Not common although it has happened to me.
Travelling to college one morning with 3 mates (in my trusty

Y reg Fiesta) the engine suddenly cuts out so im free wheeling down the road and a car coming the other way suddenly frantically flashes his lights and points at the front of the car. I think whats his problem and suddenly notice my bonnet start to go black and smoke pour from under it. I look for somewhere to pull over while 3 mates try to exit a 2 door Fiesta at 20 mph all at the same time out 1 door! I pull into a pub car park the fire brigade turn up and extinguish the fire and whats left of said trusty Y reg Fiesta. They informed me a petrol hose must have split causing the engine to stop and the fire to start.
The real interesting part would have been if it had cut out a few seconds earlier I would have been able to free wheel into a petrol station (thinking I was just out of petrol) with a car that was a fire ball a few minutes later!

how common is it for a car to catch fire whilst driving along?
it's happened to me today for the second time in a few months, I assume this isn't normal and I just run bad at cars