It would be good to have a sensor that could see bystanders so we can avoid mincing them. It also might help with location sensing.
Ultrasonic sensors might be too problematic on the playa. Problems include motor noise, reflections off dust and the limited beam pattern of ultrasonic sensors requiring many sensors, sensors interfering with one another.
This ultrasonic sensor is cheap and has a wide beam. Maybe we only need one on front and one on back? Acroname has other similar products...