D.C. police are investigating the shooting death Sunday night of a 56-year-old cab driver found on the sidewalk near his parked cab in the 800 block of 51st SE.

Timothy R. Young of 1750 16th St. NW was found shot in the head and chest just before midnight according to police. He was pronounced dead on the scene and police said they believe robbery was the motive.

The robbery-shooting is believed by police to be the first of a cab driver in Washington since three cabbies were shot in September.

The three September shootings, one of which was fatal, occurred in the 6th and 7th Police Districts. As a result, security measures in far Southeast and Northeast were stepped up.

Regulars patrol officers and special units from the robbery squad stopped cabs and checked passengers followed a news conference in which Mayor Walter E. Washington and then-police chief Maurice J. Cullinane announced the program.

That program was phased out in December, however, according to 7th District officers, after arrests were made in the September shootings and the number of cab holdups sharply declined.

"Whether or not we go back to this type of program will probably depend on how the cab companies react to this," 7th District Sgt. Michael Linster said yesterday. "This looks more like a one-time thing than a pattern, though."

Although the cab Young was driving was listed with the Eastern Cab Co., a spokesman there said yesterday that the cab was owned by Charles E. Johnson of 1721 1st St. NW and that it was listed with the company mainly for insurance purposes.

Johnson said Young had been renting the cab from him for about four years. "He usually worked the airport," he said. "I really don't know what he was doing in that neighborhood. I guess he had a fare."

Johnson said he would try to rent the cab again as soon as he completed normal maintenance.

Homicide detectives said yesterday they had no leads and no suspects.