Another possibility might be to use multiple solid state laser transmitter/receiver pairs located close together, kind of like ink-jet printer nozzles. Because

Another possibility might be to use multiple solid state laser transmitter/receiver pairs located close together, kind of like ink-jet printer nozzles. Because they're discreet, you would lose some resolution compared to a continuous scan system, but it might be OK for the purpose of staying in a road. And, since the vehicle is moving (most of the time) anyway, maybe multiple returns can be integrated something like a synthetic aperture radar system to form a more complete picture.