Not sure if this is the right place to ask, but why don't automobiles have some sort of immediate braking mechanism? Even one that plows into the ground to stop the car faster than if you just slammed on the brakes. If we were worried about abuse, it could be one time use before some costly replacement, so it would only be used in emergencies.
Simple answer - because that would be a very bad thing!
As Chenmunka says, look at Newton's Laws of Motion. Or, if you don't believe them, look at videos of crash-testing - you'll see what happens to the occupants of a car in a very rapid deceleration - it isn't nice... The car stops, quite quickly, but you don't, until you hit a solid thing (like the dashboard or steering wheel) - this is why we have airbags. Then your internal organs keep going until they smash into the inside of your ribcage, which also is a bad thing!
Lets ignore the aforementioned pitfalls of an emergency quick-stop system, and instead try to design one. (For realistic answer, see option 4)
- Option 1: Pneumatic Piston
Hollywood rollover/flip stunts are usually performed, in the absence of a ramp (though a deploy-able ramp that launches you over the obstacle isn't a bad idea..), by installing a large diameter pneumatic cylinder that fires a heavy steel piston (3-4" Ø) at high speed directly into the ground, which serves to vault the vehicle into the air.
Flipping the vehicle onto its roof in order to stop, while checking the 'one-time use' and 'expensive to repair' boxes, will probably not provide enough friction to stop the car faster than the tires would have, not to mention the air time involved, and reliability issues of takeoff/landing (see aviation.SE).
For that reason, we would want to install such a devise so as to fire forward into the ground at a shallow angle. Too steep an angle and the rod could penetrate the road surface, leading to what track & field experts refer to as the 'pole vaulting' effect.
Increasing the surface area of the piston in order to spread the force along the road might solve that problem. It may also be a good idea to make the end of the piston deformable, to gradually slow the vehicle before the piston really digs in. This system would also require a high strength yet deformable cage so as to avoid damaging the chassis of the vehicle when it literally stops you on a dime (USD).
- Option 2: Rocket Thrusters
An aviation style engine mounted on the front of the vehicle could effectively increase the wind resistance by providing reverse thrust to the vehicle. The only reverse-propulsion method that could react quick enough for an emergency stop would be a rocket thruster. These thrusters would be lighter than option 1, and would require less body modification to mount securely. Plus it'd look like you have some James Bond type s#it in your grill. Dope.
Shortfalls of this system include the risk of flame grillin' whatever obstacle you didn't want to hit. The systems endless applications would best be utilized by food trucks, as it would allow them to turn what would have been roadkill into a locally sourced roadside barbecue dining experience!
- Option 3: The Claw
If you've ever had to manually thatch your lawn or rake wet leafs, you know exactly how difficult it is to pull a toothed comb over unforgiving ground; that is the basic concept behind The Claw™.
The Claw™ is a spring loaded, articulating arm bolted to the (reinforced) roof of your vehicle. In an emergency stop situation, The Claw™ will deploy toward the rear of the vehicle (thus avoiding the 'pole vaulting' problems encountered in the prototype stages) with enough force to penetrate the tarmac. Upon penetration, the tips of the teeth will further dig themselves into the asphalt from the force of the spring loading, as they drag your car to a stop.
While not hugely popular among road repair crews, state budget officials, or any poor sap who happens to travel that stretch of road in the next 10-12 weeks (thanks, government inertia!), you can rest assured that your family will be able to return home safely to recover from their severe neck trauma, together.
- Option 4: Hydraulic Spoiler
Relatively modern "hyper cars" employ the use of hydraulically actuated rear spoilers. Take, for example, the Bugatti Veyron 16.4. In the event of a hard stop, the Bugatti's spoiler will pivot up to a 55° angle (wiki source). Through aerodynamic loading, this action provides down force, allowing greater braking traction for the rear tires, while also creating massive aerodynamic drag such that at speeds over 120 mph, the spoiler alone provides as much braking force as a compact car (roughly .70 G). To maintain such performance, the Veyron has a separate radiator to dissipate the heat created due to the immense pressure on the hydraulic system.
While average cars on public roads will (well... should) never (uhh.... rarely) see speeds that would benefit hugely from this type of spoiler, the technology has begun to trickle down into mere super car, and even sports car territory, and there is no question it has a positive effect on braking distances.
- Option 5: Parachute
Land speed record and drag racing vehicles have for years used parachutes to slow down after high speed passes. The basic concept, as seen in option 4, is to increase aerodynamic drag, but while the Bugatti is putting along at a mere 260 mph, top fuel drag cars are passing the 1/4 mile marker at 330 mph, and land speed cars are cruising through Nevada's Black Rock Desert at 760 mph. Saucy stuff.
While a small, short throw parachute would be terribly effective in preventing that fender bender on your commute home from work, the lack of sight lines for the car behind you might leave you worse off than had you accepted your fate. You might not have hit that car had you put your damn phone down.