All vehicles deteriorate in service and this can have an adverse impact upon safety and the environment. Roadworthiness testing exists to ensure that at least a minimum level of benefits in a vehicle’s original design and manufacture are retained in service. This study provides a high level overview of the likely impacts (if any) to road safety from changes to the MOT test frequency by vehicle age and time since last inspection. 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. Reducing the frequency of testing for newer vehicles will 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. Although the theoretical models 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.