summarizes the sensitivity of each FRESIM car following parameter. 

JERK VALUE (Default = 2.13)

0

20

40

60

80

100

120

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Speed (km/h)

2.13 1.83 1.52 1.22 0.91

MAXIMUM EMERGENCY DECELERATION 

(Default = 4.57)

0

20

40

60

80

100

120

1000 1500 2000 2500

Entering Volume (veh/h/ln)

Average Speed

(km/h)

4.57 3.96 3.35 2.74 2.13
 42

Table 31. General Sensitivity of FRESIM Car Following Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Car following 

sensitivity factor 

0.3,* 0.2, 

0.1, 0.0** 

No Effect Low This parameter overall has little 

sensitivity when changing the standard 

deviation of the sensitivity factor by 

driver types (but keeping the mean 

constant). 

Car following 

sensitivity 

multiplier 

100,* 125, 

150, 175, 

200 

Expected High The most sensitive car following 

parameter. Value of 125 yields 

statistically different MOEs than default 

at medium congestion levels (V/C of 

0.63 and higher). 

Pitt car following 

constant 

10,* 12, 

15, 17, 20 

Expected Medium CORSIM only allows values from 3 to 

10. A modification was made to allow 

larger values. A value of 12 yields 

statistically different MOEs than default 

at higher congestion levels (V/C of 0.83 

and higher). 

Lag acceleration 

/deceleration time 

1, 3,* 5, 7, 

9 

Expected Medium MOEs worsen as value increases. Value 

of 5 yields statistically different MOEs 

on system network than default value. 

Value of 1 much more sensitive than 

other values. 

Jerk value 2.14,*1.83, 

1.52, 1.22, 

0.91 

No Effect Low Values show no statistically significant 

sensitivity under any network-congestion 

level scenario. 

Notes: * - Default value. 

** - These values represent the standard deviation of the car following sensitivity factors for each of the 10 

driver types. The default values (range from 1.25-0.35 for driver type 1 to 10) equal a mean value of 0.80 

and standard deviation of 0.30. Each consecutive alternative has the same mean (0.80) but smaller standard 

deviation to represent more uniform driver behavior. 

The car following sensitivity multiplier parameter is clearly the most sensitive car following 

parameter and would be the most practical to manipulate when trying to alter car following 

behavior. Increasing the car following sensitivity multiplier value to 125 percent nearly always 

(more so at 1500 veh/hr/lane entering volume or higher) resulted in a statistically significant 

degradation in MOEs from the default. An interesting trend was observed with the car following 

sensitivity multiplier on the basic two-lane segment and three-lane segment networks: increasing 

the multiplier resulted in increasingly degraded MOEs for the lower entering volume scenarios, 

but increasingly improved MOEs for the at-capacity scenarios. One possible inference from this 

finding is that more conservative car following behavior results in lower quality of service in 

uncongested conditions, but improved quality of service in congested conditions. However, this 

finding was not duplicated with the other car following parameters. 

The Pitt car following constant and lag acceleration/deceleration time parameters, while not as 

sensitive as the car following sensitivity multiplier, showed consistent and expected results. They 

also should be considered when attempting to calibrate car following behavior. A Pitt car 

following constant value of 3.66 m yielded statistically degraded MOEs when compared to the 
 43

default, but generally only at the higher congestion scenarios (entering volume of 2000 

veh/h/lane or higher). (Note: The current version of CORSIM only allows values of the Pitt 

constant to vary from 3 to 10. A modification was made to allow larger values for this study.) 

The lag acceleration/deceleration time values of 5.0 s or higher showed statistical differences at 

an entering volume of 2000 veh/h/lane on the system network, but the values were statistically 

different only at the highest congestion level (2400 veh/h/lane) on the basic segment networks. 

The jerk value and car following sensitivity factor showed no sensitivity under any of the 

scenarios. However, this does not imply that these parameters have no sensitivity whatsoever. In 

fact, further testing with different networks and/or at a more detailed analysis level would 

provide a more complete depiction of the parameter’s sensitivity and might reveal a sensitivity 

that was not evident in these scenarios. 

Sensitivity of Lane Changing Parameters 

Table 32 summarizes the sensitivity of each FRESIM lane changing parameter. The lane 

changing parameters were generally not as stable as the car following parameters, as many 

produced no clear trends in the MOEs. Some parameters produced a clear trend in one network, 

but then the opposite trend in another network (e.g., anticipatory lane change speed). On the 

other hand, other parameters consistently showed no clear trend in every network (e.g., 

maximum emergency deceleration). It was interesting that the maximum emergency deceleration 

and the leader’s maximum deceleration as perceived by follower parameters had identical 

impacts on the MOEs in every network and volume level, prompting the question of why there 

are two different parameters in the model that yield identical results. 

The time to complete lane change parameter produced consistent and expected results at a 

medium level of sensitivity. Based on this finding, this parameter should be considered first 

when attempting to calibrate lane changing behavior. The advantage threshold for discretionary 

lane change, discretionary lane change multiplier, gap acceptance parameter, and percent 

cooperative drivers parameters showed no sensitivity under any of the scenarios. However, as 

stated previously, it should not be inferred that these parameters have no sensitivity whatsoever, 

but rather a more detailed analysis should be completed under different networks and/or a more 

detailed analysis level to get a more complete depiction of the true parameter sensitivity. 
 44

Table 32. General Sensitivity of FRESIM Lane Changing Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Time to complete 

lane change 

2.0,* 2.5, 

3.0, 3.5, 

4.0 

Expected Medium Value of 3.0 yields statistically 

different MOEs for most networks 

at higher congestion levels. 

Advantage 

threshold for 

discretionary lane 

change 

0.4,* 0.5, 

0.6, 0.7, 

0.8 

No Effect Low This parameter overall has little 

sensitivity, with the exception of 

average delay at the highest 

congestion level. 

Discretionary lane 

change multiplier 

5,* 4, 3, 2, 

1 

No Effect Low This parameter overall has little 

sensitivity. 

Gap acceptance 

parameter 

3,* 4, 5, 6 No Effect Low This parameter overall has little 

sensitivity. 

Percent cooperative 

drivers 

20,* 30, 

40, 50, 100 

No Effect Low This parameter has very little 

sensitivity, even at the maximum 

value (100). 

Maximum nonemergency 

deceleration 

2.44,* 

2.13, 1.83, 

1.52, 1.22 

Inconsistent Medium MOEs improve as value decreases. 

Value of 1.83 yields statistically 

different MOEs on most scenarios. 

Maximum 

emergency 

deceleration 

4.57,* 

3.96, 3.35, 

2.74, 2.13 

Inconsistent Medium MOEs improve as value decreases, 

except for a value of 2.13, which 

often yields worse MOEs. 

Leader’s maximum 

deceleration as 

perceived by 

follower 

4.57,* 

3.96, 3.35, 

2.74, 2.13 

Inconsistent Medium Identical results as maximum 

emergency deceleration parameter. 

Anticipatory lane 

change distance 

762, 610, 

457,* 305, 

152 

Inconsistent Medium No clear trend between distance and 

MOEs (different trend in each 

network). 

Anticipatory lane 

change speed 

69,* 56, 

48, 40, 32 

Inconsistent Medium No clear trend between this 

parameter and MOEs. System and 

merge networks show improvement 

in MOEs as speed decreases, but 

weave network shows degradation 

in MOEs. 

Notes: * - Default value. 
 45

Sensitivity of Free-Flow Speed Parameters 

Table 33 summarizes the sensitivity of each FRESIM free-flow speed parameter. 

Table 33. General Sensitivity of FRESIM Free-Flow Speed Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Mean free-flow 

speed 

113,* 97, 

81, 64, 48 

Expected High The most sensitive of all parameters 

tested. A value of 60 yielded 

statistically different MOEs than the 

default value for all networks. 

Free-flow speed 

multiplier ** 

0.78,* 

0.54, 0.27, 

0.00** 

Inconsistent Medium No clear trend between standard 

deviation and MOEs (different trend 

in each network). Basic networks 

have high sensitivity at higher 

congestion levels but other networks 

show little sensitivity. 

Notes: * - Default value. 

** - These values represent the standard deviation of the free-flow speed multiplier for each of the 10 driver types. The 

default values (range from 0.88 to 1.22 for driver type 1 to 10) equal a mean value of 1.0 and standard deviation of 

0.78. Each consecutive alternative has the same mean (1.0) but smaller standard deviation to represent more uniform 

driver behavior.

The mean free-flow speed parameter was the most sensitive of all parameters studied. A freeflow speed of 97 km/h yielded statistically different results under all scenarios compared to the 

default value (113 km/h). This sensitivity study confirms past research showing that the freeflow speed parameter is a crucial parameter to alter when modeling weather events on freeway 

networks. However, the free-flow speed multiplier parameter, which was tested by changing the 

distribution of speed by driver type while maintaining the same mean speed, showed no clear 

trend in its impact on the MOEs. Further, changing the multipliers so that each driver type has 

the same free-flow speed (or zero standard deviation) yielded average speeds equal to the freeflow speed on the basic segment networks, which is an unrealistic result at the higher volume 

levels because all other test scenarios showed a gradual decrease in average speed with an 

increase in entering volume. 

NETSIM Sensitivity Analysis Results 

The sensitivity results for each NETSIM parameter-network scenario, like the example results 

shown in tables 25 through 29 and figures 5 through 9, were examined and a number of trends 

from the sensitivity study were observed, including: 

• The number of lane changes was the most sensitive MOE relative to the other MOEs. 

Average delay and average speed both showed moderate sensitivity, while throughput 

and vehicle-miles of travel displayed the least sensitivity relative to the other MOEs. 

• For the basic segment networks, the parameters became increasingly sensitive as the 

V/C ratio increased. However, like the freeway parameters, the arterial parameters 

were generally slightly more sensitive at the just-below capacity (V/C ratio around 

0.8) than the at-capacity conditions. This trend is thought to occur because the at-
 46

capacity condition allowed less variability in driver behavior due to more closely 

spaced vehicles and less maneuverability. 

• For the intersection networks, the MOEs degraded dramatically when the V/C ratio 

approached 1.0. 

Tables 34 through 39 summarize the general sensitivity of each network parameter tested based 

on the sensitivity group (refer to table 30) and general level of sensitivity. 

Sensitivity of Car Following Parameters 

Table 34 displays the general sensitivity of the NETSIM car following parameter. 

Table 34. General Sensitivity of NETSIM Car Following Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Time to react to 

sudden deceleration 

of lead vehicle 

0.5, 1.0,* 

1.5, 2.0, 

2.5 

Expected High Values of 1.5 and higher yield 

statistically degraded MOEs with 

basic and system networks under all 

congestion levels. Intersection 

networks show slightly less 

sensitivity. 

Notes: * - Default value. 

As described previously, there is not a detailed car following model in NETSIM, primarily 

because the movement of vehicles on surface streets are controlled more by lane changing 

behavior and reacting to traffic control devices than by basic car following behavior. For the 

time to react to sudden deceleration of lead vehicle parameter, significant degradation in the 

MOEs were observed as the parameter value increased for the basic segment and system 

networks. However, for the single intersection networks, especially the urban intersection, the 

parameter changes had little effect on the MOEs. The small effect on the intersection networks 

is logical, given that vehicle movement likely was controlled mainly by reaction to the traffic 

signal and queues upstream of the signal. 

It should be noted that the time to react to sudden deceleration of lead vehicle parameter also 

affects lane changing behavior, as seen in the next section. 

Sensitivity of Lane Changing Parameters 

Table 35 displays the general sensitivity of the NETSIM lane changing parameters. 
 47

Table 35. General Sensitivity of NETSIM Lane Changing Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Driver type factor 15, 25,* 35, 

45, 50 

No Effect Low No sensitivity, not even at a value of 50, to 

any network-congestion level combination. 

Urgency threshold 2,* 2.5, 3, 4, 

5 

No Effect Low Very low sensitivity. In a few cases, a value 

of 5 yielded differences of 2-3% from 

default value, but never at statistically 

significant level. 

Minimum 

deceleration for a 

lane change 

1.52,* 1.22, 

0.91, 0.61, 

0.30 

Expected Medium Statistically significant decreases in number 

of lane changes (up to 20%), but none for 

other MOEs. 

Difference in 

minimum/maximu

m deceleration for 

mandatory lane 

changes 

3.05,* 2.74, 

2.13, 1.83, 

1.52 

No Effect Low Very low sensitivity. MOEs did not change 

at statistically significant level in any 

network-congestion level combination. 

Difference in 

minimum 

/maximum 

deceleration for 

discretionary lane 

changes 

1.52,* 1.22, 

0.91, 0.61, 

0.30 

No Effect Low Statistical increase in the number of lane 

changes in some cases under noncongestion level (up to 10%). Very low 

sensitivity on other MOEs. 

Safety factor 0.80,* 0.75, 

0.70, 0.65, 

0.60 

No Effect Low Moderate reduction in number of lane 

changes when safety factor is 0.6 and 

volumes are high (4–6% reduction). A few 

cases yielded differences of 1–3%, but no 

statistical significance. 

Headway at which 

all vehicles attempt 

lane change 

2.0,* 1.8, 1.5, 

1.2, 1.0 

No Effect Low Very low sensitivity. The number of lane 

changes decreased in all networks (0–4%). 

Very low sensitivity on other MOEs. 

Headway at which 

no vehicles attempt 

lane change 

5.0,* 4.5, 4.0, 

3.5, 3.0 

No Effect Low Very low sensitivity. The number of lane 

changes decreased in all networks 

(maximum 2%). Very low sensitivity on 

other MOEs. 

Time to react to 

sudden deceleration 

of lead vehicle 

0.5, 1.0,* 1.5, 

2.0, 2.5 

Expected High Values of 1.5 and higher yield statistically 

degraded MOEs with basic and system 

networks under all congestion levels. 

Intersection networks show slightly less 

sensitivity. 

Duration of a lane 

change 

3,* 5, 6, 7, 8 No Effect Low Low sensitivity. The number of lane 

changes decreased slightly (5% maximum 

at the significant level). Very low 

sensitivity on other MOEs. 

Percent drivers who 

cooperate with lane 

changer 

10, 25, 50,* 

75, 100 

No Effect Low At value of 10, the number of lane changes 

drops by up to 12%. A maximum 10% 

increase in average speed on the suburban 

intersection with a value of 100 (not a 

statistically significant difference). 

Distance over 

which drivers 

perform lane 

change 

91.4,* 152, 

214, 274, 335 

No Effect Low Very low sensitivity. 
 48

Table 35. General Sensitivity of NETSIM Lane Changing Parameters (continued). 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Distribution of 

distance to attempt a 

lane change 

17.1,* 

11.4, 6.1, 

0.0** 

No Effect Low No sensitivity was shown, not even a 1% 

difference at a standard deviation of 0.0. 

Deceleration of lead 

vehicle 

3.66,* 

3.35, 3.05 

Expected Medium The number of lane changes decreased 

dramatically, as much as 100%. The 

basic networks experienced statistical 

decreases in average speed (2–4%) and 

delay (up to 20%). 

Deceleration of 

following vehicle 

3.66,* 

3.35, 3.05 

Expected Medium The number of lane changes decreased 

dramatically, as much as 90%. The basic 

networks experienced statistical 

decreases in average speed (1–6%) and 

delay (up to 44%). 

Notes: * - Default value. 

** - These values represent the standard deviation of the distance to attempt a lane change for each of the 10 driver 

types. The default values (range from 125 to 75 percent for driver type 1 to 10) equal a mean value of 100 and standard 

deviation of 17.1. Each consecutive alternative has the same mean (100) but smaller standard deviation to represent 

more uniform driver behavior.

As shown in the table, there are 15 lane changing parameters in NETSIM. Of these parameters, 

11 had very little sensitivity overall on the MOEs. Typically these parameters had some small, 

quantifiable change on the number of lane changes, but had very little impact (and not 

statistically significant) on the other MOEs. This study does not prove that these parameters 

have no sensitivity whatsoever. Testing of different networks or use of different MOEs could 

reveal additional sensitivity not discovered in this study. For example, examination of more 

disaggregate MOEs, such as vehicle trajectory data, could reveal sensitivity of the parameters at 

a level that is not possible with aggregate MOEs such as average speed over an entire link. 

The time to react to sudden deceleration of lead vehicle parameter impacted the number of lane 

changes dramatically (up to 90 percent increase on the basic segment networks and 30 percent on 

the other networks). The other MOEs changed at a more modest, but still significant, level. 

The minimum deceleration for a lane change had a moderate impact on the number of lane 

changes (up to 20 percent change), but no statistically significant changes in the other MOEs. 

The deceleration of lead vehicle and deceleration of following vehicle parameters showed a 

medium level of sensitivity (relative to the other parameters), with a significant decrease in the 

number of lane changes and more moderate, but still statistically significant, change in average 

speed and average delay. It is interesting that CORSIM will not allow users to enter a value for 

these parameters less than 3.05 (allowable range of 3.05 to 4.57). Future consideration should be 

given to widening this allowable range, given that it is one of the few lane changing parameters 

that has a quantifiable impact on MOEs. 
 49

Sensitivity of Free-Flow Speed Parameters 

Table 36 displays the general sensitivity of the NETSIM free-flow speed parameters. 

Table 36. General Sensitivity of NETSIM Free-Flow Speed Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Mean free-flow 

speed 

72,* 64, 

56, 48, 40 

Expected High Average speed and delay changed 

significantly at all congestion levels 

(average speed reduced 12% at 64 

km/h and 46% at 40 km/h), but 

throughput was not as sensitive (no 

statistical differences). 

Free-flow speed 

multiplier 

16,* 11.4, 

5.4, 0.0** 

Inconsistent Medium Reducing the standard deviation 

improves the MOEs on the basic 

segment networks (up to 25% 

increase in average speed), but no 

statistical differences on other 

networks. 

Notes: * - Default value. 

** - These values represent the standard deviation of the free-flow speed multiplier for each of the 10 driver types. The 

default values (range from 75 to 127 percent for driver type 1 to 10) equal a mean value of 100 and standard deviation 

of 16.0. Each consecutive alternative has the same mean (100) but smaller standard deviation to represent more 

uniform driver behavior.

The mean free-flow speed parameter was the most sensitive of all NETSIM parameters studied. 

For example, lowering the mean free-flow speed from 72 to 40 km/h resulted in a 450 percent 

increase in total delay and 45 percent decrease in average speed on the basic two-lane segment. 

This finding that the MOEs are very sensitive to changes in free-flow speed is similar to that 

found in the FRESIM sensitivity analysis. 

The free-flow speed multiplier represents a distribution of free-flow speeds based on driver type. 

It was found that more uniform free-flow speeds (lower standard deviation) resulted in fewer 

lane changes. In addition, the other MOEs (except for throughput) improved on the basic 

segment networks (up to 25 percent increase in average speed), but no statistically significant 

changes were found on the other networks. The free-flow speed multiplier did not affect the 

throughput on the basic one-, two-, and three-lane segment networks. The improvement on the 

basic segment networks were similar to that found in the FRESIM sensitivity analysis. 

Sensitivity of Discharge Headway Parameters 

Table 37 displays the general sensitivity of the NETSIM discharge headway parameters. 

The mean discharge headway is a very sensitive NETSIM parameter, as shown in the table. 

Generally, as the discharge headway increased, the MOEs became more degraded. Stopped 

delay was the most affected MOE (up to 1800 percent increases), while the number of lane 

changes was the least affected MOE (maximum change of 20 percent). 

Changing the distribution of the discharge headway multiplier (while maintaining the same mean 

value) did not statistically impact any of the networks during any congestion level. 
 50

Table 37. General Sensitivity of NETSIM Discharge Headway Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Mean discharge 

headway 

1.9,* 2.2, 

2.5, 2.7, 

3.0 

Expected High Stop delay increased 1300%, and 

throughput decreased 35% at 3.0 on 

the suburban intersection. 

Discharge headway 

multiplier 

33.7,* 

20.2, 10.1, 

0.0** 

No Effect Low No statistical differences were 

observed in any network-congestion 

level combination. 

Notes: * - Default value. 

** - These values represent the standard deviation of the discharge headway multiplier for each of the 10 driver types. 

The default values (range from 170 to 50 percent for driver type 1 to 10) equal a mean value of 100 and standard 

deviation of 33.7. Each consecutive alternative has the same mean (100) but smaller standard deviation to represent 

more uniform driver behavior. 

Sensitivity of Startup Delay Parameters 

Table 38 displays the general sensitivity of the NETSIM startup delay parameters. 

Table 38. General Sensitivity of NETSIM Startup Delay Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Mean startup delay 1.3,* 1.5, 

1.7, 1.9, 

2.1 

Expected High Stop delay increased 47%, and 

throughput decreased only slightly 

(2%) at a value of 2.1 on the 

suburban intersection. 

Startup delay 

multiplier 

55,* 37, 

16.8, 0.0** 

No Effect Low No statistical differences were 

observed in any network-congestion 

level combination. 

Notes: * - Default value. 

** - These values represent the standard deviation of the startup delay multiplier for each of the 10 driver types. The 

default values (range from 218 to 23 percent for driver type 1 to 10) equal a mean value of 100 and standard deviation 

of 55. Each consecutive alternative has the same mean (100) but smaller standard deviation to represent more uniform 

driver behavior.

The startup delay parameters had a similar effect on the MOEs as the discharge headway 

parameters, but with slightly less severity because this parameter only affects the first few 

vehicles in a queue. For example, throughput and the number of lane changes were impacted 

minimally (maximum of 2 and 6 percent, respectively). Generally, as the startup delay increased, 

the MOEs subsequently degraded. Stop delay was the most affected MOE (increases of up to 47 

percent), while average speed dropped only up to 10 percent. The changes in average delays 

were impacted as well (up to 37 percent). The changes in average delays were greatest in the 

urban network, while the system network was the least impacted network overall. 

Changing the distribution of the startup delay multiplier (while maintaining the same mean 

value) did not statistically impact any of the networks during any congestion level. 
 51

Sensitivity of Turning Speed Parameters 

Table 39 displays the general sensitivity of the NETSIM turning speed parameters. 

Table 39. General Sensitivity of NETSIM Turning Speed Parameters. 

Parameter Parameter 

Values 

Sensitivity 

Group 

Sensitivity 

Level Comments 

Maximum 

allowable left-turn 

speed 

35,* 29, 

23, 16, 10 

Expected Medium The largest difference was a 12% 

increase (which was statistically 

significant) in stopped delay at 9.7 

km/h on the urban intersection. 

Maximum 

allowable right-turn 

speed 

21,* 18, 

15, 11, 8 

Expected Medium The largest difference was a 15% 

increase (which was statistically 

significant) in stopped delay at 8 

km/h on the urban intersection. 

Notes: * - Default value. 

Decreasing the turning speeds produced an expected degradation in the MOEs. The MOEs were 

most affected at the higher congestion levels, as vehicles were more closely spaced and thus 

delayed more by vehicles turning at a slower rate. Stopped delay was the most affected MOE, as 

stopped delays increased approximately 10 to 15 percent on the urban intersection during the 

highest congestion levels. The left- and right-turning speeds were approximately equally 

sensitive on the test networks. 

Summary of Sensitivity Analysis 

The purpose of the sensitivity study was to identify the most sensitive weather-related 

parameters in CORSIM. The study focused on car following, lane changing, and free-flow speed 

parameters on freeways (FRESIM) and car following, lane changing, free-flow speed, discharge 

headway, startup lost time, and turning speed parameters on arterial streets (NETSIM). Each test 

parameter was modeled on various geometric networks and congestion (volume) levels using the 

default value and then changing the value to represent incrementally more conservative driver 

behavior, as would occur under adverse weather. The MOEs produced by the default value were 

then compared to the MOEs produced with the changed parameter values to determine the level 

of sensitivity the parameter has on the MOEs. 

One interesting result of the study was that a number of parameters had little or no impact on the 

MOEs. Table 40 summarizes the tested parameters that had no effect on the MOEs. 

As table 40 shows, lane changing parameters were the majority of the parameters with no 

sensitivity. In fact, 11 of the 15 lane changing parameters in NETSIM showed no sensitivity. 

These nonsensitive parameters should be the focus of further research, because it is not clear why 

many of them did not have a greater impact on the MOEs. However, this study does not prove 

that these parameters have no sensitivity whatsoever. Testing different networks or using 

different MOEs could reveal additional sensitivity not discovered in this study. For example, 

examining more disaggregate MOEs, such as vehicle trajectory data, could reveal sensitivity of 

the parameters at a level which is not possible with aggregate MOEs, such as average speed over 

an entire link. The fact that most of these lane changing parameters had at least some small 
 52

impact on the number of lane changes shows that the parameters were affecting traffic operations 

to some degree. 

Table 40. Traffic Parameters with No Effect on MOEs. 

Parameter Category Parameter 

FRESIM 

Car following - Car following sensitivity factor 

- Jerk value 

Lane changing - Advantage threshold for discretionary lane change 

- Discretionary lane change multiplier 

- Gap acceptance parameter 

- Percent cooperative drivers 

NETSIM 

Car following - None 

Lane changing - Driver type factor 

- Urgency threshold 

- Headway at which all vehicles attempt a lane change 

- Headway at which no vehicle attempt a lane change 

- Difference in min/max deceleration for mandatory lane changes 

- Difference in min/max deceleration for discretionary lane changes 

- Safety factor 

- Duration of a lane change 

- Percent drivers who cooperate with a lane changer 

- Distance over which drivers perform a lane change 

- Distribution of distance to attempt a lane change 

Discharge headway - Discharge headway multiplier 

Startup lost time - Startup lost time multiplier 

Turning speed - None 

In addition to the nonsensitive parameters, a number of FRESIM lane changing parameters had 

an “inconsistent” impact on the MOEs, named so because they had no consistent impact on the 

MOEs. These inconsistent parameters included the maximum non-emergency deceleration, 

maximum emergency deceleration, leader’s maximum deceleration as perceived by follower, 

anticipatory lane change distance, and anticipatory lane change speed. These parameters should 

also be the focus of more detailed research to further determine how they function within the 

various model algorithms and exactly what impact they have on traffic operations. 

Table 41 summarizes those parameters that had both an expected effect on the MOEs and were 

categorized as either having a medium or high effect on the MOEs (relative to the other 

parameters). This table is important because it identifies the key weather-related driver behavior 

parameters that should be altered when trying to model weather events in CORSIM. As stated 

earlier, this study does not recommend specific values to use for these parameters during various 

weather events, but it does identify these parameters as the most sensitive, and therefore should 

be the focus when calibrating a model for a specific weather event. A traffic analyst should first 

focus on the parameters with a high sensitivity level, and if further calibration is needed, could 

use those with a medium sensitivity level. 
 53

Table 41. Traffic Parameters with Expected and Medium-to-High Effect on MOEs. 

Parameter Category Parameter 

FRESIM 

Car following - Car following sensitivity multiplier (high)

- Pitt car following constant (medium) 

- Lag acceleration/deceleration time (medium) 

Lane changing - Time to complete lane change (medium) 

Free-flow speed - Mean free-flow speed (high)

NETSIM 

Car following - Time to react to sudden deceleration of lead vehicle (high)

Lane changing - Time to react to sudden deceleration of lead vehicle (high)

- Minimum deceleration for a lane change (medium) 

- Deceleration of lead vehicle (medium) 

- Deceleration of following vehicle (medium) 

Free-flow speed - Mean free-flow speed (high) 

Discharge headway - Mean discharge headway (high) 

Startup lost time - Mean startup delay (high) 

Turning speed - Maximum allowable left-turn speed (medium) 

- Maximum allowable right-turn speed (medium) 

Note: Values in parenthesis (high, medium, or low) represent the sensitivity level of the parameter, with a parameter in italics 

representing a high sensitivity level. 

Due to the large number of networks and variables tested in the sensitivity study, a number of 

additional findings and recommendations were made that were somewhat unrelated to the task of 

determining the most sensitive parameters, but nonetheless were thought to be important for 

CORSIM users in general. These findings can be summarized as follows: 

• The minimum separation for generation of vehicles parameter is useful in calibrating 

the capacity of basic freeway segments, but users should realize that changing the 

driver behavior parameters (specifically the car following sensitivity multiplier) also 

can limit the freeway capacity in some cases. 

• The minimum separation for generation of vehicles parameter is available only on 

freeways (FRESIM) and not on surface streets (NETSIM). As a result, arterial 

volumes up to 2700 veh/h/lane can be modeled in NETSIM, which is not realistic for 

arterials. However, the capacity will likely be limited by traffic signals on arterials, 

but nevertheless, traffic analysts should be careful to model realistic traffic volumes 

on arterial streets. 

• In FRESIM, the maximum emergency deceleration and leader’s maximum 

deceleration as perceived by follower parameters are identical parameters, as they 

produced exactly equal results in the sensitivity analysis. 

• Changing the distribution of speeds for the free-flow speed multiplier is not 

recommended, because this produced inconsistent (and unrealistic for a distribution 

with very low standard deviation) impacts on the MOEs. In addition, changing the 

distribution of discharge headways and startup delays in NETSIM also is not 

recommended, because altering them had no effect on the MOEs. 
 54

• Future consideration should be given to widening the allowable range for the 

deceleration of lead vehicle and deceleration of following vehicle parameters, given 

that they are two of the four NETSIM lane changing parameters that have a 

quantifiable impact on MOEs. Currently, the allowable range is 3.05 to 4.57m/s/s, 

with a default value of 3.66 m/s/s. The allowable range for the Pitt car following 

constant should also be widened (currently 0.91 to 3.05 m with a default value of 3.05 

m) to at least 4.57 m to allow users to model more conservative car following 

behavior. 

• The HCM recommends a default mean startup delay of 2.0 s, which is defined as the 

extra time consumed by the first few vehicles in a signalized intersection queue. In 

the absence of localized field data, it is recommended that CORSIM users use this 

value of 2.0 s, which means the default mean startup delay value should be changed 

to 1.3 s (currently 2.5 s), because 0.7 s of startup delay is already hard-coded into the 

model for the second and third vehicles in the queue. 
 55

6. Guidelines for Modeling Weather Events in CORSIM 

This section provides practical guidelines for modeling the effects of adverse weather on a 

roadway network using CORSIM. The guidelines presented here are based on Traffic Analysis 

Toolbox Volume III—Guidelines for Applying Traffic Microsimulation Modeling Software, an 

FHWA guidance document on the development and application of microsimulation models.(1) 

Figure 13 shows the seven-step process recommended in the guidelines for developing a 

microsimulation model and how to apply the model to analyze various alternatives. 

Even though the process shown in figure 13 was not designed specifically for modeling weather 

events, a traffic analyst intent on modeling weather effects should not forget the importance of 

scoping the project, collecting field data, developing the base model, checking the model for 

errors, calibrating the model to local conditions, analyzing alternatives, and producing a final 

report. 

Figure 13 lays the foundation for developing and applying an accurate and valid CORSIM 

model. The same basic steps shown in this figure should be followed even when including the 

impacts of adverse weather. However, there are a few steps in the process that a traffic analyst 

should approach slightly differently when modeling adverse weather in CORSIM. These 

differences are highlighted in the remainder of this section. 
 56

Figure 13. Microsimulation Model Development and Application Process.(1)

Working Model

Before Calibration

Working Model

Before Calibration

4

7

Error Checking

1

Scope Project

-Define project purpose

-Identify influence areas

-Select approach

-Select model

-Estimate staff time 

1 -

-

-

-

-

2

Data Collection

-Traffic volumes

-Base maps/inventory

-Field observations

-

-

-

3

Base Model 

Development

-Input data

-Develop quality assurance 

-

-

-Review Inputs

-Review Animation 

Work Prior to

Actual Modeling

Initial Modeling

Yes

Calibrated Model

5

-Volumes and speeds match?

-Congestion in right places?

Compare Model

MOEs to Field Data

-

-

Calibration

-Key results

-Technical documentation 

6 -Forecast demand

-Base case

-Project alternatives

Alternatives Analysis

-Modify global parameters

-Modify link parameters

-Modify route choice parameters

Adjust Model Parameters

-

-

-

Final Report

Acceptable

Match?

No

Developed by the FHWA Traffic Analysis Tools Team and later adapted from Advanced Corsim Training Manual, Short, Elliott, 

Hendrickson, Inc., Minnesota Department of Transportation.

Model

Application
 57

Step 1—Scope Project 

It is important to define the project scope in any application of a microsimulation model. 

However, when using the model to include the effects of adverse weather, a few additional 

considerations are necessary, including: 

• Does the selected microsimulation software package have an adequate number of 

weather-related parameters (see tables 2 through 6) that can be adjusted appropriately 

to model the weather impacts accurately? 

• What type of weather event(s) will be modeled (e.g., snow, rain, fog, sun glare, or 

some combination)? 

• What is the severity of the weather event(s) being modeled (e.g., 50 millimeters or 1 

meter of snow)? 

• What is the duration of the weather event(s) being modeled (e.g., will it last the entire 

simulation period, or just for a short period)? 

• What is the extent of the weather event(s) being modeled (e.g., will it cover the entire 

simulation model area, or just a portion)? 

These are important questions that should be answered and agreed on by all parties involved with 

developing and reviewing the model before beginning the actual model coding. The first 

question listed may be the most important of the entire project, because it determines whether the 

selected software package is able to include the effects of adverse weather successfully. The 

previous sections of this report have shown that CORSIM can be applied successfully to model 

weather events and generally does have adequate parameters to account for most weather events. 

Step 2—Data Collection 

Collecting actual roadway conditions in the field is important in any application of a 

microsimulation model. Typical data collected includes traffic volumes, roadway geometrics, 

signal timing data, transit and pedestrian data, and calibration data (or MOEs) such as capacities, 

travel times, intersection delays, and queue lengths. When modeling the effects of adverse 

weather, it is important to attempt to collect data during the weather events being modeled. For 

example, if the capacity and delay at an intersection is collected during heavy rain, those values 

obtained can be used to better calibrate a model taking into account heavy rain. 

In addition to calibration data (MOEs), traffic parameter data (inputs to the traffic model) also 

are important to collect in the field during the weather events being modeled. Key weatherimpacted traffic parameters identified both in this sensitivity study and the literature review 

include free-flow speed, car following sensitivity multiplier, discharge (saturation) headway, 

startup delay, and traffic demand. 

Collecting these data in the field and using them as inputs to the microsimulation model will 

provide a more accurate starting point when going through the calibration process (step 5 in 

figure 13). 

It is recognized that resources and budgets often make it difficult to perform an exhaustive data 

collection effort for ideal weather conditions, let alone for adverse weather. However, it also 

should be recognized that the quality of data collection is proportional to the overall quality of 

the microsimulation model. Therefore, if including the impacts of adverse weather is important 
 58

to the project, then serious consideration should be given to some form of data collection during 

the weather event(s) being modeled. 

In light of this, if field data collection during the weather event(s) being modeled is not possible 

or practical, then traffic analysts could use the findings of past research (see section 3) as a 

starting point when altering traffic parameters to more accurately reflect weather events. 

Step 3—Base Model Development 

This step includes the initial setup and coding of the microsimulation model and inputting the 

data collected in the field into the model. When including a weather event in the model, the 

following additional steps are necessary during the base model development: 

1. Identify which traffic parameters are impacted by the weather event. 

2. Determine the appropriate values for these weather-impacted parameters either by (in 

order of preference): 

• Field data collection. 

• Findings of past research. 

• Engineering judgment. 

For the first step, table 42 can be used to determine which CORSIM parameters are affected by 

weather events. This table was developed by matching the generic traffic parameters identified 

in tables 3 through 7 in section 4 of this report to specific CORSIM parameters. To use the table, 

a traffic analyst must first determine how the weather event being modeled impacts the roadway 

environment. Figure 2 of this report (section 2) displays which weather events impact the 

roadway environment. 

As an example, suppose a traffic analyst wants to use CORSIM to model the effects of a heavy 

snowstorm (say four inches in an hour) on a local roadway network. From figure 2, it can be 

seen that a snow event (especially a heavy snowstorm) will likely cause a reduction in driver 

visibility and pavement friction, and the storm could block lanes or cover signs and pavement 

markings. The traffic analyst would make the final determination whether the modeled 

snowstorm should include blocked lanes and covered signs and pavement markings. Based on 

these roadway environment impacts, the traffic analyst would then use table 42 to determine 

specific parameters in CORSIM that may need to be altered due to the snowstorm. 
 59

Table 42. CORSIM Parameters Impacted by Weather Events. 

Generic Traffic 

Parameter 

Roadway 

Environment 

Impact 

CORSIM 

Parameter(s) Details 

ROAD GEOMETRY PARAMETERS 

Pavement 

condition 

Reduced pavement 

friction 

Pavement 

condition 

Available in FRESIM (freeways) only. 

Parameter creates an upper bound for the mean 

free-flow speed.*

Number of lanes Blocked lanes Number of lanes Can reduce the number of lanes based on the 

weather event. 

Lane width Blocked lanes Lane width Available in NETSIM only. Only changes the 

graphical display and not traffic operations. 

Lane taper length Blocked lanes None No parameter for length or type of taper, but can 

reduce the length of deceleration/ acceleration 

lanes (in FRESIM) themselves as surrogate. 

Shoulder width Blocked lanes None No parameter for shoulder width in FRESIM or 

NETSIM. 

TRAFFIC CONTROL AND MANAGEMENT PARAMETERS 

Reduced visibility Forward sight 

distance 

No parameter to reduce the visibility of a 

signal/meter itself. Forward sight distance 

parameter specifies sight distance from a stop 

line at a NETSIM intersection. 

Traffic signal/ 

ramp meter 

Failed traffic 

control devices 

Traffic signal/ 

ramp meter 

properties 

Can change the control to all-way or two-way 

stop to simulate flash or blackout conditions. For 

ramp meter, can turn off the meter for specific 

time periods. 

Roadway signs 

(regulatory, 

warning, traveler 

information) 

Reduced visibility Anticipatory lane 

change distance, 

off-ramp reaction 

point 

No parameter specifically for reducing the 

visibility of a sign itself. Can change the 

anticipatory lane change distance and off-ramp 

reaction point as surrogates to seeing exit signs 

on freeways. 

Surveillance 

detectors 

Failed 

communications 

Detector 

properties 

Can delete detectors to simulate failed detector 

communications. 

On-street parking Blocked lanes Curb parking Can disallow on-street parking for specific time 

periods. 

VEHICLE PERFORMANCE PARAMETERS 

Acceleration / 

deceleration 

capability 

Reduced 

friction/stability 

Acceleration 

tables 

Can change acceleration tables, including 

maximum acceleration, using record type 173 of 

the CORSIM input file. 

Turning radius Reduced 

friction/stability 

Minimum drawn 

radius of curvature 

Only changes the graphical display and not 

traffic operations. 

TRAFFIC DEMAND PARAMETERS 

Vehicle demand All** Entry volume and 

turning volume 

Entry volumes for each entering link can be 

adjusted as appropriate, and turning volumes can 

be adjusted depending on the weather event. 

Route choice All** Traffic assignment 

properties 

Available in NETSIM only. Cannot change 

impedances for individual links to simulate 

weather events. 
 60

Table 42. CORSIM Parameters Impacted by Weather Events (continued). 

Generic Traffic 

Parameter 

Roadway 

Environment 

Impact 

CORSIM 

Parameter(s) Details 

DRIVER BEHAVIOR PARAMETERS 

Car following All** See tables 11 and 

18 

See table 35 for key parameters. 

Lane changing All** See tables 12 and 

19 

See table 35 for key parameters. 

Free-flow speed All** Mean free-flow 

speed and 

multipliers 

Mean free-flow speed on all affected links 

should be changed according to the weather 

event. 

Discharge 

headway 

All** Mean discharge 

headway and 

multipliers 

Mean discharge headway (at signalized 

intersections) should be changed according to 

the weather event. 

Startup delay All** Mean startup 

delay and 

multipliers 

Mean startup delay (at signalized 

intersections) should be changed according to 

the weather event. 

Intersection gap 

acceptance 

All** Acceptable gap in 

oncoming traffic 

(AGOT), crossstreet traffic 

acceptable gap 

(CSTAG) 

Change AGOT for turns at a traffic signal 

(permitted left turns and right turns on red) 

and CSTAG for movements at stop signs. 

Turning speed All** Maximum 

allowable left- and 

right-turn speed 

Can change maximum left- and/or right-turn 

speeds in NETSIM. 

Response to 

yellow interval 

All** Amber interval 

response 

Defines the acceptable deceleration for a 

vehicle to stop at a traffic signal. 

Notes: * - Check CORSIM manual for more details.(12) 

 ** - All roadway environment changes could impact the parameter. 

After the weather-impacted CORSIM parameters are selected from table 42, then the proper 

value for them must be determined. As mentioned previously, determining the appropriate 

values ideally should be done through field data collection. Given that this is often not possible 

and/or practical for some parameters, the correct parameter values could be estimated through 

the findings of past research. See section 3 of this report for a review of relevant past research. 

It is important to use only past research that was collected on roadway facilities, congestion 

levels, and other field characteristics similar to those being modeled. Finally, in the absence of 

field data collection and past research, engineering judgment can be used to estimate the correct 

parameter values. For example, it is difficult to collect lane changing parameter data in the field, 

and there are no past studies regarding lane changing behavior in adverse weather. Thus, in this 

case, changing the lane changing parameters to represent slightly more conservative driver 

behavior (as would likely happen in adverse weather) would probably be a reasonable choice 

based on engineering judgment. 

While table 42 shows the range of traffic parameters impacted by weather events, it may not be 

possible or practical to change all of the impacted parameters for various reasons. With these 
 61

limitations in mind, a handful of key traffic parameters have been identified, based on past 

research and the sensitivity study summarized in this report, to be the most important weatherimpacted parameters in terms of their impact on MOEs. Even when resources and budgets are 

tight, these CORSIM parameters at a minimum should be altered to appropriate values when 

modeling weather events: 

• Mean free-flow speed (freeways and arterials). 

• Car following sensitivity multiplier (freeways). 

• Mean discharge headway (signalized intersections). 

• Mean startup delay (signalized intersections). 

• Traffic demand, in terms of reduced demand during more severe weather events 

(freeways and arterials). 

Step 5—Model Calibration 

Model calibration is an iterative process where the model parameters are altered until the model 

results (MOEs) adequately match the field-measured MOEs. Calibration is needed, because the 

default parameter values often do not result in model MOEs close to those measured in the field. 

This is especially true when including a weather event in the model, because all microsimulation 

software packages assume ideal weather conditions in the default values. 

Even after adjusting the weather-impacted parameters to appropriate values as described in the 

previous section, calibration is likely still needed to ensure the best model parameters are used. 

The Traffic Analysis Toolbox Volume III—Guidelines for Applying Traffic Microsimulation 

Modeling Software recommends a three-step calibration strategy:(1)

1. Capacity calibration—an initial calibration of the parameters related to capacity. 

2. Route choice calibration—a calibration of the traffic demand and route choice 

parameters to better match volumes measured in the field. 

3. System performance calibration—a final calibration of all parameters affecting the 

MOEs to better match the model-produced and field-measured system performance. 

For each step, calibrating the global parameters (parameters that affect the entire 

microsimulation model) should be done first, while fine-tuning link-specific parameters as 

necessary should follow. 

The key weather-related parameters as identified in the previous section are also the key 

parameters used to calibrate any CORSIM model, regardless if a weather event is being modeled. 

Thus, the process for calibrating a model that includes a weather event is not really different than 

calibrating a generic CORSIM model. Refer to the simulation guidelines for more detail on the 

calibration process.(1)

This section highlights the need to measure MOEs in the field, because there would be no basis 

to calibrate to if field MOEs were not measured. When including a weather event in the 

microsimulation model, the best way to calibrate such a model would be to collect field MOEs 

during the weather event being modeled. However, as stated previously, this can be difficult. 
 62

If field MOEs are not collected during the modeled weather event, then a secondary method for 

calibrating the weather-related model is possible. In this method, the first step is to calibrate the 

model during ideal weather conditions. This includes coding the microsimulation model for 

ideal weather and then calibrating the model to field MOEs collected during ideal weather. After 

developing a calibrated ideal weather model, then only the weather-related parameters would be 

adjusted to account for the adverse weather. The weather-related parameters would be adjusted 

based on the discussion in the previous section (i.e., adjustments based on field data, then past 

research, and finally, engineering judgment). While such an approach would not produce a 

model specifically calibrated to the weather event, it would at least produce a reasonably 

adequate adverse-weather model, because it was already calibrated to ideal weather, and only a 

few parameters were adjusted thereafter. 
 63

7. Conclusions 

The objectives of this study were to identify how weather events impact traffic operations, assess 

the sensitivity of weather-related parameters in the CORSIM traffic microsimulation model, and 

develop guidelines for using the CORSIM model to account for the impacts of adverse weather 

conditions on traffic operations. This section highlights the major findings and conclusions 

reached from the analysis documented in this report. Also, future research needs were identified 

as part of this project and are summarized below. 

Relationship Between Weather Events and Traffic Operations 

Conceptually, it is easy to understand that a major weather event, such as a snowstorm, will lead 

to lower average speeds and higher delays. However, it is important to understand what this 

relationship is, or in other words, what causes a weather event to degrade traffic operations. 

A weather event impacts traffic operations through a chain reaction: the event causes a change in 

the roadway environment (e.g., reduced visibility and pavement friction), which causes a 

reduction in traffic parameters (e.g., lower free-flow speeds and capacities), thereby degrading 

traffic flow (e.g., higher delays and lower average speeds). 

The qualitative impacts of weather events are seen easily through this relationship, but the 

quantitative impacts have been historically difficult to measure for a number of reasons. For 

example, there are many shades of the severity of a weather event, and the impacts are different 

regionally (i.e., a snowstorm in Florida will have more impact than the same storm in Minnesota) 

and by time of year (i.e., a snowstorm at the beginning of winter will likely have more impact 

than the same storm near the end of winter after drivers have acclimated to the adverse weather). 

Numerous past research studies have shown a quantitative link between various weather events 

and reduced free-flow speeds, saturation (discharge) headway, startup lost time, and traffic 

demand. This research shows a link between weather events and changes in traffic parameters; 

however, very little research has focused on the role of roadway environment impacts in causing 

the degradation in traffic parameters. This lack of information probably is due to the difficulty in 

understanding why motorists respond to a weather event (i.e., is a reduction in free-flow speed 

really due to a reduction in pavement friction or reduction in visibility?). 

Microsimulation Parameters Affected by Weather Events 

Past research has documented a number of traffic parameters that are impacted by weather 

events. However, many microsimulation parameters have not been measured empirically to 

behave differently during adverse weather. 

Tables 3 through 6 show the traffic simulation parameters that are likely impacted by weather 

events (through a change in the roadway environment). The selection of these parameters was 

based on the range of simulation parameters identified in table 2, the literature review, and 

engineering judgment based on the concept that driver behavior becomes more conservative 

during adverse weather conditions. Unfortunately, there is currently no empirical research 

supporting the latter concept. Therefore, the table only lists the range of potential, not proven, 

simulation parameters that may be used to model adverse weather conditions in a simulation 
 64

model. These simulation parameters may be used as a guide for traffic analysts when 

considering which parameters to adjust when modeling adverse weather. 

CORSIM Sensitivity Analysis 

The purpose of the sensitivity analysis was to identify the most sensitive weather-related 

parameters in CORSIM. Each test parameter was modeled on various geometric networks and 

congestion (volume) levels using the default value, then changing the value to represent 

incrementally more conservative driver behavior, as would occur under adverse weather. The 

MOEs produced by the default value were then compared to the MOEs produced with the 

changed parameter values to determine the level of sensitivity the parameter has on the MOEs. 

Due to the large number of roadway networks, congestion levels, and parameters tested, 

approximately 45,000 individual CORSIM runs were completed. As a result, a largely 

automated process of creating the CORSIM input files and summarizing the output files was 

created specifically for this project. 

One interesting result of the sensitivity analysis was that a number of parameters tested (19 total) 

had little or no impact on the MOEs. The majority of these were lane changing parameters. This 

finding does not mean they have no sensitivity whatsoever; rather, the finding shows no 

sensitivity to the aggregate-level MOEs used for this study. It is likely that more sensitivity 

would have been measured by using more disaggregate MOEs, or by evaluating trajectories of 

individual vehicles. 

A number of weather-related parameters had an expected effect on the MOEs and were 

categorized as either having a medium or high effect on the MOEs (relative to the other 

parameters). These parameters are important because they represent the key weather-related 

parameters that should be altered when trying to model weather events in CORSIM. These 

parameters include: 

• Mean free-flow speed (FRESIM and NETSIM). 

• Car following sensitivity multiplier (FRESIM). 

• Time to react to sudden deceleration of lead vehicle (NETSIM). 

• Mean discharge headway (NETSIM). 

• Mean startup delay (NETSIM). 

Table 41 provides a more detailed list of the key weather-related parameters identified during the 

sensitivity analysis. 

Guidelines for Modeling Weather Events in CORSIM 

A set of practical guidelines for modeling weather events in CORSIM was developed as a result 

of the CORSIM sensitivity analysis. The guidelines are based on Traffic Analysis Toolbox 

Volume III—Guidelines for Applying Traffic Microsimulation Modeling Software, an FHWA 

guidance document on the proper development and application of microsimulation models.(1) 

The guidance builds on the more general microsimulation guidance by providing additional 

considerations when modeling weather events in CORSIM. For example, the type, severity, 

extent, and time period of the weather event being modeled should be agreed before coding the 

model. 
 65

The guidelines list CORSIM parameters to consider changing when modeling various weather 

events (see table 42). Also, an alternate method of calibrating a weather event model is 

presented when field data collection during adverse weather is not possible. 

Future Research Needs 

Based on the findings of this study, a number of areas of future research were identified that 

would improve the ability of CORSIM to model weather events and improve the base 

understanding of the relationship between weather events and traffic operations. The four 

identified areas of future research are: 

1. Empirical data collection to improve base understanding of impact of weather events 

on traffic operations. 

2. CORSIM enhancements for modeling adverse weather events. 

3. Further study of “insensitive” CORSIM parameters. 

4. Real-world case study of modeling weather events using CORSIM. 

The first area, improving the base understanding of the weather event-to-traffic operations link, 

is the most important area of future research, because it will help analysts in all applications of 

traffic analysis models better understand the impacts of adverse weather on traffic operations. 

This study showed the most sensitive weather-related parameters in CORSIM and, as such, these 

parameters should be of high importance to collect during adverse weather. While the empirical 

data collection will help determine the true impact of adverse weather on traffic operations, it 

will not shed light on the impacts not measurable by video or roadway detectors, namely, the 

reason why drivers change their driving behaviors. For this reason, supplemental human factors 

studies should be initiated that would help explain the impact of the roadway environment on 

traffic operations (i.e., did a driver slow down because of a reduction in visibility, reduction in 

pavement friction, or a combination of both?). 

The second area of future research deals with improving the ability of CORSIM to model the 

impacts of weather events. Some basic improvements to CORSIM that would help analysts 

improve how they model the impacts of weather events include: 

• Separation of free-flow speed and maximum speed—Under adverse weather, drivers 

may not drive as fast as under ideal weather conditions. In the current version of 

CORSIM, to model the changes in weather conditions (e.g. sight distance reduction in 

fog), users must adjust the free-flow speed to slow down vehicles. However, delay 

and other MOEs are calculated based on free-flow speed. Therefore, a reduction in 

free-flow speed reduces the delay and causes undesirable MOE values. By separating 

the parameters of free-flow speed and desirable maximum speed, analysts will be able 

to decrease the desirable maximum speed parameter while allowing the appropriate 

MOEs to be based on the free-flow speed. 

• Adequate consideration of roadway geometry—CORSIM can account for a number 

of roadway geometry parameters that influence MOEs, but there are many others that 

are not taken into account that become more pronounced in adverse weather. 

Improving the CORSIM model to account for geometric features such as forward and 

lateral sight distance, horizontal and vertical curvature and alignment, and lane and 
 66

shoulder width would allow analysts to better analyze the full range of impacts of 

adverse weather. 

• Network vs. link-level parameters—Currently, some CORSIM parameters are 

network-wide, and all individual links must use these network parameters. With the 

ability to model large networks in CORSIM and the isolated nature of some weather 

effects (i.e., shoulder blocked by a snow bank), it may be beneficial to have certain 

parameters associated with each individual link, such as gap acceptance parameter, 

lag acceleration/deceleration time, maximum emergency deceleration, percent 

cooperative drivers, time to complete lane change, Pitt car following constant, and car 

following sensitivity factor. 

The third area of future research identifies the need for further study on 19 parameters that 

showed no or low sensitivity during the sensitivity study. The majority of these were lane 

changing parameters. An analysis of more disaggregate level MOEs would define further 

exactly what impact these parameters have on traffic flow during specific ranges of volume 

levels and network geometries. 

The fourth area of future research comprises initiating a case study using CORSIM to model a 

set of real-world roadway networks under adverse weather. Such a study would showcase the 

state-of-the-art in modeling weather and traffic events, serve as a test of the guidelines developed 

in this study for applying a microsimulation model during adverse weather conditions, and 

discover shortcomings in the process that need further refinement or study. The case study 

ideally would be done at a number of locations with varying roadway geometries (such as on 

freeway sections and signalized arterials) and varying weather events (such as during light rain, 

heavy fog, and a major snowstorm). 
 67

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