And Jonesy will be along in a minute to show us why the journeys that matter are not long ones between cities but short ones within or into/out of them. .
I'd agree but London is a big place isn't it? Although the Busway goes for about 20 miles, it passes through 8 or 9 small towns / villages on the way many of which are only 5 / 6 miles out of Cambridge - about the length of a typical London commute I'd think? The main road going that way is dual carriageway with huge lorry traffic problems, you'd be suicidal to cycle on it and nobody does. You could use the back roads but very often the route would be much longer than going direct. Again I'd agree a longer, roundabout route is great if you're out for a ride in the evening / at the weekend but if you just want to get your head down and get to work ...