Selecting a Vehicle Sensor

i am student of polytechnic college. I have selected the topic automatic control of headlight in the opposite vehicle. My idea is to have have sensor(both emitter and receiver) on each vehicle. When either one or both vehicle as switched ON the headlight it will get a signal from the sensor and cut the circuit of headlight. And activate normal light system in vehicle. SO which sensor should i use for measuring the light of the opposite vehicle at a distance of 40mts? And should use logic gate or relay system for breaking the circuit???