You know the screw on caps for Presta valves that everybody always discards? It turns out there’s a use for them.
With a sharp knife, cut off the end of the cap. Screw this cut cap onto the valve and — presto! — you have a quick and dirty Shraeder adapter. This video shows how it’s done.
Via zillions of people, most recently by iMinusD in San Jose.
And with most gas stations in CA charging for this service and the air compressors set to pump up to 35-40psi max, this and 3 quarters for will get you just as far as your next pinch flat.