Foxbat--thanks for that information-we've seen so many do the "undertake" that we were thinking it might be legal !
There's a 2 lane section of the M11 just past Stansted in u.k. with a lot of long uphill stretches--it was a favourite of the big rigs to try and pass each other- but due to speed governors they would sit out there in the outside lane and collect a fine jam of demented car drivers behind them.
If I was in the mood it was fun to get on the inside lane and set the cruise control to 58mph-invariably faster than the cars on the outside lane, so I slowly overtook them all the way up the hill. No fun to be had anymore as they've put up signs asking HGV to stay on the inside lane..
Thanks also for the mention of the slip road cash generator speed traps, you've just added to my paranoia about driving in spain--I can't relax driving hire cars anyway-and now I've got that to worry about.