Some good points have been made about diesels and air quality, and I am receiving constituents emails arguing against new penalties on owners of modern diesel cars.
One of the best points made is we need to take into account the amount of use made of various categories of dirtier vehicle. A typical privately owned passenger car spends most of its time parked. A motorist who averages 8000 miles a year, and averages 25 mph through a mixture of open road and congested town driving uses the vehicle for just 13 24 hour days or 26 12 hour days equivalent. A public service vehicle like a bus may well operate for more than ten times that amount of time, over 260 12 hour days a year. That means we will get a far bigger saving of dirty exhaust if we replace the old bus than the old car. The same is also true for many diesel trains that operate long hours, and for diesel delivery vans and lorries.
It is also important to recognise that congestion and delay cause far more pollution than allowing vehicles to make optimal progress at decent cruising speeds when the engine is not labouring, is in an economical gear, and not having to stop and start. This argues for the adoption of more policies that can reduce congestion, as have often been discussed here. Improving junctions is central to this. Parking more of the cars that are not in use off the highway is also an important aim, as often parked vehicles cause congestion and delay through straddling the highway.
Someone pointed out that vehicles often do not achieve the test specifications on emissions. This is because actual drive cycles are often different from test cycles. The more the vehicles have to slow down and speed up, and sit in traffic, the worse the emissions performance is likely to be. Older vehicles do not have cut outs at traffic light and other stops. Trains often keep their diesel engines running whilst waiting for considerable periods of time at terminus stations and to adjust service times. These are matters which newer vehicles and engines can help address.
A clumsy new tax is not the answer. Cutting emissions requires much detailed work on driving needs and conditions, road space and junctions, and ages of different types of vehicle. It is certainly important for the state to start by tackling public service vehicles, as they do so many more miles than the private car.