Speed selection at sites with restrictive alignment: the US-191 case study

An investigation into the driver’s choice of speed at roadway sites with restrictive alignment is presented in this study. Specifically, the study focused on the effect of horizontal curve radii and sight distance on speed selection. Seven sites were examined in this study that are located along a 10-mile stretch of a rural high crash corridor, US 191 north of Big Sky in southwest Montana. Two of the study sites have no restrictive geometry representing base conditions, another two have restrictive curve radii, and the other three have restrictive radii and sight distances. Vehicle speeds, classification, and headways were collected at each site. The selected speeds for free-moving vehicles were compared to the legal speed limit, advisory speed, and the speeds dictated by curve radius and sight distance when applicable. Study results found that the vast majority of observed selected speeds are notably higher than the speeds found using the alignment and sight distance design equations. Results also showed that the perceived safe speeds selected by drivers are likely to be determined by the most restrictive geometric feature.