despite all the theory about leverages etc, occasionally properly applied and seemingly perfectly working handbrakes do let vehicles roll forward, see my previous post about friday morning for one example, imho leaving the vehicle in gear on a slope is a no brainer.
tractors and ride on lawnmowers have had cluch or handbrake interlocks fitted for decades.... they can be a pain in the posterior when the switches and wiring are encased in years of accumulated mud and other materials but generally do what theyre supposed to.
