Here, they've put traffic lights on the feeder roads to motorways, to thin out the cars entering. Haven't worked that one out yet. There isn't much of an alternative route.
It's called "ramp metering" - which pretty well explains it. If the motorway is to congested, allowing more traffic to force it's way on simply slows the whole traffic stream. Restricting the number of vehicles being added to the flow is about the best solution.