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Empirical Studies on Traffic Flow in Inclement Weather

Summary Report

October 2006

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Publication No. FHWA-HOP-07-073

Prepared for Federal Highway Administration

Prepared by

Cambridge Systematics, Inc.
100 CambridgePark Drive, Suite 400
Cambridge, Massachusetts 02140

with

Virginia Tech Transportation Institute
3500 Transportation Research Plaza
Blacksburg, Virginia 24061

Table of Contents

Acknowledgments / Notice / Quality Assurance Statement / Technical Report Documentation Page / Metric Conversion Factors

Executive Summary

• Background
• Goals and Objectives
• Research Approach
• Model Form and Calibration
• Research Findings
• Future Research

1.0 Introduction

• 1.1 Background
• 1.2 Motivation for this Study
• 1.3 Scope of Report

2.0 Literature Review

• 2.1 Operations and Macroscopic Traffic Research
• 2.2 Microscopic Behavior Research
• 2.3 Human Factors Research
• 2.4 Research Needs and Approach

3.0 Research Methodology

• 3.1 Key Traffic Variables
• 3.2 Selected Traffic Stream Model – Van Aerde
• 3.3 Data Analysis and Model Development

4.0 Data Collection

• 4.1 Field Data Description
• 4.2 Dataset Construction and Data Analysis

5.0 Study Results

• 5.1 Study Results
• 5.2 Conclusions

6.0 Recommendations for Next Phase of Study

• 6.1 Lessons learned from Completed Research
• 6.2 Microscopic and Human Factors Analysis

7.0 References

A. Literature Classification and Summary

B. Model Formulation

List of Tables

Table ES.1 Research Summary

Table 2.1 Reduction in Capacity from Daylight and Dry Conditions

Table 2.2 Summary of Rain Effects on Capacity

Table 2.3 Snowstorm Impacts on Volumes

Table 2.4 Speed Reductions in Inclement Weather

Table 2.5 Impact of Environmental Conditions on Speed

Table 2.6 Summary of Rain Effects on Speed

Table 2.7 Summary of Snow Effects on Volume

Table 2.8 Summary of Snow Effects on Speed

Table 2.9 Summary of Research Results – Freeway

Table 2.10 Literature and Research on Focused on Individual Drivers

Table 3.1 Comparison of Flow Parameters for Single-Regime, Multiple-Regime Models, and Proposed Model

Table 3.2 Estimated Flow Parameters for Various Facility Types

Table 3.3 Hypothetical Capacity Adjustment Factors

Table 4.1 Precipitation/visibility Combinations Considered

Table 4.2 Detector Summary Information

Table 5.1 ANOVA Test Results for Free-Flow Speed (Rain – Twin Cities)

Table 5.2 Free-Flow Speed Regression Analysis Summary Results

Table 5.3 Ratio of Speed-at-Capacity to Free-Flow Speed Analysis Summary Results

Table 5.4 Speed-at-Capacity Analysis Summary Results

Table 5.5 Capacity Analysis Summary Results

Table 5.6 General Linear Model (GLM) – Analysis of Variance

Table 5.7 Analysis of Variance Summary of Results

Table 6.1 Reasons for Changing Routes West Sector

Table 6.2 Reasons for Changing Departure Time West Sector

Table 6.3 Stated Importance Total Sample and West Sector Results

Table A.1 Literature and Research on Studies Using Secondary Data

Table A.2 Literature and Research on Studies Using Primary Data

List of Figures

Figure ES.1 Process for Developing Empirical Models on Weather and Traffic Flow

Figure ES.2 Variation in Weather Adjustment Factors as a Function of Visibility and Snow Intensity Levels (Twin Cities)

Figure ES.3 Sample Comparison Snow and Rain Weather Adjustment Factors (Twin Cities)

Figure 3.1 Research Methodology Steps

Figure 3.2 Van Aerde and Newell Model Fit to Freeway Data

Figure 3.3 Van Aerde Model Fit to Freeway Data (Twin Cities, USA)

Figure 3.4 Impact of Precipitation on Traffic Stream Behavior and Parameters

Figure 4.1 Annual Precipitation Rates for Twin Cities and Seattle

Figure 4.2 Data Construction Overview

Figure 5.1 Statistical Analysis Overview

Figure 5.2 Residuals Normality Test for (Rain) Free-Flow Speed (Twin Cities, Minnesota)

Figure 5.3 Results of Residuals Variance Test for the Effect of Rain Intensity on Free-Flow Speed (Twin Cities, Minnesota)

Figure 5.4 Twin Cities Inclement Weather Adjustment Factors

Figure 5.5 Variation in Weather Adjustment Factors as a Function of Visibility and Rain Intensity Levels (Twin Cities)

Figure 5.6 Variation in Weather Adjustment Factors as a Function of Visibility and Snow Intensity Levels (Twin Cities)

Figure 5.7 Comparison of Raw Data versus SPD_CAL Analysis Results

Figure 5.8 Sample Comparison Snow and Rain Weather Adjustment Factors (Twin Cities)

Figure 5.9 Sample Traffic Stream Model Variation (Twin Cities – Rain)

Figure 5.10 Sample Traffic Stream Model Variation (Twin Cities – Snow)

Figure 5.11 Variation in Weather Adjustment Factors as a Function of Rain Intensity

Figure 5.12 Variation in Weather Adjustment Factors as a Function of Snow Intensity

Figure 6.1 Image of Smart Road Weather Making Capabilities

Figure B.1 Van Aerde Model Fit to Freeway Data (Twin Cities, USA)

U.S. Department of Transportation
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Washington, DC 20590
https://www.fhwa.dot.gov

Publication No.: FHWA-HOP-07-073

Publication Date: October 2006