120mph in a 1960s car? you're joking right?
(ah, you mean the ones on the main carriageway, without warning signs? ok

)
well i suppose the law and emergency services are allowed to exceed the Joke (um, i mean, 70 limit

) so at least they should be able to round most "unmarked" corners safely at the ton in the wet on the motorway...
Personally i think the autobahn 150-odd mph is probably far too much for most people to cope with unaided, but 80 is well within the capabilities of pretty much anyone who doesnt freak when presented with a 3-lane divided highway, and is even a decent cruising speed in a 1L (though not really any faster!). Should be higher, but with that, it needs really to be more like the states and many parts of europe, where there might be a general idea of what speed most motorways / autoroutes/strada/bahns / freeways are set at, but it's still allocated on a case-by-case basis in real life. I've taken journeys in france and italy where the limit on the same road has gone between 130 (would be 150 in some cases now!), down thru 110, 100, 90, 80 even 70kmh and back up again depending on the road's location, prevailing traffic conditions, curves & junctions etc. Some of them seemed a little low, and the blanket max of 130k got exceeded by some margin in various places, but on the whole a fairly sensible arrangement.
Maybe an idea to start with switching to kilometres, and in general, converting each MPH limit to twice the existing number but in km/h? That would be closer to many people's general driving speeds
Highest limit on motorway perhaps 90mph/140-150kmh (with an unwritten understanding that you can get away with the ton but not really any higher - and over ~125/200kmh is a ban unless you can prove you were travelling safely) on the best sections but with many being lower, with ~80/125-130kmh being the most widespread, and 75-thru-55 (90-110/120kmh) for special cases, less well designed/constructed/maintained sections and tricky junctions.
Stick with around 60-70 for A-roads (keeping head-on collision speeds down) and 70-80 for dual carriageways, again depending on quality. That most dual carraigeways, with minimal fencing, semi-urban routes and decidedly rubbish junctions can be the same limit - with LOWER fines/penalty points! - compared to a full on motorway is just plain daft...
Thing is, with more sensibly set limits - and possibly, as an experiment, properly explained ones where it's lower than the max? a little hazard sign underneath the limit? - they'll get more respect and more people adhering to them, rather than thinking... "70? sod that... floor it!". Traffic moving at a more even pace and less frustration at the slow bugger in front of you, hopefully less bunching (hopefully...) and an actual improvement in road safety and economy with less hard acceleration/braking.
(also stricter bans on trucks overtaking etc - have to be progressing a certain reasonably increased speed compared to the vehicle in front to make an attempt at passing, maybe get all truck speed limiters VERY finely adjusted, using GPS, or just abolished... perhaps even differing speed limits and MINIMUM speed limits per lane in particularly busy or trunk-freight routes... why is the M6 Toll so great, so quick, and so safe? Very few trucks, so you can have 3 lanes of traffic at 75, 80 and 85..

... funny how the motorway doesnt have a minimum speed limit for "clear/flat road" conditions, so any tosspot can trot down there at 40mph, legally)
there's so much work could be done on this whole thing but the usual response seems to be "slap a limit on it 10mph lower than what's expected and garnish it heavily with cameras". time to get the red flags out and give them to a man to walk in front of you...