Stuck in the slow lane?

I've been unlucky enough to get a couple of buses - most often, the relatively reliable 38 - where the driver has been unwilling to drive faster than 20mph. Last night, we crawled from Tottenham Court Road, through Holborn and Islington, to my patch in Hackney at a snail's pace. I could have run faster, to be honest... but what the hell are the drivers playing at? Twenty miles an hour? Give me a break? The roads weren't even busy.