Project: Rear Signal Lights: Commercial Vehicle Poor Performance

Reference: S270D/VD

Last update: 03/12/2003 14:48:52


To determine the causes of the discrepancy between the light output of rear signalling lamps on commercial vehicles in use on the road and the values specified for type approval, and to quantify the significance of the various causes.

To offer costed solutions to the problem and demonstrate an improved rear lighting system.


This project investigated the cause of poor performance of rear signal lamps on commercial vehicles and whether it was feasible to establish a test of the
performance as part of the annual roadworthiness test.

The poor performance of rear signal lamps on large vehicles is well recognised as a problem that compromises safety. Large, slower moving vehicles present a real hazard to other road users if the rear lights are difficult to see. Previous studies have indicated that, even when approved lamps are fitted to vehicles, they may not satisfy the minimum performance requirements when installed. In some countries, checks on the intensity of lamps are carried out as part of the annual roadworthiness test.

This project has confirmed that the in-service performance of the rear lamps on a significant number of vehicles is still below even the minimum requirement for the components at the time of approval. A brief investigation of these lamps on vehicles inservice indicated that low voltage at the lamp holder was the most significant reason for poor performance.


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Contract details

Cost to the Department: £96,250.00

Actual start date: 01 February 1998

Actual completion date: 31 December 1999


Vehicle Lighting: Rear Signal Lamps on Commercial Vehicles. Phase 1 Final Report
Author: BSI Product Testing
Publication date: 01/12/1999
Source: Contact:

Summary of results

  1. There has been, for some time, public concern about the inadequacy of the rear lamps on commercial vehicles. A roadside survey undertaken by the Transport and Road Research Laboratory in 1989 also found that the performance of rear lamps on commercial vehicles was poor.

    The purpose of the study reported here was to see if the situation had changed during the intervening time and investigate the possible reasons for any poor performance.

    The main part of this work is a survey of commercial vehicles taken from the road to measure the performance of their rear lamps. A suitable piece of portable equipment that would measure the luminous intensity of rear lamps was sourced and measurements on rear lamps using this equipment undertaken in the laboratory confirmed that it was suitable and quantified its accuracy.

    A survey of commercial vehicles was taken, measuring the luminous intensities of rear, stop and fog lamps by linking into the Vehicle Inspectorate's regular lorry checks on the road.

    The design of all rear lamps on vehicles undergo a component type approval test to European specifications that includes a measurement of the luminous intensity and therefore, at least when newly installed on a vehicle, lamps should meet specified minimum conformity of production light outputs.

    The survey showed that a high proportion of lamps on commercial vehicles on the road did not meet the required performance. Investigating further showed that on average cleaning the lamps increased the light output by 10%. However, by far the largest factor was the voltage measured at the lampholders. This was far lower than that specified for testing the lamps for their approval. Moreover, the voltage was lower the longer the vehicle with the average articulated vehicle giving over 4 volts less than the specified 28 volts. Supply of the correct voltage would increase the light output for articulated vehicles by 70% and by 40 % and 27% for heavy and light goods vehicles respectively.