Either way the root cause is the same; lots of vehicles
Actually, I don't think that is the root cause.
IMO, it's poor driving standards. I cannot count the number of times I've driven along a 3-lane motorway with some moron doing 60mph in lane 2, with nothing whatsoever in lane 1. That leaves substantially all the motorway traffic having to merge from three lanes into one.
And people aren't good at merging; there's this *need* to be one space ahead of someone you've never met before. So when two lanes merge together, both slow to a crawl or complete standstill. A little self-discipline, and *both* lanes would get through the merge much faster.
Then there's the bunch that leave huge gaps in front of themselves I slow traffic. I cycled across the Itchen Toll Bridge this morning, watching several drivers leave a good 30 yards ahead clear. This just pushes the queue backwards by that amount - and when a queue hits a junction/roundabout, it propagates up all the feeder roads to that junction. It doesn't take many people like this to cause significant congestion.
There are far more examples of crap driving, but those will do to get going with. TL;DR: our roads are able to deal with higher numbers of vehicles than they currently do, if only said vehicles weren't driven by people deliberately trying to stop all traffic.