Factors affecting driver speed choice along two-lane rural highway transition zones

The primary objective of this research was to develop speed prediction models to explain the relationship between the roadway features present along a two-lane rural highway transition zone and driver operating speeds. Two general model specifications were considered based on the available speed data. These included point speeds based on the "tracked" vehicles, and speed differentials between successive data collection points in a transition zone.;In the point speed analysis, four repeated speed measurements were collected on each of the 2,859 drivers across 20 different sites. Longitudinal models were used to model these data and compared to the more traditional operating speed modeling approach, ordinary least squares (OLS) regression. Use of OLS regression violates the assumption of independent observations. The longitudinal models considered in this research were panel data models using both the fixed and random effects estimator, multilevel models, and generalized estimating equations (GEE). From the results of the analyses it was concluded that a three-level model in which speed observations were nested in drivers and drivers were nested in sites is more appropriate in explaining the influence of highway characteristics on driver speeds along two-lane rural highway transition zones. Key relationships between highway features and mean operating speeds in transition zones are as follows: (1) When compared to a posted speed limit of 55 mph, a speed limit of 45 mph is associated with a mean operating speed reduction of approximately 3.5 mph. A speed limit of 25 mph is associated with a mean operating speed that is approximately 10.5 mph lower than the baseline of 55 mph. Similarly, a posted speed limit of 35 or 40 mph is associated with a mean operating speed that is approximately 2.4 mph lower than the baseline of 55 mph. (2) Wider travel lanes and lateral clearance distances are associated with higher operating speeds along two-lane rural highway transition zones; a mean operating speed increase of 2.4 mph is expected per one-foot of lane width increase while a one-foot increase in lateral clearance is associated with a mean operating speed increase of 0.15 mph. (3) The presence of curb is associated with a mean speed reduction of approximately 4 mph while the analysis indicated that a mean speed reduction of 1 mph is associated with a one-unit increase in driveway density. (4) The presence of Intersection Ahead and School/Children warning signs were associated with 2 and 1 mph mean speed reductions, respectively, while the presence of a Curve Ahead warning sign was associated with a mean speed increase of almost 1 mph, when compared to the baseline of other warning sign types. (5) Finally, the presence of a horizontal curve was associated with a mean speed reduction of 1.5 mph; if the horizontal curve is combined with a warning sign, a mean speed reduction of almost 3 mph is expected when compared to the baseline of a tangent roadway section.;A second data set was created in which the response variable was change in speed along the transition zone. By considering speed change as the response variable, only one data point per vehicle (driver) was available; however, a site cluster could still be considered in the model specification. Use of the speed differential as the dependent variable in a statistical model eliminated part of the repeated observation issue present in the point speed analysis. As such, two general modeling methods were considered. These included OLS regression and multilevel models in which speeds were nested in sites. The variables that were consistently associated with speed reductions across all models were changes in the posted speed limit, reduction in paved shoulder width (1 mph reduction per one-foot reduction in paved shoulder width), number of driveways (0.36 mph reduction per one-unit increase in driveway density), school/children related warning signs (8 mph mean speed reduction), length of transition zone (0.8 mph average speed reduction per 100 foot increase in transition zone length), and presence of horizontal curve that warrants a warning sign (3.2 mph mean speed reduction is expected with this type of horizontal curve). The presence of a Curve Ahead warning sign and tangent sections were consistently associated with a speed increase along transition zones across all models (3.2 mph average and 2 mph average, respectively). (Abstract shortened by UMI.)...