There is no basemap under this. No terrain, no counties, no coastline — the only ink on the sheet is 48,471 miles of Interstate, and the country appears anyway, because the system was built to describe it. That is the whole idea borrowed from the 1957 Yellow Book, the slim federal volume titled General Location of National System of Interstate Highways that showed the thing as pure linework before a yard of it was poured.

Every road is tinted by the weather on it. Values come from the National Weather Service's NDFD grid at 2.5 km, sampled at 5,580 points spaced about eight miles apart along the pavement, then blended between those points so a front crossing a route moves down it smoothly instead of stepping from marker to marker. Nothing here is interpolated from city observations: it is the gridded forecast, read where the road actually is.

The temperature scale is diverging about 32 °F rather than running a rainbow from cold to hot, because on a road the freezing point is not one value among many — it is the value. Sub-freezing pavement is also drawn heavier, so the hazard is carried by a second channel and never by colour alone.