Well, first there is simple mechanics involved. If you make a car that won't go over 80 mph, then you also have a car that won't carry very much stuff, and will be limited as to how much weight it can take. If you have a car that is capable of carrying a fair amount of weight and passengers, then it will also have the horsepower to go fast. So, having manufacturers limit the HP on their vehicles is pretty much a non starter. Everyone likes to take their families with them on road trips, as well as have enough gear to make the trip worthwhile and enjoyable. Make cars that are not capable of much, and people won't buy them.
As has been noted here further up the thread, you CAN put limiters on them, but, there will also be the people who will simply remove the restrictions placed on the engine, or find someone to do it for them, resulting in a car that will go fast again.
You need to have a good amount of HP to make the vehicle useable. Me? I have a Honda Civic that is good on gas, and won't win any drag races, but it also won't be able to drag a trailer behind it that is over 1000 lbs (which is only a pop up camper trailer, anything bigger and it won't be able to tow it). Lots of people out there need a vehicle that can tow trailers or carry lots of cargo for whatever reason. It's their right. But, that also means those vehicles will be capable of high speeds when empty. No, limiting the vehicles won't work.
And, even if you did, it's too late to put the genie back in the bottle. There are MANY people out there (myself included) who know how to get more HP into an engine. Even though I might not need it all the time, it's nice to have if I do. Someone further up also noted that there were times that having lots of HP and able to hit high speeds quickly saved them from an accident. I've had an experience or two like that myself, and was grateful that my vehicle was able to get me out of a bad situation. I had a motorcycle (Harley Sportster), that I modified for HP. It was capable of hitting 130, and only took it up that high a few times. Once was to see how well I'd done on the engine (which is why I know how fast it was), and other times were doing dyno drags in Sturgis (which I won handily). BUT, having that extra "oomph" kept me from getting ran over by cars a time or two. And, it also allowed me to keep up with the big twins that I rode with.
I don't speed (much), and I try to keep it no faster than what the flow of traffic is. Here in SC, on interstate roads, there are times that if you're not doing at least 75, you are actually holding up traffic. I've seen many days where I'd be sticking with the flow of traffic, only to look down at my speedometer and see that I'm doing 85 to 95 and STILL getting passed by other cars.
If people want high HP cars that go fast, let 'em have them. They'll figure out how to make them go fast no matter what the regs are. I mean, look at all the street racers out there, many of them modify their cars themselves. All that can really be done is make the punishments fit the crimes of exceeding the speed limit. Go 10 to 25 over? You should only get a fine. Go 30 plus over the limit? You should have your car taken away and license suspended for at least a year, because you've proven yourself to be a danger to others as well as yourself. But even then, there are still places in this country (Montana, where I'm from for one) where speed is needed if you want to get somewhere without spending all day getting there. In Montana, police won't pay much attention to you if you're doing 85, since it's such a big state with a small population.
But, even then, I can see exceptions being made. If I had a seriously injured person who needed medical ASAP with me? You can bet that I'm gonna get them to a hospital as quick as I can, even if that means going 100 plus mph to get there, as I don't want them to die on me.
Sorry to say, but I don't agree with the OP. There are times that high HP and speed are needed, some where it's wanted, and sometimes (in the case of approved racing) where it's just plain fun.
