repetition frequency is a direct measure of the relative distance to the light source since the light incident on the source from a source at a distance is known to be approximately inversely proportional to the square of the distance further the time rate of change of the pulse repetition frequency or its first derivative is also a measure of the velocity of the light source relative to the present system an important aspect of the invention is then that the drivers of approaching motor vehicles may see a flashing light to clearly identify the location of the bicycle 10 further the rider of the bicycle is provided with a flashing light 22 which indicates to him that a motor vehicle is approaching by watching the rate of flashing of the light the rider of the bicycle may be provided with an indication of the closeness of the motor vehicle and the speed of the motor vehicle the lights 22 and 24 are particularly located so as to be on a direct line of sight to each nearby driver of a motor vehicle the photosensitive resistor 42 is connected to the circuit such that when the general ambient light level is at least as great as the light incident upon the photosensitive resistor 34 the flash rate of the oscillator circuitry is reduced to zero in this state the system is converted to a very low power consumption state with power loss occurring only because of the resistances of resistor 44 and the interbase resistance of transistor 38 the system is not thus required.