The two-wheeled stand-up scooters have popped up at more than a dozen airports as authorities discover their utility in policing terminals and traffic.

They're winning favor at airports because they move relatively quickly and their electric motors give off no emissions. They also stop easily and allow users who stand about eight inches off the ground to stand out, a useful quality for seeing and being seen in crowded terminals.

Detroit Metro deploys its three Segways, each bought for $5,400, at McNamara Terminal, which is nearly a mile long. Used by police officers past the security checkpoint, they came in handy recently in aiding a passenger having a heart attack, and in chasing a traveler accused of stealing money and electronic items, says airport spokesman Michael Conway.

Airport officials who use the so-called human transporter say they've proved to be safe and effective not only in emergencies but in crowd control, maintenance and parking enforcement. At some airports, operations workers use them.

But Segway's critics question the mixing of heavy vehicles with large airport crowds.

"If you mix (them) with people, it's a recipe for danger," says David Snetman of Transportation Alternatives, an organization lobbying to outlaw Segways on New York City sidewalks. "Airport terminals are very much akin to a sidewalk."

Some travelers are concerned. Fred Pfeifer, a product manager in Baltimore who's noticed Segways at Baltimore/Washington International, says the vehicle seems unlikely to improve an officer's response time much. The "downside is the possibility of a collision with a passenger," he says. "The (airlines' electric) carts are bad enough."

Made by New Hampshire-based Segway, the self-balancing transporter uses computers, motors and gyroscopes to keep it upright and balanced. Users simply lean forward or backward to go in that direction. Top speed is about 13 mph.

Fight-stopper:

Phoenix Sky Harbor was one of the first airports to use them. Two years ago, it bought eight.

Most operations employees at Phoenix have taken 16 hours of training on Segways. Its Segways are programmed for a top speed of 6 mph, the lowest of three speed settings.

Dan Cummings, an operations employee, says he was able to stop a fight recently between two drivers outside of a terminal before it escalated. "I was on the other side of the terminal and I got there in less than two minutes," he says. Running, he said, could have been more disruptive "It's also a real conversation starter."

Orlando International bought four Segways last year and uses them to manage traffic and terminal crowds. Because the city hosts many large conventions, lines in the terminal easily get tangled at peak times.

Carolyn Fennell, an airport spokeswoman, says Segways allow workers to move quickly between problem areas to keep the lines moving.

Traveler Chris Byrd, a clinical research consultant in Phoenix, says he doesn't mind their presence in airports. "I'm all for police using them to get around faster," Byrd says. "They don't take up any more space than someone running, and it is a lot faster."