From a US Military helicopter pilot. I asked him if there were lateral limits to these helicopter routes? Here’s his answer,
There are routes and zones around DC. The routes you have to stick to, if you need to deviate laterally you're supposed to get permission to deviate. The zones, you can fly wherever your heart pleases but you have yo stay below the maximum altitude. You're never supposed to go above the altitude listed on the map.
As for the lateral confines for the routes, they're marked on the map buy a solid blue line. Otherwise known as the route.