The 'main' roads end up getting backed up and then people naturally start drifting over to a bunch of side-roads to get to the destination. This then causes further traffic issues as the locations where side-roads intersect the main roads get backed up as people on the side roads try to merge into the main ones.

A solution ends up being closing some side roads to funnel the temporary traffic into the main thoroughfare while still allowing some local traffic through the non-closed side roads at the cost of some side roads being inaccessible.