"park" gear engages brakes

When you place the gear shift into "park" it should close a small valve on the hydraulic lines going to your front brakes. Since presumably your foot was on the brake as you shifted into park, the valve traps pressure in the lines and keeps the car from rolling several inches until the pawl in the transmission engages into a tooth.

Even if the pressure gradually leaked out, the transmission would eventually engage to keep the car from rolling away, yet it would still hold the car long enough to prevent inconveniencing or possibly injuring your passengers if they happen to be exiting your vehicle just as you take your foot off the brake.

Nearly all cars today have electric motors and valves to regulate braking pressure by the ABS system. It might be possible to implement my idea on such cars by doing nothing more than modifying the firmware program in the ABS computer - no new hardware needed.

Sure you *could* just sorta use the emergency brake for the same result, but: 1) people are lazy and most don't apply the emergency brake 2) using the emergency brake every single time you park (especially if you actually apply it hard enough to prevent the car from rolling) stretches the wire cables it uses to activate the brakes. This reduces their effectiveness in the event that you really do need them (ie in an actual emergency). E-brakes can be easily adjusted to take up the inevitable slack in the cables, but few people get that done regularly.