The District's Public Service Commission voted yesterday to allow riders to choose smoke-free taxis and drivers to keep smokers out.

In a unanimous vote, the commission voted to allow the owners and operators to put a six-inch-diameter decal with the universal no-smoking symbol on the outside of taxis, putting smokers on notice before they step into the vehicles that they are expected not to smoke.

Commissioners cited the health of cab drivers as the main reason for allowing drivers to put smoking clearly off limits in their cabs. But passengers who don't want to sit in clouds of smoke also should benefit from the rule.

Although a growing number of taxis sport no-smoking decals and signs inside, it has been hard for a passenger to judge before entering a cab whether he or she might be riding with smokers. The problem is particuarly acute during rush hours when cabs may carry large groups of riders picked up at separate points.

The commission acted on an application for an exception to the regulations by independent cab driver Andrew Johnson, who asked for permission to put a no-smoking decal on the outside of his cab. In acting, they went further, making the ruling applicable to any driver who chooses to display the decal.

A special assistant to the PSC, Melvin Washington, said that under the regulations a cab driver whose taxi bore the decal apparently would be within his rights to ask a passenger who insisted on smoking to get out of the cab.

In other actions, the commission voted against allow Yellow Cabs to display "Thank you for riding Yellow Cab" decals inside vehicles. Commissioner Elizabeth H. Patterson also noted that a number of bumber stickers being displayed on taxis violate commission regulations.

They also rejected an application by Johnson for permission to display advertising in a metal frame atop his cab. The commission took a similar position against outside advertising on cabs when legislation to permit it was pending last year.

"We've tried very hard to keep cabs in D.C. free from unnecessary clutter," Patterson noted.