Actually you can't go from 40 miles/ph to 0 miles/ph in an instant. At least not with a passenger car. The reaction time is typically a few seconds. If one throws themselves in front of a car, the car would need a few seconds to react. Based on the speed, distance and other parameters, I don't think any car would be able to stop in an instant.

Thinking about it seriously, it may be shouldn't. Also these things could lead to a crash pile up with other vehicles coming in from behind.