A new assessment of the likely effects on road accidents of adopting SDST (TRL Report 368)

  • Published: TRL, 1998
  • Authors: J. Broughton and M. Stone
  • Date Added: 18 Mar 2013
  • Last Update: 31 Oct 2016
  • Format: html


The aim of the work described in this report is to explore the effect of the level of natural light on the incidence of casualties, and to predict the effect on casualty totals of altering the system of timekeeping in Great Britain.


An important conceptual advance, relative to the 1989 TRRL study, lies in the use of trigonometrical equations to calculate the altitude of the sun at any date and time for any point in the country. The main part of the report describes two separate studies which have been made of the level of natural light on the incidence of casualties. These studies have been conducted in parallel, embodying different assumptions and techniques and exploiting the varying altitude of the sun in different ways. The results achieved by the two studies are independent, except that they use the same body of data.

Key Findings:

  • Proposals to amend the system of timekeeping have a long history in the United Kingdom.

  • At present, clocks follow Greenwich Mean Time (GMT) from October to March and are set forward one hour to Summer Time, i.e. GMT+1 hour, from March to October.

  • In recent years the case for adopting Single/Double Summer Time (SDST) has been advocated: with SDST, clocks would be set to GMT+1 from October to March and to GMT+2 from March to October, so that with respect to the clocks the sun would rise and set one hour later than at present throughout the year.

  • Two alternative statistical models have been used to analyse RTI data for Great Britain for periods between 1969 and 1994 to investigate the effect of darkness on the number of casualties. These show that darkness leads to more casualties, and that the effect increases with casualty severity.

  • The adoption of SDST in Great Britain would transfer an hour of daylight from the morning, when there are relatively few casualties, to the afternoon and evening when there are more. It is predicted that this would reduce the number of people killed and injured in RTIs.

  • The estimates of the reduction in the number of deaths per year range between 104 and 138, depending upon the assumptions made.


Casualty reductions, Modelling, SDST.


In-depth statistical research.