Chrysler’s Pacifica Hybrid minivan, a plug-in hybrid that can go 33 miles on an electric charge before the gasoline engine turns on, uses speakers when driving 22mph or slower in forward or reverse.

Following a delay on all new regulations mandated by President Donald Trump, rules have been finalized requiring hybrid and electric vehicles to create sounds to warn pedestrians of their presence. The hybrid rules had been set to become official in December 2016, but Trump’s action delayed finalization until the end of the first quarter of 2017. The implementation timeline has not changed – hybrid and electric vehicles must begin making warning sounds in 2018 with full implementation by 2019.

Hybrid and electric vehicles are 1.18x more likely than gasoline- or diesel-powered vehicles to collide with pedestrians and 1.51x more likely to hit cyclists. National Highway Traffic Safety Administration (NHTSA) researchers attribute that statistical variation to pedestrians’ inability to hear vehicles coming without loud engine noises to serve as a warning. Researchers say the new rules could prevent up to 2,400 accidents per year.

Nissan’s Leaf electric vehicle, a car capable of driving about 100 miles on a charge, already includes sound equipment to warn pedestrians and cyclists of its presence. All hybrid and electric cars will need such equipment by 2019.

All hybrid and electric light vehicles with four wheels and a gross vehicle weight rating (GVWR) of 10,000 lb or less will be required to make audible noise when traveling in reverse or forward at speeds up to 30km/h (about 19mph). At higher speeds, the sound alert is not required because other factors, such as tire and wind noise, provide adequate audible warning to pedestrians.

The rules give automakers leeway on what sorts of sounds to add to vehicles. Recommendations for complex sound profiles, such as increasing the pitch of the sound to match acceleration or deceleration, did not make the final rule. Although the regulation does mandate adjusting the volume of the signal as speed increases to give pedestrians and cyclists an idea of how quickly a hybrid or electric car is moving.

Because hybrids and electric vehicles are relatively rare and are not expected to dominate the automotive market soon, costs of implementing the rules are relatively low – an estimated $39.3 million for the industry by 2020.

NHTSA researchers estimate adding sound equipment will add about $130 to the cost of each hybrid vehicle and about $55 for electric vehicles. Several hybrids and electrics, such as Nissan’s Leaf, already include such equipment. The weight of the sound equipment is expected to make hybrid cars infinitesimally less fuel efficient – 2.3 more gallons of gas throughout a vehicle’s lifetime for hybrid cars, 2.5gal for hybrid trucks. Administration researchers estimate that eliminating or minimizing accidents will save pedestrians and cyclists $247.5 million to $320 million per year in medical and lost-time costs.