TWENTY YEARS AGO, Arlington County's Crystal City area had two motels and a drive- in movie. It has since become a massive high- rise and underground development complex with 37,000 workers and nearly 5,000 residents. It has 7.6 million square feet of office space, 500,000 square feet of retail space and 3,700 hotel rooms. How many officers does the Arlington County police department have to patrol its labyrinthine underground tunnels and offices and parking lots, its walkways and bridges, and all the densely packed alleys and nooks and crannies above ground?

Four, sometimes five.

The county police department wants to more than double that contingent with a budget request to add six more police officers for Crystal City. It is easy to show why they, and probably more, are needed. Crystal City has become a focal point for serious crimes in the county. It is first in the county in robberies and stolen motor vehicles. County-wide, there has been a 30 percent increase in the number of robberies this year, a 68 percent increase in the number of vehicle thefts, and most, say police, occurred in Crystal City. At the same time, patroling the area has become much more complicated.

"The same number of police officers patroling 10 years ago would be seen two or three times a day by residents and employes," said Arlington County Deputy Police Chief David Reiten. Now, with so much more space to cover, residents or workers are lucky "to see a police officer once every month or so." The same situation emboldens criminals, especially purse snatchers, who can also find countless entrances, exits and places of concealment in Crystal City. Increased crime is a consequence of such dense and easily accessible development, and more police are badly needed.