I stumbled upon this paper
from the Taxpayers Alliance and the 'Drivers' Alliance', which seem to be one and the same rightwing thinktank with close links to the Tory party. In it, they claim that road spending gives better value for money than rail spending, on the basis that many more journey kilometres are made by road vehicles than by rail, despite the government spending being roughly equal. They further claim that road users pay more in tax (fuel duty and vehicle excise duty adding up to £30.3 billion) than the combined total of road investment (£8.3 billion) and the cost of carbon emissions due to road vehicles (£3.54 billion).
This is rather silly. If you're comparing the relative cost-efficiencies of two systems, you would need to take into account not just the government subsidies, but also private outlay by individuals and companies. This would include total fares and freight charges for rail, and for roads would include the cost of all fuel, insurance, servicing and depreciation of vehicles.
Let's overlook this basic error, however, and look at what the real cost to the government is. Clearly, there's a lot more costs involved in the road system than just infrastructure. There's the cost of policing the road network for one. There's the massive costs of disability benefit, lost taxes, NHS treatment and so on accruing from the annual 30,000 road crashes in which people are killed and seriously injured. There's the 5000+ lives lost annually to pollution, much of which is caused by road vehicles.
Let's overlook that as well. The premise of the paper is that road infrastructure spending gives better value for money than rail. They give no evidence to support this. The years of predict-and-provide road investment show that building more roads doesn't reduce congestion. At best, it just moves the congestion elsewhere. At worst, it simply generates more journeys and creates more congestion. There is plenty of evidence that roadbuilding gives spectacularly poor value for money. The 27-mile M6 toll road cost around £500M at 1998 prices.
Let me give an example of what can happen when you build a road. We'll wave a magic wand and widen the M4 between Reading and Chiswick. Cars would get to the traffic jam at Chiswick a lot quicker, and wait a lot longer there. So let's build a new motorway onwards from Chiswick to central London. We'll build a massive car park at the end. Now people can get from Reading to central London quickly, and more cheaply than by train. Sounds like a no-brainer, so a lot of people will now take that option, and pretty soon, the new motorway will be at full capacity, and there will be massive tailbacks every day. Bear in mind that the peak capacity of a single motorway lane is 2,100 vehicles/hour, so 6,300 vehicles per hour per carriageway for a 6-lane road. A single light-rail line can carry 20,000 passengers per hour. You would need a road 18 lanes wide to achieve the same capacity.
The fact is, the whole transport system is both interconnected (many people drive to a station and take the train) and interdependent - if you change one transport option you will affect the other. If you make rail unattractive by increasing fares, reliability, journey time or through overcrowding, passengers will switch to using the car, until the level of congestion on the roads makes driving equally unattractive. In other words, if you invest in rail, you're investing in road. It's a networked system - you can't take a reductionist approach and consider one element in isolation.
The biggest error the TPA make however, is to assume that roadbuilding is actually possible. In many of the most congested parts of the UK (like London), there is no more land. People don't want rat-runs through residential areas any more than they want new roads built through the countryside - both damage quality of life on so many levels. I've never understood the idea that building roads is a 'Conservative' thing to do. It helps destroy the things we value most in this country. While most people accept that ease of travel is essential to quality of life, there comes a point where we all have to consider: is your journey really necessary?