is often powered by the input side of the transmission and is therefore not running when the transmission is in Neutral. The vehicle may be started in Neutral as well as Park. Drive (D): This position is the normal mode for driving forwards. It allows the transmission to engage the full range of available forward gear ratios. Low (L): This position, required by U.S. law, provides for engine braking on steep hills. It also provides for a lower gear ratio for starting out when heavily loaded. Some automatic transmissions, especially General Motors, from 1940-1964 used a layout with reverse as the bottom position (eg P-N-D-L-R). However this layout led to the risk of the driver accidentally shifting into Reverse while the vehicle is travelling forwards (especially during engine braking maneuvers). Other positions and modes Many transmissions also include positions to restrict the gear selection to the lower gears and engages the engine brake. These positions are often labelled "L" (low gear), "S" (second gear) or the number of the highest gear used in that position (eg 3, 2 or 1). If these positions are engaged at a time when it would result in excessive engine RPM, many modern transmissions disregard the selector position and remain in the higher gear. In descending order of the highest gear available: 3: Restricts the transmission to the lowest three gear ratios. In a 4-speed automatic transmission, this is often used to prevent the car shifting into the overdrive ratio. In some cars, the position labelled "D" performs this function, while another position labelled "OD" or a boxed "[D]" allows all gears to be used. 2 (also labelled "S"): Restricts the transmission to the lowest two gear ratios. In some cars, it is also used to accelerate from standstill in 2nd gear instead of 1st, for situations of reduced traction (such as snow or gravel). This function is sometimes called "winter mode", labelled "W". 1 (also labelled "L"): Restricts the transmission to 1st gear only, also known as a "low gear". This is useful when a large amount of torque is required at the wheels (for example, when accelerating up a steep incline) however usage at higher speeds can result in excessive RPM for the engine, which may cause overheating or damage. Many modern transmissions also include modes to adjust the shift logic to prefer either power or fuel economy. "Sport" (also called "Power" or