To determine if changing to white light street lamps improves road safety: A multilevel longitudinal analysis of road traffic collisions during the relighting of Leeds, a UK city

Large-scale installation of broad-spectrum (white) road lighting has been claimed to bring about a substantial reduction in road traffic collisions (RTCs). This confirmatory study estimates the effect on personal injury RTCs of a relighting programme that installed nearly 80,000 new white lamps, between the years 2005 and 2013, throughout the large UK city of Leeds. Time series of weekly RTC personal injury counts in 107 areas, within the city over nearly 9 years when its road lighting was almost completely relit, were analysed, using multilevel modelling. The background change in each area when and where no lighting was being changed was separated from that associated with when and where new replacement white lamps were installed by including a polynomial for the underlying time-trend. The key interest is how the installation of the new lamps affects the ratio of the rate of collisions occurring in darkness to those occurring in daylight. The measure sought is given by the daylight adjusted darkness collision rate ratio (CRR) for the specified amount of relighting, that is the factor by which the daylight adjusted collision rate changes by the specified change in lighting. The daylight adjusted darkness collision rate ratio (CRR) has a point estimate of 0.990 and a 95% confidence interval, CI (0.971, 1.010) for 100 replaced lamps; representing a range between a 3% reduction in the collision rate to a 1% increase. Using the series truncated at the end of 2011 that seems more trustworthy, gives a revised point estimate of 0.993 and a CI (0.971, 1.015). Both CIs include one, therefore no effect on road safety was detected. The CIs for any other number of lamps installed will also include one. Typically, the plausible range of road safety impact brought by the new white lamps for a typical area with 700 lamps, roughly spans 20% reduction to a 10% increase for the daylight adjusted darkness collision rate. No convincing evidence was found for an improvement (or detriment) in road safety by relighting with white lamps, despite the extensive, city-wide installation efforts and associated costs.