Cars with automatic transmissions dating from the mid-1990s or later generally have shift locks, also sometimes called shift interlock systems --- safety features designed to prevent unintended vehicle acceleration. Both automatic and manual transmissions with a floor-mounted gearshift can also employ theft prevention devices sometimes called shift locks.
Audi designed and installed the first shift lock following an investigation by the National Highway Traffic Safety Administration addressing reports of unintended acceleration by the Audi 5000. Originally thought to have stemmed from a vehicle defect, these incidents were all found to have resulted from operator error. Drivers starting their cars would accidentally hit the gas pedal instead of the brake pedal. Audi gave automakers around the world the rights to its shift lock patent, and the shift lock became a standard feature on automatic cars.
Since many cases of unintended acceleration occur when a vehicle first starts, the shift lock system prevents this issue by requiring drivers to depress the brake pedal to move the gearshift from park to another position. The gearshift will remain immobile until the foot depresses the brake. A car with a manual transmission does not require a shift lock since even if a driver accidentally steps on the gas instead of the brake, the car will not go until the driver also lets up on the clutch.
Shift lock systems also prevent drivers from turning off the car and remove the keys from the ignition until the gearshift lever is in the park position. This safeguard prevents the driver from turning off a car accidentally before putting it safely in park to prevent it from accidentally rolling. In most cars with a shift lock system, key removal constitutes a two-part process. The driver must first rotate the key to the lock position, which can only occur when the transmission is in park. Only once the key is in the lock position can the driver remove it from the ignition.
Some aftermarket modifications have eliminated the shift interlock system. The National Highway Traffic Safety Administration found that many Ford vehicles used by law enforcement agencies that were previously modified to have brake lights flash when police lights were in use no longer had active shift lock systems when the lights were in use. Drivers can test the shift lock system by attempting to shift out of park without pressing the brake pedal. If the vehicle shifts, the shift lock system is not functioning. In vehicles with police lights, drivers should perform this test should both with and without the lights in operation.
