In reply to Loughan:
Done it a few times years ago, too scared these days!
The time I remember best was coming round a roundabout and getting tucked in behind a lorry going up a long incline. I hadn't got a cadence meter on the bike but my speedo showed me doing over 30mph at the top of the hill
I was pedalling like crazy and didn't want to take a hand off to change gear - it was in the days of shifters on the down tube.
These days, drivers are just too erratic and don't drive smoothly enough to do it effectively.
ALC