I think the main problem is that stroads do not handle surge traffic well at all. They tend to end up sending blocks of cars 100-400 yards forward at a time, in intervals of 30 seconds to 2 minutes. With extra deadlocks happening when left turn lanes fill / jackasses block lanes when trying to sneak through lights. Highways at least can take measures to mitigate surges with feeder lanes, surge pricing, carpool lanes, etc.