Hybrid electric vehicles (HEV) and electric vehicles (EV) operating at low speeds may be a hazard to pedestrians and bicyclists. Research by the National Center for Statistics and Analysis (NCSA) found that the incidence rate of pedestrian crashes was higher for HEVs than for internal combustion engine (ICE) vehicles. Some HEVs can operate solely on their electric motor when operating at low speeds, such as backing out of a driveway or slowing down to make a turn. While these vehicles emit less air and noise pollution than ICE vehicles, they are quiet at low speeds, and thus present a safety concern for pedestrians, particularly the visually impaired, who rely on the sound of vehicles to safely navigate.

The National Highway Traffic Safety Administration (NHTSA) recently announced proposed minimum sound specifications for EVs and HEVs. This proposed motor vehicle safety standard is designed to improve the ability of pedestrians and bicyclists to detect the movement of these vehicles and thereby reduce injuries and fatalities. The development of these proposed specifications was informed, in part, by a body of research about EV/HEV detectability that was conducted over the past four years at Volpe, The National Transportation Systems Center, under sponsorship of NHTSA.

HEV Detectability: Three Phases of Experiments

Phase 1

Work on HEV detectability was conducted in three phases. In phase 1 experiments, Volpe scientists recorded ICE and HEV pairs in the field and conducted laboratory experiments with human subjects to measure the detectability of vehicles moving at low speeds in a variety of different operating conditions. Subjects listened to audio recordings of three vehicle maneuvers: a vehicle backing out of a driveway, a vehicle approaching at a constant 6 mph, and a vehicle moving parallel to the pedestrian and slowing from 20 mph to 10 mph. This experiment evaluated the detectability of ICE vehicles and HEVs under a variety of conditions and studied the extent to which HEVs were less detectable than the ICE vehicles tested.

Phase 2

In phase 2 experiments, Volpe scientists evaluated nine different synthetic sound profiles to understand how well they alert pedestrians to the presence of a vehicle and how detectable these sound profiles are compared to the sounds produced by ICE vehicles. Both sighted and blind persons served as subjects in the experiments. Vehicles outfitted with loudspeakers that broadcast the sounds were driven toward subjects to determine the distance at which the sounds could be detected. The synthetic sound profiles were primarily a variety of engine noises with different frequency compositions and amplitudes, as well as some artificial sounds designed to maximize detection range while minimizing sound pressure level and annoyance. The results studied the extent to which some sounds were more detectable than other sounds.

Phase 3

In phase 3 of this research, Volpe researchers explored in detail various approaches that can be used to identify potential performance specifications for alert sounds that would ensure that they are detectable and recognizable.

Developing specification criteria to improve transportation safety is a complex undertaking that often relies on experimentation and technical analysis to inform the process. Volpe, with its unique mix of cross-disciplinary expertise in a wide variety of disciplines, tapped experts in human factors, acoustical engineering, experimental design, environmental measurement, modeling, and statistics to conduct this complex series of studies for NHTSA.

Dr. Michael Zuschlag is an engineering psychologist specializing in the human factors of electronic displays and automation, computer-operator interfaces, and organizational change and evaluation in transportation systems.

Mary Stearns, Ph.D.

Dr. Mary D. Stearns is the Chief of the Surface Transportation Human Factors Division. Her professional contributions focus on the social factors influencing the use, operation, and organization of transportation systems and equipment and she has a particular interest in the issues of special user groups.