From 1887 through 2004, I traveled Interstate 15 from Downtown to Rancho Bernardo (about 24 miles). The good thing was that if you were driving away from the downtown area in the morning, and toward the downtown area in the evening, it wasn't too bad. Traffic is a headache going south from Mira Mesa in the morning, and going north to Mira Mesa in the evening. BUt now it seems like it's gridlocked and busy from 6:00 am to 8:30 am, and again at around 3:30-4:00 pm unitl 6:00 to 6:30 PM.
Here is a link to NBCSan Diego. They have real time traffic so you can get an idea of how traffic is like.
San Diego Traffic, Transit, Autos and Road Conditions | NBC San Diego
Hope you know how to drive.