On the day a 19-year-old man was sentenced to prison for killing an Oahu taxi driver, a cabbie for one of the island?s largest taxi companies said he?s happy his firm has installed new GPS technology that allows drivers to communicate an emergency to dispatchers with the touch of a button.

Michael Robles was sentenced Wednesday to 20 years in prison, the maximum allowed for manslaughter, in the killing of cab driver Charlys Tang last year.

Driver Malig Williams works for Charley's Taxi, which is not the firm that Tang worked for when he was killed. Williams said cabbies often fear for their safety -- especially at night.

"A lot of drivers look in the rear-view mirror to see if the other person in the back is about to choke them or stab them or do something to them. So you're always on full alert all the time," Williams said.

This fall, Charley's Taxi installed GPS units in its nearly 200 cabs. Besides helping dispatchers send cabs to customers more quickly, the new system allows drivers to call for help with a panic button, so dispatchers will know their exact location to send help.

"Now with the computer system, I know that somebody else is watching for my back, besides myself," Williams said.

Williams said the GPS system also has a drop-down menu, which taxi drivers can use to quickly send word of an emergency.

One number sends the message, "Getting robbed, call police." Another code tells dispatchers: "Had an accident need ambulance."

Williams said he's refused service to people because he feared for his safety only a handful of times in the seven years he's driven a cab on Oahu.

He said he tries to immediately evaluate a customer's behavior and whether they're acting suspicious or want to go somewhere far away.