In the last 5 months there have been at least 71 kids killed by guns. Average just under 6 years old

[sarisataka]

73. I can see the economy taking a hit

though since so much is electronic I question the overall effect. People would have to leave earlier to go to work. Goods would need to be shipped more efficiently as in by train. It seems like it would be unpleasant but we could adapt.

Disease- why? I fail to see any connection between the means to travel and the spread of disease except in enhancing the spread. Slower travel should slow the spread disease
Starvation- there are no hi-speed tractors. Most farm supplies are delivered by rail then locally by road. The food supply uses it in the reverse to a lesser degree. Leveraging that infrastructure would enable food to move from farm to market.
Suicide- I would seriously dislike travelling at 20 mph but it would not make me suicidal. As society adapts it would reduce the short term pressure. The exception being oil company executives.
Violence- I would expect a short term spike in road rage due to frustration at change but that too would recede as the new becomes normal

20 mph may be extreme but we can ask the question now, why do we have cars that can exceed twice the highest posted speed limit in the country? Would there not be a measurable benefit to require cars governed to 80 mph or less?