Measures to control emissions from in-use vehicles
are an essential complement to emission standards for
new vehicles and are the subject of chapter 4.
Appropriately-designed and well-run in-use vehicle inspection
and maintenance programs, combined with
remote-sensing technology for roadside screening of
tailpipe emissions, provide a highly cost-effective
means of reducing fleet-wide emissions. Retrofitting engines
and emission control devices may reduce emissions
from some vehicles. Policies that accelerate the
retirement or relocation of uncontrolled or excessively
polluting vehicles can also be of value in developing
countries where the high cost of vehicle renewal