My son is 28 now and lives down in Texas. He lived with us until he finished college at age 22. I didn't stop listening for the sound of his vehicle until the day he moved into his own place. I don't know if you will ever be able to get over the apprehension while they're still under your roof.
Even now, if he doesn't update his blog on a daily basis or if I don't see him posting on his own bbs I worry a little, just not like I used to when he lived at home. He's still single so is out in the evenings more than he probably would be if he was in a relationship. However, I expect he'll be spending more time at home since he's signed up recently to take additional college courses online.
We have a few years yet before our boys start driving and I am dreading it.