A study conducted for the BBC, "Secret Life of the Cat," tracked a number of cats and their roaming distances; while they varied by personality, the average cat roamed 40-200 meters (43-219 yards). Male cats, cats in more rural areas, and intact cats tended to roam even farther, with some roaming up to 3 km (nearly 2 miles) from their homes. (source: Petplan)
An Australian study equipped "volunteer" cats with GPS units and mapped their movements at night for up to 10 days (welfare of the cats themselves was given higher importance than all cats finishing the ten day study). While some of the cats did stay close to home, others were observed roaming quite far from their homes, as seen here:
(Image source: Central Tablelands Local Land Services)
More of the maps can be found in this article by Mashable.
While you may not have observed your cat roaming very far, the owners of the cats in the Australian study had not observed theirs roaming very far, either; at least one owner, after being presented with the map of their cat's activities, chose to keep their cat indoors at night after seeing the distance traveled.
If you wish to protect your cat from the highway, it's best to keep her indoors. Any number of factors could cause her to head across the highway, including chasing prey, fleeing a predator, looking for a mate (if she's intact), or trying to escape an overly aggressive suitor.