1. Report No.
FHWA-HOP- 07-073

2. Government Accession No.

3. Recipient’s Catalog No.

4. Title and Subtitle
Empirical Studies on Traffic Flow in Inclement Weather

5. Report Date
October 2006

6. Performing Organization Code

7. Author(s)
Robert Hranac, Emily Sterzin, Daniel Krechmer,
Hesham Rakha, Mohamadreza Farzaneh

8. Performing Organization Report No.

9. Performing Organization Name and Address
Cambridge Systematics
4800 Hampden Lane, Suite 4000
Bethesda, MD 20814
Virginia Tech Transportation Institute
3500 Transportation Research Plaza (0536)
Blacksburg, VA 24061

10. Work Unit No.

11. Contract or Grant No.

12. Sponsoring Agency Name and Address
Road Weather Management Program
Office of Operations
Federal Highway Administration
400 Seventh Street, S.W.
Washington, D.C. 20590

13. Type of Report and Period Covered
Guidelines

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract

Weather causes a variety of impacts on the transportation system. While severe winter storms, hurricanes, or floodings can result in major stoppages or evacuations of transportation systems and cost millions of dollars, day-to-day weather events such as rain, fog, snow, and freezing rain can have a serious impact on the mobility and safety of the transportation system users. These weather events can result in increased fuel consumption, delay, number of accidents, and significantly impact the performance of the transportation system. The overall goal of the research work undertaken in this study was to develop a better understanding of the impacts of weather on traffic flow. The research was intended to accomplish the following specific objectives: 1) Study the impact of precipitation on macroscopic traffic flow parameters over a full range of traffic states; 2) Study the impact of precipitation on macroscopic traffic flow parameters using consistent, continuous weather variables; 3) Study the impact of precipitation on macroscopic traffic flow parameters on a wide range of facilities; 4) Study regional differences in reaction to precipitation; and 5) Study macroscopic impacts of reduced visibility.

The work documented in this report was conducted in two parts: 1) literature review and development of a data collection and analysis plan, and 2) analysis and interpretation of the results. The recommended plan combined the use of macroscopic traffic data archives with archived weather data in order to meet the research goals that include achieving better understanding of the impacts of weather on macroscopic traffic flow. The results of the research conducted for this study were helpful in identifying weather impacts of traffic flow in the three cities studied, Minneapolis-St. Paul, Baltimore and Seattle. No impacts were found on traffic stream jam density, but both rain and snow did impact traffic free-flow speed, speed-at-capacity and capacity and parameters varied with precipitation intensity. The results of these analyses are documented in the report.
This report concludes with some recommendations of future research related to weather and traffic flow. Several ideas are presented including enhancing the macroscopic analysis used in this study. Additional work is proposed related to human factors and microscopic traffic modeling.

17. Key Words
Weather, traffic flow, statistical models, human factors

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
114

22. Price