Time and distance halo effects

The one common finding in the literature is that automatic speed enforcement effects are limited in terms of both time [64] and space [10] [36]. ‘Time halo’ can be defined as the length of time that the effects of enforcement on drivers’ speed behaviour continue after the enforcement operations have been ended. ‘Distance halo’ is defined as the distance over which the effects of an enforcement operation last after a driver passed the enforcement site.

Physical policing

With regard to time halo, the effects of physical policing methods vary largely, ranging from effects lasting 1 hour to up to 8 weeks after the police activity has ceased [15]. It was also found that less than 6 days of police activity at a given location will have little or no time halo effect after the enforcement effort has been stopped. With regard to distance halo, Elliott and Broughton [15] conclude that:

• The effects of visible and stationary policing on driving speeds are halved for every 900 metres downstream of the enforcement site
• The effects of police presence on driving speeds typically last between 2.4 and 8 km
• Police enforcement can have considerable larger distance halo effects (e.g. up to 22 km), if visible, stationary enforcement is used randomly on a large part of the road network, as such suggesting a large scale enforcement effort and high unpredictability of checks.

Speed cameras

• A few studies indicate that 500 metres is about the maximum distance halo of a speed camera. More studies, however, indicate a much larger distance halo effect, even up to 10 km [15].
• Larger distance halo effects seem to be associated with physical policing, especially if the policing method is ‘randomised’. The minimum distance halo effect found at speed camera sites (500 m) is almost five times smaller than the minimum distance halo effect of physical policing (2.4 km) [15]
• When the roads are checked by 24-hour operating speed cameras, the effects on speed and safety are larger in sections within 1 km of the camera site than sections within 2 km [34].
• When visible and invisible mobile camera operations are used the effects are more widespread over the road network [39]
• The effects of visible camera operations along the road side tend to dissipate after 3 days [14].