The above map, made by Eric Fischer at Mapbox using federal data, shows in yellow every road in the country, from Interstates to local arterials, that's part of the Federal-Aid Highway Program. This is the road infrastructure that's supported by the Highway Trust Fund and, in short, your gas taxes.

Zoom in, and Fischer has mapped each road by how heavily it's used. The 14th Street Bridge connecting Washington and Virginia, for instance, saw on average more than 175,000 cars traveling across all of its lanes in both directions each day in 2012 (the thickness of each yellow line corresponds to the heaviness of its traffic flow):

This is another way of visualizing a problem the government has struggled to solve for years: The user fees paid through gas taxes by all these drivers crisscrossing the city (and the country) no longer cover the full costs of maintaining these roads. And if Congress doesn't come up with a long-term solution by the time its current funding stopgap expires in May, the result won't play out in some abstract political fight. It'll impact, eventually, this fundamental infrastructure used on a daily basis by thousands of commuters:

The 2016 budget President Obama unveiled on Monday proposes to cook up an additional $238 billion to fund public works by taxing corporations that currently stash their profits overseas. That revenue would make up about half of a $478 billion, six-year plan to upgrade highways, bridges and transit. The rest would come from the gas tax.

While Congress prepares to fight about that, you can play with Fischer's map below. He has layered the roads we're talking about on top of the street network mapped by the open-source collaboration OpenStreetMap (local roads that don't benefit from federal highway funds but that have been mapped by OpenStreetMap are shown in blue):

(Hat tip Planetizen)