This flashing brake light circuit can be used for motorcycles. When brake-light switch S1 is closed, power is applied to U1 and U2. Two inverters of U2, a 4049 hex inverting buffer, are connected in a low frequency oscillator that feeds clock pulses into U1, a 4017 decade counter/divider. Outputs 0, 2, 4, 6 and 8 of U1 are coupled to the gate of Q1 through a 1N914 diode.
As the 4017 counts, it turns the brake light on and off four times and then leaves it on until the brake switch is released. The on/off rate can be set by potentiometer R2.
For best results, the on/off rate should be set so that it is rapid.