[Zigurd]

Hard numbers seem to be hard to come by, but this article gives at least a qualitative balance of costs and benefits: http://www.telegraph.co.uk/news/uknews/road-and-rail-transpo...

What's interesting is the case for a subsidy based on reducing government costs when people are killed and injured. reduced insurance costs could also make the time-to-payback for individual purchasers very short.