Citation

Kyte M, Khatib Z, Shannon P, Kitchener F. Transp. Res. Circular 2000; (E-C018): 108-119.

Copyright

(Copyright © 2000, U.S. National Academy of Sciences Transportation Research Board)

DOI

unavailable

PMID

unavailable

Abstract

The estimation of free-flow speed is an important part of the process of determining the capacity and level of service for a freeway. The "Highway Capacity Manual" (HCM) notes that the free-flow speed depends on both the traffic and roadway conditions found on a given freeway facility. Particularly important are lane width, lateral clearance, number of lanes, interchange density, and vehicle stream composition. The draft chapter on freeway facilities, to be included as chapter 22 of the HCM 2000, notes that "adverse weather can affect not only capacity, but also reduces operating speeds significantly." The chapter cites several studies that investigated the effects of rain, snow, and fog on both capacity and speed. The authors have studied the effects of a variety of weather-related environmental factors on driver speeds as part of an Intelligent Transportation Systems project that has been on-going in Idaho since 1993. Visibility and roadway sensors were installed on a segment of I-84 in southeastern Idaho in 1995. This project has generated substantial data on traffic flow rates and driver speeds during periods of reduced visibility and other hazardous driving conditions. While capacity is not an issue along this section of rural interstate freeway, the sensor infrastructure now in place provides an opportunity to determine the effects of various factors on free-flow speed. This study reports on data collected during two winter periods, 1997-1998 and 1998-1999.