[lotsofpulp]

https://en.wikipedia.org/wiki/Induced_demand

If you have 10 cars wanting to use a stretch of road capable of carrying 10 cars at 8AM at speed X with the safest distance gaps, and you add 10 more cars, then there will be some combination of congestion behind this stretch of road, the distance gaps getting smaller, or cars driving faster now that there are 20 cars.

If you add a lane so that 20 cars can travel at the safest driving distance gaps at 8AM at speed X, but this then causes 10 more cars to come on the road because they see the possibility of using a road that is maybe just under maximum capacity, then you now have 30 cars on a 20 capacity road, and once again you end up with previous situation as 20 cars on a 10 car capacity road.

Obviously these things are impossible to exactly predict and constantly in flux and so engineers and politicians and drivers are all crudely estimating the shifts of supply and demand curves, and the overestimating and underestimating will cause congestion, or complaints of excess road, etc.