E-bikes are becoming more common on roads and bike paths, and the number of teens riding them is growing. But the recent deaths of several teen riders have raised concerns about the safety of certain types of vehicles and whether they legally qualify as e-bikes. Here's what's known about e-bikes and their risks.

What is an e-bike?

The Consumer Product Safety Commission, the federal agency responsible for regulating the safety and sale of low-speed e-bikes, defines an e-bike as a two- or three-wheeled vehicle with pedals and an electric motor.

The motor must be rated at less than 750 watts, which is about twice the amount of power a professional cyclist can produce. Riders can use either the pedals or the motor alone or in combination. When using only the motor, the bicycle may not travel faster than 20 miles per hour on a level surface. State law regulates where electric bicycles can be ridden, the minimum age of riders, and other rules about how the vehicles can be used.

To meet federal regulations, bicycle manufacturers have developed a three-tier classification system for e-bikes.

Class 1: Maximum speed, 20 mph; the motor provides power only when the rider pedals. (This is called "pedal assist.") Age restrictions: Most states have no restrictions, but some states (e.g., Oregon) do not allow riders under the age of 16 to use an e-bike in any category.

Class 2: Maximum speed, 20 mph; motor can be powered independently of pedals. Age limit: Most states have no age limit. (These e-bikes have been particularly criticized because of the instant bursts of speed they can achieve by relying on the motor alone.)

Level 3: 28 mph top speed - but only if both pedals and motor are used. These vehicles are designed for commuters and other riders interested in traveling longer distances than traditional bicycles. In many states, riders under the age of 16 are not allowed to use them.

Notably, federal consumer agencies do not recognize the three-tier system.

What are the state regulations?

According to PeopleForBikes, a trade organization that helped manufacturers develop the three-tier system, 42 states have laws that are generally consistent with the classification system. In most states, Class 1 and Class 2 e-bikes are available to riders under 16, while riders of Class 3 e-bikes must be 16 or older.

But local and state law enforcement officials say enforcing these rules is tricky. It's hard to tell by looking whether a teen rider is too young for the e-bike he or she is riding. One look at an e-bike's motor doesn't determine whether it can travel faster than 20 mph.

This has led some jurisdictions, such as Bend, Oregon, to design public service campaigns to remind riders and parents to obey the law. In Orange County, California, officials impounded some models of e-scooters, such as the Sur-ron, which the county considered to be unlicensed and unregistered e-scooters.

Why does 20 mph make sense?

Safety experts say the origin of the parameter is unclear, but it appears to have emerged from legislative debates as a way to balance the risks associated with increased speeds.

"Congress, the Consumer Product Safety Commission and the Department of Transportation are deciding the line between consumer products and motor vehicles at this point in time," says Chris Cherry, a professor of civil engineering at the University of Tennessee who advises the federal government. On e-bike safety.

By all measures, the risk of serious injury and death rises sharply at speeds around 20 mph, although most studies involve collisions between cars and pedestrians. For example, according to the AAA Foundation for Traffic Safety, the risk of serious injury to pedestrians is 25 percent when a car is traveling at 16 mph, and rises to 50 percent when a car is traveling at 23 mph. The risk of fatal injury follows a similar curve. But e-bikes are new, so there is much less data on the relationship between speed and injury risk.

Mr. Cherry said the 28-mile-per-hour limit appears to be an attempt to comply with the European standard of 45 kilometers per hour so that e-bike manufacturers can serve both markets.

But don't many e-bikes travel faster than 20 mph?

They do.

If the rider pedals at the same time as the motor, the e-bike can travel at speeds in excess of 20 mph, and up to 28 mph for Class 3 bikes.

But in many cases, these limits can be easily bypassed. For example, some e-bikes are sold with a speed "governor" that limits the speed at the point of sale to 20 mph, but the electronic governor can be eliminated by cutting the cord or using a smartphone app to change the limit. Unrestricted, some models can exceed 55 mph Law enforcement officials and industry experts say that e-bike manufacturers selling these products are aware that the governors are periodically removed.

"Some products are ostensibly compliant, but it's easy for users to make modifications based on knowledge and manufacturer support," said Matt Moore, general counsel for PeopleForBikes, a trade organization that represents bicycle and e-bike manufacturers. "The real question is what to do."

What is being done about this vulnerability?

"PeopleForBikes has been pointing out these issues to regulators for some time," Mr. Moore said. "Unfortunately, there seems to be a lack of resources at the federal level to investigate and address EV products that may actually be motorized vehicles."

The federal government doesn't seem to have a clear answer as to whether some of these products cease to be e-bikes (regulated by the Consumer Product Safety Commission (CPSC)) and become motorized vehicles (regulated by the National Highway Traffic Safety Administration.

A spokesperson for the federal consumer protection agency responded in an email that the higher speed products "would fall under motor vehicles outside of the CPSC's jurisdiction," adding that the highway transportation agency "has jurisdiction over motor vehicles "

In response to an inquiry from the Times, the Highway Traffic Agency issued a written statement, "Because emerging e-bike designs vary in terms of speed capability, the way motor power and pedal power are combined, and other design factors, NHTSA is evaluating, in cooperation with the U.S. Consumer Product Safety Commission (CPSC), how best to oversee e-bike bicycle safety."