A regional system of high-quality buses (like tour buses) along the Front Range sounds like a viable alternative.  It's a relatively compact region of large and medium cities.  Busses can change routes in response to population shifts.  If you think about the massive amounts we pay as individuals (car payment, gas, insurance, repairs and maintenance) and a society (massive land use for roads and parking, urban sprawl, pollution, stress-filled commutes with road ragers) we pay for our car habit, maybe, just maybe there is a better way.