17000 E 14th St San Lorenzo, CA 94541, USA
There does not appear to be any signage indicating that the onramp to westbound 238 is on 170th Ave. If I hadn't've had a map, I wouldn't've found it. The onramp is one (short) block up 170th, so "Freeway Entrance" signs on the corner would be inappropriate, but "to I-238" or something, ideally hanging from the traffic signal.
This intersection is in unincorporated Alameda County (called "Ashland" on most maps, but whatever), but because E 14th St is also state highway 185, this is probably under the oversight of the state.