The formal highway distinctions are:
- Interstate Hwy system (numbers start with I)
- Federal hwys (numbers start with US)
- State hwys
- local and county
Interstates meet standards on number of lanes (at least 2 per direction), divided, limited access (interchanges), lane width, grade, etc.
Some of the other highways are built to the same standards, especially ones that connect important cities. Some are divided, but still allow surface access. Others are 4 lanes, without dividers. But most are 2 lane with occasional passing lanes.
On print maps colors help distinguish the build standards of the highways. On digital maps it may be harder to distinguish, though with enough zooming you can see all these details on Google Maps.
The distinction between Interstate and 'blue highways' is a bit crude. It may be useful in some areas, but less so others. Across west Texas and flat eastern Colorado it may not matter much which you take - all are flat, straight and lightly traveled. But in the Colorado mountains there are big differences in highways, and numbers are only the start. Crossing the mountains on I70 is very different crossing them on a US550. US50 and US160 are a bit more like I70.
