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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 EFFECTS OF ROADWAY FACTORS ON SPEED Introduction The effects of roadway factors on speed refers to the impact of geometric, environmental, and traffic factors on driving speed under free-flow conditions in tangent roadway sections. Speed in curve entry is covered in Chapter 6. Free-flowing speed is defined as conditions in which a driver has the ability to choose a speed of travel without undue influence from other traffic, conspicuous police presence, or environmental factors. In other words, the driver of a free-flowing vehicle chooses a speed that he or she finds comfortable on the basis of the appearance of the road. Typically this involves a minimum headway time of 4 to 6 s (1). Note that although posted speed is often found to be one of the factors that is most strongly correlated with free-flow speed, this correlation is somewhat misleading, because driver compliance with posted speed can be low if the posted speed is set too low (see the guideline "Effects of Posted Speed Limits on Speed Decisions," on page 17-8). In contrast, the strong association between posted speed and free-flow speed typically occurs because the 85th percentile speed is often used to set the posted speed limit. Design Guidelines The following factors that appear to be associated with drivers' choosing a higher travel speed should be considered when designing roadways. Strength of Factors Associated with Empirical Evidence HIGHER Free-Flow Speeds Low-Speed Rural Highways Urban Streets Higher Design Speed Solid Solid Grade Solid Solid Wider Lane Width -- Mixed Higher Access Density Solid Mixed Separated Bicycle Lanes -- Mixed Less Pedestrian/Bicycle Side Friction -- Mixed No Roadside Parking -- Mixed Number of Lanes Solid -- Shoulder Width Mixed -- Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-6
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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion As the table on the previous page makes clear, the empirical record is far from conclusive with respect to the ability to predict drivers' speed choices associated with relevant geometric, environmental, and traffic factors. Nonetheless, some relationships between drivers' speed choices and these factors--however tentative--have emerged from the literature and are worth presenting here. Currently, there is insufficient research to provide more quantitative guidance about how much the factors listed in the guideline table increase free-flow speed. As seen in the figure below, roadway factors impact both the driver's choice of speed, as well as overall crash probability and severity. In Fitzpatrick, Carlson, Brewer, and Wooldridge (3), data were collected at 24 horizontal curve sites and 36 straight section sites to identify roadway factors that influence speed. Data collected included details of alignment (e.g., curve radius, curve length, straight section length), cross section (e.g., lane width, superelevation, median characteristics), roadside details (e.g., access, density, pedestrian activity), and information on traffic control devices. Laser guns were used to collect speed from vehicles at the 60 (total) sites. Multiple regression techniques, using 85 th percentile speed as a "quantifiable definition of operating speed," were used in the analysis. The alignment (downstream distance to control) and cross section (lane width) factors explained about 25% of the variability in the speed data for both curve and straight road sections. Roadside factors were not significant for the straight road sections, but accounted for about 40% of the variability in the speed data for curves. Additional analyses conducted without using posted speed limits resulted in only lane width as a significant variable for straight road sections, with both median presence and roadside development as significant variables for curves. Source: Milliken, J.G., Council, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (2) Design Issues None. Cross References Design Consistency in Rural Driving, 16-8 Behavioral Framework for Speeding, 17-2 Key References 1. Parker, M.R., Jr. (1997). Effects of Raising and Lowering Speed Limits on Selected Roadway Sections. (FHWA-RD-92-084). McLean, VA: FHWA. 2. Milliken, J.G., Council, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (1998). Special Report 254: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits. Washington, DC: Transportation Research Board. 3. Fitzpatrick, K., Carlson, P., Brewer, M., and Wooldridge, M. (2001). Design factors that affect driver speed on suburban streets. Transportation Research Record, 1751, 18-25. 17-7