Once off the Interstates and U.S. and state highways in the "designated highway" system open to commercial over the road traffic (where you can expect at least 10-foot lanes), there really is no legal standard, and 8-foot width for each paved lane is still pretty common.
What often matters more is what is off the edge of that pavement. 6-8 feet of paved shoulder, 4 feet of gravel or other improvement, or is it grass, or just 6 inches to the edge of the embankment of the bar ditch?
Not all the roads are paved. Many county and local roads may be unpaved, 12 to 16 feet total width, but folks on these tend to run down the center until they meet traffic, so we have to slow down and move over into the softer stuff when we meet each other coming.
Then there are the rural roads, and quite a few old bridges, one lane for short stretches, where oncoming vehicles have to figure out how to take turns. Narrowest bridges I've encountered have been 8 feet at the pavement (this has been a minimum standard for about 80 years) but usually have enough clearance on the side rails or superstructure to carefully drag 8.5 feet through. From time to time an oversize vehicle gets jammed into one of these, then we have to shut down that road until we get funds to build a new bridge.
City streets are a whole different matter. Major thoroughfares, expect 10 feet usable except for bad parking behavior. Other streets, particularly with street parking, might be less than 16 feet with parking on one side, 24 with parking on both sides, and it is entirely possible that someone may be parked so as to close the road to vehicles much more than 6-6 1/2 feet wide. Delivery vans, work crews particularly cause this problem because they are so much wider than the space assumed to be used for parking.
For prediction, the best source I've found is the Rand McNally Motor Carrier's Road Atlas, because it shows the 8.5 wide, 55-65 foot long OTR rigs what highways are designated for their use. Off those highways, you are on your own, sort of. Actually, each state publishes more detailed maps for the trucking industry, showing additional roads with tighter restrictions than the Federal standard, and details the restrictions for each road segment. This information is not easy to get together for all states, and it changes constantly as road conditions change. Changes can be sudden, if an overpass gets knocked down or collapses, two roads might just close for a few months to a couple of years.
As you approach towns, you will often find truck restriction signs as you enter, truck routes designated to follow. Sometimes this is traffic control, sometimes it is because large vehicles can't get through. If you are piloting a large vehicle, the truck routes are usually safer. If you have a need to get into the town, local knowledge helps, it doesn't to stop and ask.
Tom Test
Itasca Spirit 29B
