Empirical Collision Model for Four-Lane Median Divided and Five-Lane with TWLTL Segments

Abstract

There is a great debate between the use of raised median and two-way left turn lane (TWLTL) treatments on arterial roads. The raised median is generally believed to be the safer of the two treatments for both vehicular and pedestrian traffic, while the TWLTL is considered more beneficial to businesses since it allows direct access to left-turning traffic from an arterial. The North Carolina Department of Transportation (NCDOT) has been using studies conducted in other states as well as collision rates from case studies in North Carolina to make decisions in such matters. Studies conducted in other states, however, are generally not considered relevant to North Carolina drivers and the use of collision rates is criticized because they do not account for driveway densities or land use.

This research was conducted to definitively determine which of these cross-sections is safer on North Carolina's highways and under what conditions. A random sample of 200 four-lane roads with raised median and five-lane roads with a TWLTL was visited and data were collected on the geometric, volume, land use, and collision characteristics of each site. Of these 200 sites, 143 were found to meet all necessary criteria and were used to calibrate and validate a collision model for each cross-section using Poisson regression procedures. The form of the collision models developed by Bonneson and McCoy were used since they were judged to give logical and consistent results.

The final regression equations showed that the factors contributing to collisions on these two segment types were average daily traffic (ADT), length, predominant land use, and approach density.

For predominantly residential and industrial land uses, the raised median design was found to be associated with fewer collisions than the TWLTL across all values of traffic volume and approach densities. The raised median design also has a safety advantage over the TWLTL for predominantly business or office land uses with low to medium approach densities (0-25 approaches per mile). For business and office land uses with medium to high approach densities (25-90 approaches per mile), the TWLTL appears to be safer at low traffic volumes and the raised median appears to be safer at high traffic volumes.