Arlington offers immediate access to some of the D.C. area's most popular trails, as the Mount Vernon, Custis and Washington & Old Dominion (W&OD) trails all join together within Arlington County.  These trails attract enormous numbers of users - cyclists, walkers and runners - every day.  

The trails are a regional treasure, and they're meant to be shared. The rules are pretty straightforward - stay to the right, signal when passing and don't speed. For the most part, the sharing goes well when all users observe these rules. Unfortunately, not everyone appears to understand them (or if they do, they do a good job of ignoring them).

So a little review might be in order:

• First, stay to the right.  It's a simple concept, really - slower traffic should stay to the right, just like on the road.  This allows faster traffic - usually runners and cyclists - to safely pass walkers on the left.  Walking or riding down the middle of the trail simply doesn't leave enough room for others to pass you safely.
• Next, signal when passing.  This is probably the most broadly interpreted (and ignored) of the rules.  Generally, it is expected that cyclists will warn other users with a bell or by calling out "on your left" when passing other trail users.  Yes, "on your left" sometimes confuses users who don't expect it (often tourists, who occasionally take it as a direction to step left), but a bell is a pretty universally understood signal.
• Finally, don't speed.  This means you, cyclists.  While other trail users regularly break the first two rules, going too fast is almost exclusively the province of cyclists.  The speed limit is 15 miles per hour, but even that is too fast in some locations (e.g., Gravelly Point, near the airport) and conditions (e.g., crowded weekends).  If you want to go faster, get on the streets.

There are, of course, lots of other good behaviors for the trails - slow to a crawl around kids, ride single file, and always use lights after dusk.  If we all managed to reliably observe the just the three rules above, however, the trail would be a safer and more pleasant place for everyone.

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The Mount Vernon Trail is managed by the National Park Service, and trail rules can be found here.

The W&OD trail is part of the Northern Virginia Regional Park Authority, but the best source of info for it is Friends of the W&OD Trail.  Here are some safety tips from Friends of the W&OD.