of leaving power applied to the windows until a passenger door is opened at which time the window power is removed. Hydraulic drive systems could lower the windows at rest, since pressure from the hydraulic system was merely released to lower the window. Raising the windows required an electrically operated pump to operate and introduce pressure at the appropriate cylinder. These systems also required pressure lines to each cylinder (in the doors, as well as on certain cars, to the power seat and a power operated convertible top). Because of the complexity, the system could also leak fluid. Many modern cars have a time delay feature, first introduced by Cadillac in the 1980s, called "retained accessory power". This allows operation of the windows and some other accessories for ten minutes or so after the engine is stopped. Another feature is the "express-down" window, which allows the window to be fully lowered with one tap on the switch, as opposed to holding the switch down until the window retracts. Many luxury vehicles during the 1990s expanded on this feature, to include "express-up" on the driver's window, and recently, some manufacturers have added the feature on all window switches for all passengers' convenience. This is done by activating the switch until a "click" response is felt. Power windows have become so common that by 2008, some automakers eliminated hand crank windows from all their models. So many vehicles now have power windows that some people no longer understand the (formerly) common sign from another driver of using their hand to simulate moving a window crank to indicate that they wish to speak with someone when stopped at a light or in a parking lot. The 2008 Audi RS4 sold in Europe, however, still has roll-up windows for the rear doors although its counterpart sold in the U.S. has power windows for all doors. Safety Power windows have come under some scrutiny after several fatal accidents in which children's necks have become trapped, leading to suffocation. Some designs place the switch in a location on a hand rest where it can be accidentally triggered by a child climbing to place his or her head out of the window. To prevent this, many vehicles feature a driver-controlled lockout switch, preventing rear-seat passengers (usually smaller children) from accidentally triggering the switches. This also prevents children from playing with them and pets riding with their heads