"Classical" filament lights are 6V/2.4W/50 Ohm and 6V/0.6W/15 Ohm (front and rear light respectfully). When connected in parallel they account as an equivalent 12 Ohm resistor.
As a 12 Ohm resistor they draw 500 mA approx and that why the voltage drops to a nominal value of 6V (at 20 km/h).
Human body has a much higher impedance and the voltage will not drop at all!!!