The Federal Motor Vehicle Safety Standard 102 declares that shifters for automatic transmissions follow the Park-Reverse-Neutral-Drive-Low (PRNDL) sequence for the sake of consistency across vehicles.
This sequence provides separation between Drive and Reverse to reduce the chance of accidentally selecting one when the other is intended. When the standard originally went into effect in the 1960s, automatic transmission shifters were mechanical devices with a cable running to a lever on the planetary automatic transmission that selected gears.
Over the years, as drivetrain technologies have evolved to include electronically shifted transmissions, dual-clutch automatically shifted transmissions, hybrid-electric power, and automatic engine stop-start systems, the regulation has been amended to address factors such as when the engine can automatically stop and start. The standard also requires a visible indication of gear position.
However, the development of electronic shifting technology has produced a proliferation of shifter designs, which defeats the purpose of commonality for the sake of safe familiarity of operation.
This includes the spring-loaded joystick shifters that are described as “monostable” and their steering column-mounted equivalents such as those that are used on BMW and Mercedes vehicles, and by Tesla, which uses Mercedes switchgear. A traditional PRNDL shifter that moves through discrete positions for each gear and remains there is called a polystable shifter.
It was a monostable shifter in his Jeep Cherokee that led Star Trek actor Anton Yelkin to accidentally leave his car while it was in Neutral rather than Park, with the result that it rolled down a hill and crushed him to death. Jeep made a financial settlement with Yelkin’s family and changed the shifter to a less confusing polystable design.
“Monostable shifting requires the driver to devote time to confirm the desired gear position on a display,” states a study by Sweden’s Luleå University of Technology. “In a worst-case scenario, this could have fatal consequences. The NHTSA concluded that the operation of a monostable shifter is not intuitive, which increases the potential for unintended gear selection.”
Here’s why the polystable PRNDL is superior:
- Familiarity: If they are the required standard there are no instances of cars where the drive pushes the shifter forward (or upward on a column shifter) to engage reverse instead of pulling it back one notch.
- Haptic feedback: With a PRNDL, the driver can feel the shifter move through the notches of each position as confirmation of gear selection. Additionally, they can feel the position of the shifter without even seeing it to know which gear is selected.
- Visual feedback: Without looking directly at the shifter, the position of a polystable shifter is visible in the driver’s peripheral vision.
Monostable shifters deliver none of these benefits. Worse still are the pushbutton shifters. These require the driver to take their eyes off the road every time they want to change gears. The gear selected is never obvious. In an emergency, it could take too long to locate the intended button and select it.
This is why pushbutton shifters should be specifically prohibited in the FMVSS 102. Pushbutton shifters also have no Neutral position as a buffer between Drive and Reverse, helping avoid inadvertent selection of one when the other is intended. Alas, this criticism applies even to gorgeous eye candy like Aston Martin’s glass buttons, as seen in the photo at the top.
The best current examples in the contrast in useability between a polystable PRNDL and a pushbutton shifter are the Kia Telluride and the Hyundai Palisade. These vehicles roll on the same underpinning platform but have different executions above that. Kia employs a user-friendly PRNDL shifter. Driving the Palisade immediately following the Telluride is a rude awakening in ease of use that underscores the issue for reviews and consumers cross-shopping the two vehicles.
Another alternative is the rotary dial shifter. These are also monostable and polystable, and again, the polystable variant is preferable. Monostable dials spring back to a center position and require a counterclockwise turn to select Reverse and a clockwise turn to select Drive. Polystable rotary shifters start in Park and turn clockwise to progress through Reverse, Neutral, and Drive.
These shifters lack the benefit of having their position be visible to the driver without looking closely. They also can’t tell the position just by reaching for the shifter to feel its location. They can, however, tell its position if their hand is still on the shifter, as when clicking one notch for Reverse to back out of a space and then turning it two more for Drive to drive away.
It seems like there is a good case for polystable rotary dial shifters to be allowed, but monostable shifters of all types should be prohibited. And pushbutton shifters should be banned with extreme prejudice.