How come I keep wanting to fall asleep on the expressway? I get enough sleep everyday!?

several medical conditions could cause get yourself checked out

sleep apnea - could be cause

It's very common. The brain gets bored and in a daze. Keep yourself occupied.

Windows down. Radio up and you singing. Have a snack next you like beef jerky to keep you occupied. Chew on ice or something similar.

WHY because you do.
Suggest you have a medical check to determine the reason.

Maybe your car has a leak in its exhaust system.
Of course you know your exact need for sleep and always have the correct amount at all times and never need an alarm clock to wake up.
There is no magic number of hours of sleep for EVERYONE.

You have not said if you get sleepy after ten minutes on expressway or ten hours.

If it is the latter then Obi Wan Knievel has that answer.

It's called road hypnosis or highway hypnosis, commonly known as white line fever. And trust me, it's not just you.

It's a combination of retinal fatigue, mental fatigue, the sensations of constant movement, the hum of the tires mixed with the rhythm of the engine, and those broken white lane markers zipping at you for hours on end. White line fever has been around as long as the white lines have.

What you need to do is leave earlier, and take more breaks while driving. The leaving earlier will compensate for the more breaks.

I drove medium distances for a living for a few months, and I've been on more than a few dozen half-day and multi-day road trips, so I know what you're talking about. And unless you take those breaks every couple of hours, you will (not might, not could, will) succumb to the white line fever. You don't want to do that.

The breaks don't have to be long, but they do have to be every two hours. For every two hours on the road, you need at least (minimum) ten minutes of break time. Break time means get out of the driver seat and do something / anything else for ten minutes. Drink a coffee, smoke a cigarette, get a BJ, take a shower, write a letter, compose a sonnet, whatever. As long as it isn't driving, it's a break from driving.

Narcolepsy. Get a sleep study. Narcolepsy is much more common than is realized.
Do you snore? You may have obstructive sleep apnea (stopping breathing while asleep). It deprives your brain of oxygen, makes poor sleep quality even if you get enough hours.

Drink some coffee before you kill somebody.

we will never know

It is boredom.