Chevron Markings on Freeways: Effect on Speed, Gaps and Safety
af Poul Greibe
45% of all injury accidents on Danish freeways are categorised as accidents with vehicles driving in the same direction. On a year basis, these accidents results in more than 120 killed and injured persons. Rear-end accidents typical occur in situations with very short headways and/or in combination with high speed levels.
Inspired by positive results from the use of chevron markings in UK and France, a trial of chevron markings on Danish freeways, to help drivers to choose a safer distance to the vehicle in front, was initiated by the Danish Road Directorate in 2007. The chevron markings were establish on 5 road sections (each 4 km long), and consist of a series of white arrowheads on the road surface at 36 m intervals. The chevron markings were accompanied by road side signs advising drivers to keep a distance of 2 chevron markings to the vehicle in front.
In order to evaluate the use of chevron markings, traffic flow, speed and gaps were measured by use of loop-detectors at 3-4 locations on each road section. One detector was situated upstream from the markings; one detector on the road section with markings and one or two detectors were situated downstream from the markings (4-12 km downstream from the chevron markings). Data on speed, traffic flow and headways were collected 1 month before and 2 month after the installation of chevron markings. In addition, in order to estimate the long term effect, the plan is to collect traffic data again 2 year after the installation.
The short-term effect by chevron markings (2 month after) showed significant fewer vehicles with small gaps (gap<1sec) compared to the before situation. The largest reduction was found in the left lane. The number of vehicles with gap less than 2 seconds was also reduced, but the reduction was smaller. Speed was reduced slightly. In general, the largest effects on gaps and speed were found on road sections with chevron markings, but also an effect 4-7 km after the marking (downstream) could be found. Further downstream (>10 km), no effect from the markings could be measured. The long-term effect (2 year after) has not been estimated yet.
A questionnaire survey among drivers indicated that the purpose of the markings was understood, and that majority of drivers have changed behaviour (greater gap to vehicle in front).
A small before-after accident analysis, based on two road section with chevron markings on freeways established back in 2004, showed a reduction in accidents. The analysis is based on very few accidents though, and the estimate is uncertain. The accident analysis will be repeated again using a longer before-after period.