Project: Effect of Vehicle Defects in Road Accidents

Reference: FIL101N

Last update: 10/05/2011 14:29:13


The objectives of this project are to determine:
a) the prevalence of vehicles with roadworthiness defects in the UK vehicle population;
b) the prevalence of vehicles with roadworthiness defects in the UK vehicle crash population;
c) the frequency and nature of crashes caused by vehicle defects in the UK;
d) the contribution of MOT assessed vehicle defects to crash causation in UK; and
e) the contribution of MOT assessed vehicle defects to casualty injury outcome in UK.


This project aims to understand the extent to which passenger car, light vans (up to 3.5 tonnes) and motorcycle roadworthiness, particularly defects, is involved as a primary or contributing factor in crash causation and subsequent injury outcome.


TRL Limited
Crowthorne House, Nine Mile Ride, Wokingham, Berkshire, RG40 3GA
+44 (0)1344 773131

Contract details

Cost to the Department: £9,996.45

Actual start date: 08 December 2010

Actual completion date: 08 April 2011


PPR565 Effect of Vehicle Defects in Road Accidents
Author: R W Cuerden, M J Edwards, M B Pittman
Publication date: 31/03/2011
ISBN: 978-1-84608-950-0
Source: TRL
More information:

Summary of results

  1. The study has concluded that:
    - There is uncertainty with respect to the number of accidents which occur in the UK where vehicle defects are contributory. This is because no recent studies have been specifically undertaken to investigate these issues.
    - This study has estimated that vehicle defects are likely to be a contributory factor in perhaps 3% of accidents in Great Britain.
    - On average in 2009, approximately 40% of vehicles tested failed their initial (Normal) MOT test.
    - In general,
    - as vehicles age, the rate of MOT failure increases, for cars this reaches nearly 60% when they are 13 years old; and
    - the greater the cumulative distance travelled, the higher the rate of MOT failure, for example all cars which had driven over 90,000 miles experienced above a 50% failure rate.
    - There is no established link between MOT measured roadworthiness and vehicle defects contributing to accidents, other than the common sense approach, where the greater the number of defects, especially the most safety critical ones in the fleet at a given time, the greater the likelihood of accidents being caused, at least in part, by roadworthiness issues.
    - This study investigated the effect on road safety (if any) associated with a change to MOT testing frequency and found that the greater the distance between inspection dates, the greater the likelihood of adverse road safety consequences. Two different theoretical models were developed and used to provide an estimate of the magnitude of the number of accidents and casualties which may occur annually due to less frequent MOT testing.
    - The first model consisted of a prediction based on a hypothetical relationship between MOT defects in the fleet and casualties. The 4,2,2,2 option yielded the largest predicted increases, with an additional 1,200-2,200 accidents per year, 16-30 fatalities and 180-330 serious casualties, based on 2009 road injury statistics.
    - The second model was based on a prediction based on a comparison with the German roadworthiness testing experience. For change to a bi-annual inspection regime it was estimated that there would be a 1.65 % increase in the number of accidents and casualties which equates to an additional 37 people killed and 407 seriously injured.
    - Although both approaches are not ideal, largely due to a lack of data upon which assumptions have been based, they consistently indicated an increase in accidents and casualties. However, it must be stressed that these are estimates only and further work would be required before a genuine quantification of the scale of these adverse road safety impacts will be known.
    - It was not possible to quantify the nature of the likely impacts (if any) to road safety from changes to the MOT test frequency, with a transition to retest on the basis of miles travelled since last inspection, rather than time or combination thereof.
    - Although, on the data reviewed to date we believe the vehicle age is more important than miles travelled, partly because these two factors are related and partly because new vehicles which travel large distances are still likely to follow manufacturers‟ maintenance schedules and have regular service checks.
    - Reducing the frequency of testing for newer vehicles is likely to have adverse road safety consequences, but these would be substantially greater for older vehicles as the data presented in this report already indicates their high MOT failure rates.