Revisiting the Design Parameters Related to Stopping Sight Distance and Vertical Curve Length using Probabilistic Analyses

Stopping sight distance (SSD) is one of the 10 controlling criteria for the geometric design of highways. The current design guidelines related to SSD are detailed in the American Association of State Highway and Transportation Officials’ A Policy on Geometric Design of Highway and Streets . This guidance assumes conservative values for brake reaction time and deceleration rates that are based on data from the late 1990s. This study revisits these design parameters to assess the adequacy of current design policies related to SSD and length of vertical curves. Data from crash and near-crash events were obtained from the second Strategic Highway Research Program Naturalistic Driving Study. This provided data on brake reaction times and deceleration rates. Field data were also collected on the distribution of driver eye height, headlight height, and taillight height for passenger cars and trucks. Monte Carlo simulation was utilized to conduct probabilistic-based analyses for various design parameters related to SSD. The study results showed that current design policy tends to be relatively conservative. Further, changes in several of these parameters have occurred over time. Collectively, these findings led to a series of recommendations to update guidance related to brake reaction time, driver deceleration rate, driver eye height, and object height for SSD applications under various design scenarios.