Expressways are known as controlled-access roadways because:

The explanation as to why expressways are known to be a controlled access roadways because of the fact that they are designed to allow vehicles to safely travel in a controlled and fast speed by which it has various terms around the world.

Vehicles move in a much higher speed than usual when they're in express ways. Express ways are meant to provide a faster option for vehicles which have a very distant place as a destiny that would most likely take a ridiculous amount of time if combined with traffic and traffic signals. Since cars move really fast inside an expressway, getting in one at the middle of it might be quite dangerous already. Therefore, the access points must be carefully controlled for safety issues, hence the name: "controlled-access roadways".

a is the final ! : )