Are motorized scooters a dangerous urban plague or the future of local transportation? Here's everything you need to know:

Where did this start?
In September 2017, a startup called Bird began renting battery-powered electric scooters in the beachside city of Santa Monica, California. The scooters proved so popular that Bird moved to other cities, and in less than a year it reached a $1 billion valuation, the fastest that a company has ever done so. Bird is now worth double that, providing more than 10 million rides in 100-plus cities. Lime, the San Francisco–based scooter and bike-share startup, is growing neck and neck with Bird, while Ford, Uber, and Lyft have made major investments in scooters. These companies believe they're ushering in a "micromobility revolution," pitching scooters as a fast, ecofriendly alternative to driving in cities and congested suburbs. "I also use bikes, but this is a much more fun way to get to where you want to be," says Washington, D.C., college student Ria Montgomery. "You don't need to pedal harder uphill and you can fit sidewalks easier." But the sudden emergence of thousands of scooters in cities has triggered a backlash, with the two-wheeled vehicles frightening pedestrians, injuring hundreds of riders, and alarming municipal officials.

How do these companies work?
Their scooters don't have docks like many bike-share services; instead, they're usually just ditched on the side of the street. Adult customers unlock them for $1 each ride using a smartphone app, then typically pay 15 cents per minute of use. Most scooters can go up to 15 mph. Some cities require riders to stay on the street, while others confine them to the sidewalk. Few riders bother to wear a helmet. At night, company employees or contract workers round the scooters up, charge and repair them, and return them to heavily trafficked spots around town.

Are they safe?
Emergency-room doctors across the country say they've witnessed a surge of serious accidents since electric scooters were introduced, The Washington Post recently reported. "Injuries are coming in fast and furious," said Michael Sise, chief of medical staff at Scripps Mercy Hospital in San Diego. So far, four people are known to have died in the U.S. from scooter-related injuries. Doctors say scooters look simple and fun to ride, giving a false sense of security. The vehicles' small wheels are no match for potholes, and inexperienced riders can lose control as they speed downhill or need to stop suddenly. A January study analyzing ER patients who'd suffered scooter-related injuries found that 80 percent of them were hurt in falls, while 11 percent collided with an object and 9 percent were hit by a moving vehicle or object. A third of patients suffered bone fractures and 40 percent had head trauma. Last July, John Montgomery, 47, was riding a Bird scooter when the accelerator got stuck in place, causing him to lurch into a Los Angeles intersection. Thrown from the scooter, he suffered a broken jaw, a fractured wrist, and a head injury that left blood pouring from his ears. "Companies are just getting these scooters out there as fast as they can," says Montgomery, who plans to sue. "I honestly don't think they give a damn if I lived or died."

What else can go wrong?
Earlier this month, the Chinese scooter company Xiaomi admitted that a flaw in its Bluetooth module made it possible to hack the scooters and make them suddenly brake or accelerate. Lime recently recalled about 2,000 scooters from San Diego, Los Angeles, and the Lake Tahoe area after it found that a battery defect could cause them to catch fire.

Are cities regulating scooters?
They're trying. Scooters were temporarily banned after contentious rollouts in cities such as Indianapolis, Denver, San Francisco, and Nashville. "A lot of these companies roll into town, flout local regulations, [and] see what they can get away with and how far they can push cities to accommodate them," said Chloe Eudaly, a Portland, Oregon, city commissioner. Pedestrians and residents also resent having scooters left on sidewalks and in streets, which they find both unsightly and a threat to cause tripping and falling. In Los Angeles, angry residents have protested the influx of scooters by stuffing them into public toilets, throwing them into the ocean, tossing them off balconies, smearing them with dog poop, and setting them on fire.

Can scooters share the road?
For these vehicles to be ridden safely, most safety experts and city officials agree, they will need to be used in an extensive network of bike lanes — not on sidewalks or in the midst of car and bus traffic. In most cases, the existing bike lanes are not sufficiently extensive, so for scooter riders, it's still the Wild West. In a commentary accompanying the first major study of scooter safety, physician Frederick Rivara says the vehicles can be a viable part of the urban transportation mix — if properly regulated, and if riders are required to wear helmets. Electric scooters are "here to stay," Rivara says. "Action, however, is needed."

The rise of 'Bird hunters'
Carelessly discarded scooters are a blight to some — but a business opportunity to others. In the Los Angeles area, teenage "Bird hunters" drive around town after school, picking up stray scooters, charging them at home with equipment provided by the company, then returning them to designated "Bird nests" in the morning. Bird pays between $5 and $20 for each charged scooter. "Charging scooters for Bird is like Pokémon Go," says 21-year-old Nick Abouzeid of San Francisco. After work, he and his girlfriend like to walk around the city collecting scooters to charge in their basement. "In the end, it pays for a fancy dinner," Abouzeid says. Some entrepreneurs resort to ransom: Last year, two men in San Diego launched Scooter Removal, which people can call for free to pick up stray scooters and rental bikes that are left on private property. The company "impounds" the vehicles, then bills scooter companies $40 or more to return their property, depending on where it's found. They've already impounded 6,000 vehicles.