There are a lot of drivers today who need to be taught, it seems, that when they take their foot off the accelerator, their cars will naturally slow down. Too many times I see phantom traffic jams caused by inattentive or inexperienced drivers. The M25 around London is constantly full of these fools.
So what's a phantom traffic jam, you ask? Simple - it's a clogging of the motorway for no reason. Phantom jams are caused not by accidents, road works or any other physical reason but for a mental reason. Someone will come up a little fast behind a car in front, and rather than simply taking their foot off the accelerator, they'll brake a little. Their brake lights come on and the person behind them is then under the impression that they're slowing down properly, as oppose to adjusting their speed. So they brake too, only a little harder. Not much - 2 or 3% maybe. Multiply this scenario back up the line of traffic and eventually you'll get to one car that comes to a complete stop for no reason. Even if that's only for a second or so, it's enough. The phantom traffic jam is born and within minutes, the whole 3 or 4 lanes will be parked.
This java applet shows the effect wonderfully : Traffic Flow Simulator. Choose the ring-road simulation and after a few seconds you'll see a phantom traffic jam form and start to crawl backwards around the road. In this case caused by a single car that slows down fractionally before pulling out from behind a truck. This is exactly what happens on the M25 all the time.
So come on - try not using your brakes from time to time - traffic will flow much more freely.