Investigation of drivers speed selection behavior: A naturalistic driving approach
Research studies have generally shown that increased travel speeds result in higher crash frequencies and greater traffic fatalities. On the other hand, road users generally favor higher posted speed limits due to the resulting increases in travel speeds and reductions in travel time. Therefore, the influence of speed limits, traffic characteristics, and roadway geometry on driver speed selection, as well as the interrelationship between speed and crash risk, continue to be critical areas of interest for transportation agencies across the United States. To better understand the differences in driver behavior that may result from speed limit policies, this study involved a detailed assessment of the behavior of individual drivers using data collected as a part of the second Strategic Highway Research Program (SHRP2) Naturalistic Driving Study (NDS). The SHRP2 Safety Data from the NDS includes very detailed data on individual driver behavior, as well similarly detailed information regarding the roadway environment from the related Roadway Information Database (RID). By leveraging these data, drivers’ speed selection behavior was investigated under three different settings including: 1) Contiguous road segments with constant posted speed limit zones; 2) Transition areas where the speed limit increased or decreased; and 3) Horizontal curves, with particularly emphasis on those with advisory speed signs in place. Speed profiles of study participants under each setting were examined through the estimation of a series of mixed-effect linear regression models. For the first two settings, separate models were estimated for freeways and two-lane highways, whereas the latter analysis of horizontal curves was solely focused on two-lane highways. The impact of drivers’ behaviors, as well as roadway geometry and other environmental conditions on crash risk were examined.
Drivers were generally found to vary their speeds with respect to changes in the roadway geometry and weather condition. On segments with constant speed limits, drivers were shown to increase their speed based on the posted speed limit and on segments with limited access points (e.g., ramps, intersections, driveways). Conversely, reduced speeds were observed under traffic congestion, adverse weather conditions, and along horizontal curves. Younger drivers tended to travel faster, and, in addition, significant differences were observed between individual drivers, with some tending to drive consistently faster or slower than other similar drivers. A subsequent crash risk analysis showed that safety critical events (i.e., crashes and near-crashes) were more likely under increased traffic congestion, along horizontal curves, near access points, and through work zones.
Speed profiles demonstrated similar patterns across speed limit transition areas. Drivers were found to begin adjusting their travel speeds upstream of the new regulatory speed sign. More pronounced changes were seen where limit reductions were introduced as compared to more limited changes when limits were increased. In all cases, the changes in actual driving speed were significantly less pronounced than the magnitude of the speed limit change. Weather and traffic flow conditions demonstrated significant impacts on travel speeds, like the first analysis, and speeds also varied by driver age. The third series of analyses revealed more complicated patterns in drivers’ behavior and how they negotiate curves. Generally, drivers were found to reduce their travel speeds across horizontal curves, especially when advisory speed signs were present. Increased reductions were observed when negotiating with sharper curves. However, as with the analyses of regulatory limits, the speed reductions tended to be much lower than what was suggested by the advisory speed signs. Individual locations were examined by deploying functional data analysis (FDA) methods, which showed much of the speed reduction occurred upstream of the sign, between the advisory sign and the point of curve (PC). Where smaller reductions were advised, drivers tended to begin accelerating back to baseline speed within the curve. In contrast, they maintained the reduced speed throughout the curve where greater speed reductions were suggested.