Apparently they are uncommon in US? I guess there are enough general airstrips and open space to make them not necessary
I understand that Ukraine put to good use its highways after the initial Russian attack that temporarily put out of order many of its airports.
The actual "failure" loads are orders (yes, plural) of magnitude higher and most failures aren't the kind of catastrophic thing that people on the internet typically try and imply unless you get really unlucky and your mountainside shelf road turns into a landslide. Failure usually means you park a heavy object somewhere and come back hours/days/weeks/months later and find that it sank or slid slowly downhill in that time.
Short of maybe, maybe, landing a fully laden C5 on everglade highway or redefining landing to include crashes where the tires happen to be the first bit to crash you are going to have a hell of a time displacing the road bed a meaningful amount with an aircraft without destroying the aircraft. They are simply not strong. They have to be light enough to fly you know.
Never-mind the fact that they used to fly these things off of dirt strips...
At best you might displace the asphalt a little and leave some ruts behind you. Remember, these are vehicles on pneumatic tires. Their point loads aren't high enough to really cut through pavement. All you can do it smoosh it out of the way. And the road bend was made of gravel of particular size and roughness and then gone over with compacting equipment to prevent exactly that from happening, albeit over time.
Later, WWII lead to the construction of even more airfields for pilot training. So many pilots were being trained that the training bases didn't have the capacity for the pattern practice, so many training airfields had multiple "outlying fields" that were just extra sets of runways a few miles away. Some of these turned into municipal airports later, but many more were abandoned for lack of need. By the time the freeway system was built, the US was positively lousy with airports, much denser than seen in most other countries.
https://www.af.mil/News/Article-Display/Article/2721902/a-10...
Sounds like it wouldn't work for an emergency landing?
There’s still the middle barrier etc., so in a planned conversion that would be removed, a control tower of sorts set up etc.
But in todays traffic, you’d probably not find a gap large enough to squeeze an airliner in. And I don’t think these are still maintained as air strips, at least in Germany.
Trivia: Ramstein Air Base was originally also an autobahn air strip used by the nazis during the last months of the war, then the US forces occupied it, and the autobahn had to be rerouted.
Wider runways also make it easier to land/takeoff in a wider range of weather conditions. If you plan to routine use something, it's worth removing the barriers.