An excellent source of information on trails that permit mountain biking is the Web site of the Mid-Atlantic Off-Road Enthusiasts,; choose "where to ride" from the menu on the left side of the page. The group helps maintain more than 300 miles of trails in 24 parks around the region.

Following are listed some of the more popular multi-use trails in the area. Most of these are paved, so they attract more road bikers than mountain bikers. Under standard protocol on these shared-use trails, slower traffic stays to the right and bikers yield to pedestrians. Walkers torment bikers by dawdling on the left, and cyclists anger walkers by passing on the right, or passing pedestrians at high speeds and without announcing their approach. And you dog-walkers? Keep them on a leash, please.

* Baltimore & Annapolis Trail, 13 miles. Links Annapolis and Glen Burnie, with a connection to the BWI Trail.

* BWI Trail, 11 miles. Circles around Baltimore-Washington International Airport.

* Chesapeake & Ohio Towpath, 184 miles. Historic earth and crushed-stone path runs from Georgetown to Cumberland, Md.

* Custis Trail, 4 miles. Runs through Arlington.

* Mount Vernon Trail, 18 miles. Narrow, winding trail hugs Potomac River from the District to Mount Vernon.

* North Bethesda Trail, 4 miles. Connects Rockville and Bethesda.

* Washington & Old Dominion Trail, 45 miles. Rail trail from Shirlington to Purcellville, Va.

* Capital Crescent Trail, 13 miles. Paved section from Georgetown to Bethesda is thick with walkers, joggers, cyclists and bladers, especially on weekends. Crushed-stone portion from Bethesda to Silver Spring gets lighter traffic.

* Rock Creek Hiker/Biker Trail, about 23 miles. Stitched-together arrangement of roadway (some sections closed to cars on weekends) and narrow winding trails from Lake Needwood near Rockville to the Potomac waterfront near Georgetown. Rock Creek Park's unpaved hiking trails are closed to mountain biking.

* Sligo Creek and Branch Trails, about 20 miles. Ties Mount Rainier to Wheaton, College Park and Greenbelt.

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