PeterE wrote:
Assuming you leave at least a 2-second gap when pulling in or out, is lane-weaving every 11 seconds really a sensible behaviour?
Having given this further thought, it's more like 5 seconds than 11.
If you're travelling at 70 mph 100 yards behind a truck doing 56 mph, you will take 15 seconds to catch him. But you are travelling at 34 yards per second, so your two-second gap is two-thirds of the distance between you and the truck. The two-second gap is the time by which you will pass a fixed point, not a moving object.
Therefore, if you move into Lane 1 when you're 100 yards back, if you want to maintain your speed and pass the truck, you’ll have to move out again as soon as your distance from the truck has fallen below 68 yards, which will take you approximately five seconds (not 11 seconds as I said before). This, I would suggest, is not a good idea.
Every time you want to make a lane change, to do it safely you need to have a two-second gap both in your own lane and in the one you’re moving into. If you move into a smaller gap, then traffic behind will need to back off a little to achieve a safe separation distance, with the effect potentially rippling back along the road. A lane change effectively needs twice as much road as staying in lane.
It follows from this that, in busy conditions when there is a steady stream of traffic in all three lanes of the motorway, frequent lane changes serve to reduce the capacity of the road, and capacity is maximised by minimising the number of lane changes.
The thing that reduces capacity is not that drivers won't move over after having passed other vehicles, but that not enough car drivers are prepared to
stay in Lane 1 in busy conditions.
It may be difficult to grasp, but the rules that are entirely sensible in free-flowing conditions of moderate traffic actually serve to reduce capacity (and possibly safety too) in busy conditions. That is why the signs in the M25 VSL section say "stay in lane".