Until relatively recently, roadside emission testing coupled with enforcement was restricted to the former Vehicle Inspectorate (VI). However, under the provisions of the Environment Act 1995 this role was extended to any local authority declaring an Air Quality Management Area (AQMA). The Department for Transport provided a fund of eight million pounds to contribute to the cost of roadside vehicle emission testing of light-duty vehicles (LDVs) within AQMAs for a three year period to the spring of 2004. The fund provided local authorities with the opportunity to test vehicles either within, or on roads leading to, an AQMA under the Road Traffic (Vehicle Emissions) (Fixed Penalty) (England) Regulations 2002. The overall aims of the roadside testing were to determine vehicle compliance with the MoT emission standards, and to raise public awareness of the issues concerning polluting vehicles.
Against this background, the BOC Foundation commissioned the National Society for Clean Air and Environmental Protection (NSCA) and TRL Limited to undertake a review of in-service emission tests for LDVs, and specifically the roadside emission testing activities undertaken by UK local authorities.
Data from the MoT and the VOSA roadside test programme indicate that emissions failure rates are tending to decrease with time, and failure rates for private cars and light goods vehicles now tend to be less than 5 to 6%. However, for diesel taxis the failure rate appears to be increasing towards 10%. Within the local authority test programme, the lowest failure rates were associated with petrol catalyst equipped cars (3%), while the highest overall failure rates were associated with the petrol non-catalyst cars (8%). The proportion of test failures generally increased with vehicle age and mileage accumulation. Failure rates amongst diesel vehicles were between 6% and 7%. The targeting of specific vehicles during roadside emission testing could help to increase the rate of identification of high-emitting vehicles.
The costs calculated for the local authority roadside testing, at between around £70 and £90 per vehicle. These costs included capital expenditure, staff time and training, as well as scheme management, publicity campaigns, data archiving and reporting. When publicity costs were excluded, the average cost per test under the local authority testing scheme reduced to £36.
The introduction of a random roadside emission test programme could reduce CO emissions (in g/vkm) from petrol vehicles by between around 10% and 20%, and HC emissions by between around 5% and 20%, under the premise that all high emitting vehicles are rectified. Changes in NOX emission associated with roadside testing would be minimal. These changes in emissions would tend towards a relatively small improvement (a few percent) in ambient CO and HC concentrations near to the roadside.
However, the value of roadside testing is not just the benefit from the repair of a few faulty vehicles

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