David Cameron's motive in this is easily understood. He wanted to portray himself as a man of the people, sharing our traffic frustrations, etc.
I think most of us could understand that - it's a simple enough concept.
The problem lies in the fact that he hadn't thought it through sufficiently, or that he was ill-advised. Either way, the decision isn't world-shattering news, and will, I suspect, be reversed, once the Prime Minister understands the wider implications of his move. Everyone drops a few clangers during the first few weeks of a new job - let's not be too hasty to make enormous mountains from tiny molehills.