A few comments:
In regards to those who ride mountain bikes on the road regularly - get some slicks for it. A huge increase in efficiency (not as much as a true road bike and wheelset, but the next best thing), and is much easier to pass off to a significant other than a whole new bike.
Ideally if you have a bike with discs, a spare set of wheels and tyres for quick changes for when you want to hit the mud, if not then new tyres and a decent pump so you can switch them over in a matter of minutes.

As for red lights and general naughtiness - bikes are required to follow the laws of the road, full stop. I may disagree at times and wish I could keep momentum when faced with an empty road or a pedestrain crossing with nobody on my side, but if they allow fair use of ignoring the laws, then the arses will take advantage as usual and continue to ride dangerously and put others in danger (getting hit by a bike travelling at speed sure ain't nice) - if they police catch you, you *will* be fined £30 for committing such an offence, as found out by various friends of mine.
If you do want to get a head start, hopping off the bike and pushing across a pedestrian signal is acceptable (as pushing a bike you are of course counted as a pedestrian), and places you on the other side of the junction and moving by the time the cars get to you.
(442, Sat 24 Sep 2011, 14:06, 1 reply)

Re using a mountain bike on the road...
...some tyres roll better than others, but I can't be bothered with swapping out wheels each time I choose to ride on the road or offroad. Besides, the physical challenge keeps me fitter.

For the bike geeks out there, a Maxxis Ardent on the back and a Nevegal on the front roll acceptably well and are also great three season off road tyres.

The lower gearing of a mountain bike means that you can normally get away very cleanly and quickly when the lights turn green too.
(Bicycle Repairman "you're also a bit of a wanker", Sun 25 Sep 2011, 11:46, closed)