IF MAYOR BARRY approves a bill just passed by the City Council, Washington will be graced by 16,000 mobile billboards-on the roofs of taxicabs. The four-foot-long illuminated advertisements will be trucked right behind that little light the driver never turns on to let you know the cab is in service. For a fee, a hacker could get a permit to sell advertising, which would be subject to review before display and which would include public-service messages. But speaking of public service, there's a message for all hackers that should be included in any legislation before it is put on the books: no additional financial relief without better policing of the industry.

Mayor Barry could-and should-exact improvements in cab service by vetoing this cabe bill and insisting on legislation that would consolidate and simplify the procedures for enforcing regulations. In return for additional financial relief, cab drivers should be under a greater obligation to improve their service generally. As it stands, the courteous, a law abiding drivers are being hurt by others, including many foreign student part-time drivers with a total disregard for the regulations, who treat the public shabbily.

Some tightening of enforcement could be accomplished by fielding more inspectors and posting telephone numbers andother basic information that customers need to register complaints. But the council members should review all the laws and regulations governing taxi service, hold public hearings and then approve any necessary changes. Cabbies who are working hard for a more equitable fare system and other ways to improve their incomes know that each instances of poor service tends to hurt the whole industry. In turn, most passengers who are treated fairly, courteously and efficiently are more than willing to accept changes that will encourage these drivers to stay in the business and make a living at it.