Since my commute is over 20 miles one way, thanks to the advice of others here, I've been investigating using Park and Ride lots. However, the purpose of these lots is to catch busses (in Minneapolis, at least). Is there any problem parking there and riding my bike instead? Do they check at all or would they tow cars that do not appear to be using the lot for the intended purpose? I'd hate to ride back to find my car missing!

Not sure if it's the same everywhere, but here in Seattle metro area people use the PnR lots for bi-modal commuting all the time. Before I built my endurance up to doing my entire commute distance, I used to drive to the PnR lot halfway to work, then ride the rest of the way. Never had any problems.

The official answer is probably on a web site for whomever operates the P&R lots. But you are parking & riding. And around here, the P&R are used by carpoolers as well as bus riders (and drive/bike/bus commuters). I'd not worry about it.

I'm sure it depends on the locale, but here, the NFTA covers "transportation". The Park and Rides are administered by them, for use in multi-modal transport.

Bike-Bus
Car-Walk
Bus-Car
Walk-Bus
etc etc

Another option, one that I use, is to leave your car in the parking lot of one of the big box stores like Home Depot, WalMart, etc.

Everywhere that I've been the purpose of Park&Ride lots is to reduce the number of cars on the road - use of mass transit, carpooling, bike/car, bike/bus/car, etc. have all been legitimate uses of the lots.

OTOH, I'd be more concerned about using private parking lots at stores. Many of them are ok, but in areas where parking space is limited they sometimes enforce parking restrictions for customers only and may have cars towed.

Big Box store parking lots are huge! I'm not talking mom n pop.

Not all of them, it really depends on where you live. I am in Los Angeles, and there is not the case for many of them. Growing up in Iowa, it was the case for all of them.