Assessing vertical curve design for safety: case study on the N1/R300 Stellenberg Interchange, Western Cape

Abstract

Paper presented at the 31st Annual Southern African Transport Conference 9-12 July 2012 "Getting Southern Africa to Work", CSIR International Convention Centre, Pretoria, South Africa.

This project assessed the safety of a crest vertical curve on a four-lane freeway by examining the required stopping sight distance at operating speeds. The basis of the safety concerns were identified as the geometric properties of the road, such as grade and K-values, where the latter is an indication of the stopping sight distance (SSD). The vertical alignments in both the inbound and outbound directions were determined from GPS surveys. It was found that the available SSD (170 m) along the curve, using 150 mm object height, was significantly less than the required SSD (320) needed on the curve in order to achieve a safe stop, when including the gradient of the road. The analysis was based on the measured 85th percentile speed of 125 km/h. The K-value of the vertical curve at the Stellenberg Interchange does not conform to standards of the Geometric Design Guidelines of South African National Roads Agency Limited (SANRAL, 2002). Collision data was examined to confirm the safety concerns. Collision data was compared with that of a control site, which met all geometric standards for vertical curves. It was found that significantly more collisions, totalling 42 collisions, occurred at the Stellenberg Interchange site compared with the control site that totalled 4 collisions over the 17 month analysis period. The majority of collisions at the test site were head/rear end collisions which typically occurred as a result of inadequate SSD. It is recommended that a 100 km/h speed limit be imposed on the N1 road section through the Stellenberg Interchange as interim measure until reconstruction can be commenced.