M D Wooldridge

DESIGN FACTORS THAT AFFECT DRIVER SPEED ON SUBURBAN STREETS

Many roadway factors have an effect on driver behavior. Geometric, roadside, and traffic control device variables that may affect driver behavior on four-lane suburban arterials were investigated. Traffic signals and traffic volume were considered within the study site selection and data collection criteria and, therefore, were not included in the analysis. Regression techniques were used to determine how selected variables affect operating speed on horizontal curves and straight sections. When all variables were considered, posted speed limit was the most significant variable for both curves and straight sections. Other significant variables for curve sections were deflection angle and access density class. In another series of analyses performed without using posted speed limit, only lane width was a significant variable for straight sections, but median presence and roadside development were significant for curve sections. The analysis that included posted speed limit, however, produced stronger relationships between speed and significant variables than did the analysis that excluded posted speed limit.

Effects of horizontal curvature on driver visual demand

A consistent design allows drivers to perform safely the task of driving, allowing attention or capacity to be dedicated to obstacle avoidance and navigation. A measure of the consistency of a design is the amount of visual information needed by a driver to maintain an acceptable path on the roadway. Vision occlusion is a technique that measures driver visual demand on a roadway. It allows a more direct evaluation of the effects of various geometric elements on the driver. Studies of the effects of variations of curve radius, deflection angle, spacing, and sequences revealed several relationships between roadway geometry and visual demand. Curve radius and its reciprocal were found to be significantly related to visual demand in both on-road and test track studies. Small changes in visual demand were also found between types of curve pairs (S and broken back) with differing spacing between the curves. Visual demand was found to be a promising measure of effectiveness for use in studies of design consistency.

PERFORMANCE COMPARISON OF THREE FREEWAY GUIDE SIGN ALPHABETS

Modern fully retroreflective legends can create a blooming effect that reduces legibility. In this research project three alphabets were evaluated to determine whether the legibility of a white high-intensity legend on a green high-intensity background could be improved by reducing the blooming effect. The three alphabets were: Series E(Modified), Clearview, and British Transport Medium. Overhead and ground-mounted sign positions were evaluated in both daytime and nighttime conditions. There was no sign illumination for the nighttime conditions other than the vehicle headlights. Both legibility (unknown word) and recognition (known word) distances were measured. A total of 54 subjects participated in both daytime and nighttime trials. There was significant variability in the results of the various experimental conditions. In general, the results indicated that Clearview was slightly more legible than Series E(Modified) in the overhead position in both daytime and nighttime conditions. The extent of improvement over Series E(Modified) was generally in the range 2 to 8 percent. The greatest improvement was achieved for worst-case drivers. Clearview ground-mounted signs were less legible than Series E(Modified) in daytime conditions. In nighttime conditions, the ground-mounted Clearview signs did not demonstrate a consistently better performance than Series E(Modified). A greater degree of improvement was realized in the recognition of Clearview in the overhead position for both day and night conditions. British Transport Medium was generally less legible than Series E(Modified).

Evaluation of Zero-Length Vertical Curves

To evaluate the use of zero-length vertical curves with respect to Texas Department of Transportation design practice, construction results, and vehicle dynamics, and to compare zero-length vertical curves with minimum-length design vertical curves, 20 zero-length vertical curves and 15 minimum-length vertical curves were evaluated. Evaluations performed included examinations of sight distance for zero-length vertical curves, surveying roadway profiles, and measuring vertical accelerations. Sight distance criteria were found to be generally inapplicable below 2 percent for sag vertical curves. For crest vertical curves, sight distance criteria were inapplicable below 1 percent for design speeds of 90 km/h or less and inapplicable below 0.5 percent for design speeds of 100 to 120 km/h. The results of vertical acceleration testing showed that, below 0.5 percent grade change, no practical difference was found between zero-length and minimum-design-length vertical curves. Between 1.0 and 0.5 percent grade change, significantly higher accelerations were measured for high-speed tests on zero-length vertical curves. Finally, it was found that zero-length vertical curves were more likely to meet drainage grade requirements than minimum-design-length vertical curves were.