You can take the city bus as far East of El Cajon as you can, then there's a grocery store / strip mall right in front of the highway. The onramp there on the I 8 has plenty of space for cars to pull over.
It's a short (60 MI?) hitch to El Centro, where you can catch a city bus up to Niland for a dollar, then you're a 3 mile walk to Slab City if you're interested in that. The bus out of El Centro only leaves like 2-3 times a day so you might have to wait a few hours for it.