Daytime running lamps executive summary

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Daytime Running Lamps (DRL): A review of the reports from the European Commission

Executive summary

The European Commission (EC) has published the results of research into the effectiveness, costs and benefits of introducing Daytime Running Lights (DRL). This research suggested that a substantial number of casualties could be prevented if this was introduced across the EU. Further, it showed a positive benefit to cost ratio when the costs of fitting lamps and the environmental cost of running them was considered, (i.e. benefit/cost ratios >1). On the basis of this research the EC has now entered a consultation exercise on how best to implement DRL.

In the past, a number of researchers have found flaws with some of the studies carried out into the effectiveness of DRL and some road safety groups are concerned that they could potentially have adverse affects for some road user groups, particularly for motorcyclists. For this reason, the DfT commissioned TRL to carry out a critical review of the research carried out for the EC in order to inform the DfT's response to the EC consultation exercise. The work has involved TRL experts reviewing specific parts of the research reports produced for the EC, comparing with other related research and carrying out a sensitivity analysis on the cost benefit model used in the EC research.

Overall, the research reported in the EC DRL reports represents a thorough and comprehensive analysis of the available data. Although it is possible to be critical of several specific aspects of the work, very substantial evidence has been presented that the introduction of DRL would result in a net casualty reduction effect. However, there appears to be greater scientific uncertainty concerning the size of the expected effect. Some of the parameters in the statistical analysis were not found to be statistically significant and should, therefore, be treated with some caution. In particular, the evidence for assuming a 15% improvement on fatal accidents is weak and it was considered that it would be more technically defensible to assume that a mean effect of between 3.9% and 5.9% (depending on which biases and assumptions are considered) applied to accidents of all injury severities and that there would be no effect on damage-only accidents.

The investigation of the effect of DRL for passenger cars on the conspicuity of vulnerable road users appeared, in general, to be a well controlled experiment. It is possible to criticise several specific aspects of the research but, in most cases, it was not considered likely that these would have substantially affected the main results and conclusions of the work. However, a few more serious concerns with this work were identified:

  • The conspicuity of motorcycles in the presence of differing intensities of DRL and different ambient lighting conditions was not investigated.
  • There was some concern that the photographic methods used may potentially not have replicated the real world environment sufficiently realistically.
  • The relative positions of cars and motorcycles that were evaluated by the work did not include situations at a junction where the motorcycle was approaching from the side and was positioned in front of a car equipped with DRL. All road scenes considered appeared to place the motorcycle to the side of the car such that daylight was visible between the two to physically separate them in the image.

However, when this research was compared with other experiments carried out in this area, it was considered that, if the limitations of scope of Interim Report 3 (Brouwer et al, 2004) were accepted, then the three studies actually presented consistent conclusions. These were that DRL with high light intensities could impair the conspicuity of motorcyclists but it was possible to design DRL that could improve the conspicuity of cars in the dim ambient light conditions of most relevance without adversely affecting the conspicuity of motorcyclists. This shows that it is very important that the technical details of the implementation of DRL are considered very carefully since it may be that a policy option which involved the use of existing passing beam headlights (or high intensity dedicated DRL) as DRL could have an adverse effect on motorcyclist conspicuity. Further research to assess the concerns identified above will be necessary to gain confidence that implementation of any particular DRL policy option would not have an adverse affect for motorcyclists.

There was very little evidence presented in the EC reports on the justification of the estimates of environmental dis-benefits in terms of increased fuel consumption and emissions. However, an independent assessment of those effects using a sophisticated computer modelling technique has suggested that the values of 0.5% to 1.5% increase in fuel use and carbon dioxide emissions appear reasonably accurate and possibly even slightly higher than justified.

The topic where the greatest scientific uncertainty was found was in the cost benefit analysis. It was noted that the cost benefit analysis presented showed benefit/cost ratios considerably in excess of 1. However, the confidence limits calculated reported for these ratios were not statistically significantly different from a ratio of 1. It had been assumed that the accident reduction effect of DRL would be 15% of fatal accidents, 10% of serious accidents and 5% of slight accidents. This review suggests that the relationship between size of effect and accident severity was very weakly supported by the statistical studies of accidents and it was considered more technically defensible to assume that the mean value of 5% (or perhaps 5.9%) applied to all injury accidents. It can be seen that changing the assumptions from the very weakly supported estimate of effect in relation to accident severity (15%, 10%, 5%) to the more technically defensible 5.9% mean effect for all severities has a very large effect on the conclusions, changing the benefit/cost ratios from substantially greater than one to substantially less than one (i.e. a negative cost benefit). In addition, this review found that the statistical analysis also acknowledged a publication bias in the data and analysis has shown that this would in fact reduce the size of the mean benefit by 2% to 3.9%.

The size of effect estimates discussed above were found to be crucial to the cost benefit argument in favour of DRL. It seems clear that in terms of the cost benefit analysis, considerable technical uncertainty remains.

One further issue that was considered noteworthy was the fuel cost assumptions. The analysis had been carried out using a cost per litre for fuel that excluded tax, which is the technically correct approach to an analysis of the effect on "Europe plc". However, if the increased cost to UK motorists were considered the fuel cost would be approximately four times that assumed in the cost benefit analysis.

In summary, the review reached the following conclusions:

  • There is substantial evidence that the mandatory use of DRL would provide a net accident reduction. However, the evidence concerning the magnitude of the effect and particularly the relationship with accident severity is considerably weaker.
  • The estimates of the fuel and emissions increases as a result of implementing DRL are reasonable and possibly slightly conservative (high).
  • The research into the potential of DRL on cars to impair the conspicuity of motorcyclists and other vulnerable road users was well controlled but limited in scope and did not consider some important variables. However, some consistent conclusions could be drawn which were that it should be possible to design dedicated DRL of low intensity (e.g. about 200cd) that are beneficial to the conspicuity of cars without adversely affecting the conspicuity of motorcyclists. However, DRL of higher intensity (potentially including standard passing beam headlights) could have an adverse effect on motorcyclist conspicuity in some circumstances.
  • There is considerable scientific uncertainty inherent in the values of the benefit to cost ratios presented in the EC work. The key variable is the assumption that the accident benefits would be considerably greater for fatal accidents (15%) than for serious (10%) or slight (5%) accidents. This assumption was very weakly supported by the available data and changing it to a more technically defensible assumption that the mean effect of 5.9% remained the same for all accident severities reduced the benefit to cost ratios to much less than 1 indicating that the costs would be greater than the benefits.
  • It was considered that it would be more technically valid to present a range of possible benefit to cost ratios within which there could be confidence that the true answer would lie, thus reflecting the technical uncertainty. The analysis showed that a ratio of 1 would fall within this range meaning that, although an accident reduction potential exists, it is not possible to say with certainty whether the benefits of implementing DRL would outweigh the costs.

Full Report

The full report is available in PDF format by using the following link:
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Reference

Report by I Knight, B Sexton, R Bartlett, T Barlow, S Latham & I McCrae

PPR170