DCA walk/bike access route
The yellow route is the signed route, without any grade crossings. It's reasonably direct for cyclists approaching from the south, but for pedestrians from the north it adds almost 1/2 mile (and even more for people headed to the south pier or Terminal A).
The red route is much more direct for those coming from Crystal City (to the north) but requires jaywalking across a three high-speed roads, each one 1-2 lanes and with okay sight lines.
There's also a south route to the airport for those headed to Terminal A or B or the parking garage (e.g., for car rentals). Start at the sand volleyball courts and walk along the northbound exit ramp, over the Airport Access Road bridge, and follow the signs around the offices to the terminal.