The U.S. National Highway Traffic Safety Administration (NHTSA) in early October distributed a notice soliciting industry and public input on whether to allow “camera monitoring systems” to replace rear- and sideview exterior mirrors mandated since 1968 in U.S. auto safety standards. The agency said in a notice in the online Federal Register that it is seeking outside research and data about the potential safety impacts of replacing conventional fixed mirrors with camera-based vision systems. The action is one step in the process to inform a potential proposal to alter the mirror requirement for future production vehicles sold in the U.S.

Mirrorless vehicles have long been advocated by automotive stylists and engineers seeking sleeker looks and improved safety. They also can help improve a vehicle's fuel efficiency because the camera setups – multiple exterior cameras feeding a flat-screen array inside the cabin – have lower aerodynamic resistance than traditional stalk-mounted fixed mirrors.

The gains from eliminating the drag-inducing mirror assemblies will be on the order of nine to 10 “counts” of aero per mirror, according to experts. Scott Miller, GM’s director of Global CO2 Strategy, Energy, Mass and Aerodynamics, told Automotive Engineering that about 1 mpg in real-world vehicle fuel efficiency is typically gained by each 12-count reduction of aerodynamic drag. Removing two exterior mirrors is worth 1.5 to 2 miles per gallon in real-world vehicle use – on average, a 20-count drag reduction, adjusting for vehicle frontal area, Miller said. The larger the conventional mirror assembly’s surface area, the more aerodynamic efficiency can be gained.

Significant aero impact
One count of aerodynamic drag is equal to 0.0001 Cd. Drag coefficient (Cd) is a dimensionless parameter used to quantify the drag or resistance of an object in a fluid environment such as air or water. For automobiles (and various other objects) the reference area is the projected frontal area of the vehicle. The product of the Cd and frontal area is the optimum measure of a vehicle’s aero efficiency. The book Aerodynamics of Road Vehicles published by SAE International, notes that exterior side-view mirrors contribute to the vehicle’s total aerodynamic drag by an average of 2 to 7%.

In 2014, the Alliance of Automobile Manufacturers (AAM) and Tesla petitioned the NHTSA to change the law requiring passenger vehicles to have a mirror on the door and another on the windshield and instead permit cameras to replace them. They cited improved side vision and increased fuel efficiency as the major benefits; NHTSA did not rule in favor of the petition. Advancing technologies and growing pressure on the industry to increase both occupant safety and fuel efficiency are major factors influencing this important front.

The vision-system technologies needed for a mirrorless driving future are increasingly being added to new vehicles as key elements of the new-generation safety, advanced driver-assistance systems (ADAS) and navigation systems, the experts acknowledge. U.S. safety regulators have been criticized for “lagging” their counterparts in Europe and Japan in approving the replacement of mirrors by vision systems. A number of OEMs already have launched, or are readying, production vehicles using exterior cameras in place of mirrors, including Audi’s 2019 e-tron EV, the 2020 Honda e city car and the Lexus ES sedan sold in Japan.

Tesla’s Model X was designed to have no exterior mirrors, instead using small cameras in the mirrors’ door locations. The NHTSA and state requirements for traditional fixed-glass mirrors kept it off the road. Tier 1 suppliers including Continental, Valeo and Magna also have vision systems ready for production and camera-based systems have been approved for commercial-vehicle pilot programs.  

NHTSA has long argued that camera monitoring systems may introduce new safety risks. The agency’s 2017 tests of a prototype camera monitoring system found it was “generally usable” in most situations. It produced better-quality images than mirrors at dusk and dawn. But the test also exposed potential flaws, including displays that were too bright at night, distorted images and camera lenses that would become obscured by raindrops.