There already pretty good answers (Even Mine is a great one!). It's a glorified comment to add a few points.
One side effect of a entire population running around is calories needs increase. This makes land even more valuable and food production even a bigger business.
Car and other vehicles can still be necessary for long distances depending of how much a human can endure to run at top speed. Cargo vehicles like trains and ships are still common. Anyway oil consumption will decrease a bit an can even not be biggest business around.
A interesting side effect can be high demand for footwear. Running at 40 mph all day ill make people buy running shoes weekly. Footwear industry can be a lot larger in this hypothetical world.
Running at 40 mph a platoon trained to run carrying heavy logs can crush gates and break enemy lines with easy. All tactics can change since mobility is a key factor since the first pre historian tribal skirmishes. At that speed spearmen and javelins throwers can be fearsome.
Since cars are more a utility for cargo and long trips streets ill be build for fast running people. That means maybe a grass lane with a damn good drain system.
Law enforcement in transit ill don't need the arise of automobile or the big confluence of horse powered wagons. Each ancient civilization ill create his transit code and in modern days we ill got specialized transit lawyers.
Alcohol can be strict prohibited for everyone!
Even in medieval eras communication ill be a lot better between tows and feuds. Also depending on human endurance trips can be a lot more common increasing trade letting people in touch that with the mobility of armies and the need for more land can even "enlarge" a lot the size of the small medieval feuds and nations ill be likely to form early.
Need is the mother of invention That's true so steam power => Combustion => Aviation can arise a bit later for example but A increased commnucated humanity can need telegraph early.
Now trying to respond the question:
How to keep stupid people alive
Just make a better peripheral vision. the human ability to drive at 40 mph proves we don't need a lot more upgrades.
Can strict prohibited to move and use a mobile device for obvious reasons. In some places they can be entirely banned.
Prohibited since ancient times!
Can even be a taboo
Sorry for the lack of beer & wine guys.