#33 Tailgating

Tailgating, or following too closely behind a leading vehicle, is a ubiquitous driving behaviour associated with rear-end crash risk. While often deliberate, tailgating also occurs accidentally, particularly when traffic flow and related conditions change suddenly. A safe following distance has been defined as following with at least a 2-second gap to the vehicle in front, where the distance covered in 2 seconds depends on vehicle speed. The 2-second gap represents a distance of approximately 56m at 100km/h, or 61m at 110km/h. While tailgating is a major contributor to rear end crashes – attributed to at least 8,000 injury or fatal crashes yearly in Queensland, for example[1] - enforcement of safe following distance is difficult and consequently limited. Therefore, self-regulation and vigilance by drivers are critical in the management of rear-end crash risk.

Treatment

While speed reduction measures can reduce both the risk and severity of rear-end crashes, they are not designed to reduce tailgating and can negatively impact the utility and performance of high-speed roads. Educational measures have potential to deter tailgating and may target specific groups such as younger drivers who are found to be overrepresented in relevant crashes. However, engineering options are limited.

One treatment in limited use internationally and in Australia involves the use of suitably spaced chevron markings on the road surface, with drivers advised to keep a spacing of at least 2 chevrons between their vehicle and the vehicle in front. The chevrons are spaced according to the distance covered in 1 second at the posted speed limit. Published evaluations of this specific treatment are limited. However, trials on Danish 130km/h freeways using chevrons at 36m intervals (equivalent to 31m at 110km/h) over 4km sections were documented, showing positive results[2]. Pre- and post-treatment comparison showed significant (7-11%) reduction in small gaps (<1 sec), and smaller reductions (1-4%) in gaps <2 seconds. The study also found small (1-3km/h) speed reductions associated with the treatment. Positive results continued 1-4km downstream from the treatment end, but were not observed at a greater distance (7-12km) downstream, indicating that drivers soon revert to baseline behaviour in the absence of the chevrons. The authors of the Danish study concluded that, while safety was improved, particularly in the reduction of small gaps, there was still potential to further address the issue in future trials and associated research.

[1] Nicolls, M. et al., (2022). Self-regulation and tailgating behaviours among young drivers. Transportation Research Part F: Traffic Psychology and Behaviour 89, 347-354.

[2] Greibe, M. (2010). Chevron markings on freeways: Effect on speed, gap and safety. 4th International Symposium on Highway Geometric Design, 2-5.