So you observed 21 cars with issues out of the 1000 you saw, a marginal hit rate.
Rebuilding just a mile of a side street would cost $10 million. If it is a major road the cost goes up, and if you need to put in bridges the costs is very high. A useful change to a single intersection can costs more than a million dollars.
We have the ability to do more testing and training. However may drivers would hate the fact that they can't pass a license tests - enough to vote out anyone who tries to pass such things and get the law changed.
Long term running great public transit is the cheapest answer, but anything less than great won't move the needle, so your minimum costs are in the mulit-billion dollar range for tiny cities - cost goes up greatly from there.
This claim gets made a lot but, while I'm not sure how you would test this, I'm not really convinced that an expensive driving school and more rigorous driving test moves the needle much relative to just a lot of hours on the road and perhaps becoming more mature with age. My recollection is that my driving almost certainly approved with a year or two of driving after I got my license; you're not going to replicate that amount of driving with training/testing.
Commercial (not private) pilots would be a good example. They regularly take refresher courses to get updated on the latest laws. They have regular simulator training so they can practice situations harder than anything they will ever encounter. They have to provide proof they are in good health. (I'm not what is legally required, or what something all airlines require but isn't legally needed).
I've seen driving simulators - not video games, but actual driving simulators. They are very good, we can require everyone spend a couple hours per year in one to show they know how to drive. We can setup situations where the other car does something stupid. We can setup situations where you will crash and have to choose the least bad crash. We can setup bikes and pedestrians to ensure that you drive safe around them. We can setup bad weather, ice on the roads. We can setup mechanical car failures. These are things that rarely happen and so you need refreshers to ensure you do the right thing should they happen to you.
A rounding error?
The point is human drivers aren't a proportions problem. It's that any unsafe driver is bad.
Adding more safe drivers doesn't lower the risk of unsafe drivers, even if they're a smaller proportion of drivers.