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A light in the darkness - daytime car lights


One of the areas of health improvement which has been targeted in recent years is that of preventing injuries. A bulletin from R&D Wales [1] has gathered together a mass of evidence on injury prevention strategies and categorised it in terms of the strength of evidence and whether interventions are beneficial or harmful ( http://www.uwcm.ac.uk/uwcm/lb/pep ). Bandolier 's eye was drawn to the question of the use of running lights on cars in daytime, and a meta-analysis [2] which might explain why all those Volvos and Saabs irritatingly have their lights on all the time.

Meta-analysis


This drew together all the known studies on the use of daytime running lights (DRL). Some of these were randomised (some cars had lights on, others did not, usually from a fleet of cars), some were before-and-after studies with a comparison group, and some were simple before-and-after studies, for instance when a new law came into force.

The distinction was drawn between the risk of accident for an individual car using DRL, and aggregate effects on the total number of accidents. The outcome was accidents occurring in daytime between cars, or between cars and pedestrians or cyclists. This makes sense, since at night-time all cars should have lights on, and use of DRL should make little difference for single-vehicle accidents.

Results


The design of studies seemed to make little difference on the magnitude of the reduction in accident rates for individual cars (Table 1). The estimate of the mean effect was about a 15% reduction in accident rates whether studies were randomised or not. There was also data showing that variability in accident rates between studies was found predominantly in small studies - only to be expected really - and that large studies gave consistent results.

Table 1: Effect of study design on accident reduction rates
Study design Percent reduction in accident rate (95% CI)
Randomised 15 (32 to +6)
Before-and-after with comparison group 18 (28 to 5)
Simple before-and-after design 14 (16 to 12)


There was also good agreement between the estimates of reduction in accident rates for different types of accidents, and overall, between individual car studies and those on aggregate effects of accidents (Table 2). The one point of disagreement was for rear-end collisions, where the aggregate data seemed to suggest a small increase while the individual car estimate was for a small decrease.

Table 2: Effects of DRL on accident rates for multiparty daytime accidents
  Percent reduction in accident rate (95% CI)
Type of accident Individual car Aggregate estimate
Front or side impact 13 (16 to 10) 13 (14 to 12)
Rear end collision 16 (21 to 11) +3 (0 to +6)
Pedestrian accident 25 (38 to 8) 20 (22 to 18)
Cyclist accident no data 6 (9 to 4)
Motorcycle accident no data 20 (23 to 18)
Type not stated 14 (20 to 7) 7 (9 to 4)
Mean of all types 14 (16 to 12) 12 (13 to 11)


The beneficial effects of DRL were maintained whether the proportion of cars using DRL was 30 or 50% at the start of a study, and from 60% to 95% at the end. There was a weak effect of latitude. Higher latitudes may mean more 'low sun' conditions, or longer twilight, where DRL may be more effective at reducing accidents. Since the aggregate of about 15% reduction was across all latitudes, it might be that in the UK DRL could be slightly more effective than the average.

Comment


This is well done review, thoughtful, detailed and impressive. It makes a good case that using daytime running lights right now in the UK should reduce the effect of an accident for an individual car of about 15%. Accidents are not that uncommon, and this is an appreciable reduction. Bandolier has started using lights in the day.

There is also a lesson for public safety. In the UK there are many thousands of injuries caused by traffic accidents. Universal use of DRL could effect a reduction of perhaps 10%, with perhaps the same sort of reduction in death and injury. Simple 'back of stamp' economics would indicate that the cost of universal DRL in the UK might be a one-off £130 million for conversion (£5 per car at a service?), and trivial amounts thereafter. Putting that against a 10% reduction in road traffic deaths and injuries makes it an effective intervention whose cost may be offset by lower insurance premiums. Must ask the garage and the broker.

References:

  1. R Lyons et al. Injury Prevention. Health Evidence Bulletins Wales, September 1998.
  2. R Elvik. A meta-analysis of studies concerning the safety effects of daytime running lights on cars. Accident Analysis and Prevention 1996 28: 685-94.



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