WE HAVE ONLY one reservation about the Secret Service's proposal to close off Pennsylvania Avenue to traffic from 15th to 17th streets as a way to increase security for the president and to create a "campus-like" atmosphere around the White House. It doesn't go nearly far enough. Certainly it's appropriate that the White House -- as a unique educational institution for expanding the fortress mentality -- be in the safest possible setting. But why stop with the closing off of the "Grand Avenue"? If it's safety first here, you need more than just a quick-barricade job around the president's house. To heck with a Band- aid approach -- we're talking big security.

Better to seal off the entire District of Columbia at the borders, allowing no civilian vehicular traffic at all. Any pedestrians choosing to set foot inside the boundaries should be required to produce official "walking papers" or a student I.D., color-coded to prevent unauthorized ventures into areas off limits. Rivers, of course, would need heavy patrolling.

There are aesthetic as well as security reasons for expanding the presidential sanctuary. Last year, when the White House began by closing East Executive Avenue between the White House and the Treasury, this block proved easily convertible into spaces for cars of the executive staff. If the same change is made on Pennsylvania Avenue, this wider strip could be designed to resemble spectacular campus areas, such as the University of Maryland's main parking lot.

Some parts of this new security zone wouldn't take any conversion at all to add to the campus setting, particularly the absorption of American, Catholic, Georgetown, George Washington and Howard universities and the University of the District of Columbia. Other buildings would have to be gutted and razed for security reasons, the most obvious being the tallest tower, the Washington Monument.

We realize that the safety of the president, unlike our suggestions, is a serious matter. But sealing off a famous and important artery in the center of the capital city is not, as any driver would say, an acceptable approach.