Ceri JC

I believe the speed limits (and the theoretical braking distances) are also massively flawed in the way they are (with a few exceptions for lorries/caravans on motorways) not specific to vehicles. The differences in braking distance for a superlight like a Caterham compared to an Austin Allegro is massive. Similarly, handling, width of the vehicle and even acceleration play a part. When driving, most people take these factors into consideration (I don't bomb down narrow alleys in a wide van the way I do on a bike/a small car). Speed limits don't take these things into consideration (nor would it to realistically be able for them to do so).

I believe this is why many double white lines are irrelevant to bikes, they are in place because 90%+ of cars on the road could not perform a safe overtake there, whereas a lot of bikes could.

There's also the practical considerations to consider- we couldn't have speed signs every 20 yards (which is probably around about the frequency with which changing road conditions, let alone traffic/pedestrians etc. would cause the maximum 'safe speed' to fluctuate). Aside from the cost of implementing it, it'd be massively distracting to drivers. However, there are short stretches in some built up 30s where it's save to do 40, but then after that 50 yard stretch, it may only be safe to do 20. For bikes (and similarly, high performance cars) it might be worth quickly accelerating up to 40, then braking to 20, but again, for most vehicles (ie your bog standard car) it's not worth it.

I could go on, but it boils down to the need for traffic police, who would use reasonable discression, and pull people for dangerous speed when it exceeds the posted limits. Traffic police are quite capable of turning a blind eye to speeding when it's safe to so (they did to me, today).