THE FIRST TRAFFIC appears on the Mt. Vernon bike path well before dawn. The last stragglers are out there long after dark. Some are hardy souls who commute by bicycle. Most are simply out for exercise, fresh air and the view of the Potomac. They pedal. They trot. They walk and watch birds. Sometimes the traffic gets heavy on summer afternoons, but many of those people will be out there next winter. It's an all-weather path, and addictive.

You can hardly think of another case in which the National Park Service has provided as much pleasure to as many people for so little money. Running from Memorial Bridge to Mt. Vernon, the path generally lies in the narrow strip of park land between the right bank of the river and the roadway. Most of that land was inaccessible and unused until the Park Service began to open the path a little more than a decade ago. Despite the path's heavy use, the Park Service does a superior job of maintaining it. The only serious protest to the path currently comes from an exceedingly possessive red-wing blackbird who inhabits a tree on the edge of Dykes Marsh and angrily buzzes the joggers plugging along the causeway.

At present the path cuts inland from National Airport into Alexandria. The Park Service is now at work relocating it to the shoreline along Daingerfield Island and around the Pepco plant. The new route is to be open next spring. The expanding network of bike paths throughout the metropolitan area has become a cherished resource and escape for city dwellers who occasionally feel a desperate impulse to get out of doors.