* Slowdowns, sure, but many drivers also exceed the limit, and the walking:driving ratio is still massively lopsided.
* Pedestrians are not immune to red lights.
* While there are certainly pedestrian shortcuts that are faster than driving, this is definitely not the case for most combinations of start and end points. Far more often, while the pedestrian might gain distance, the car is still literally 10x faster.
* In fact, unless you live in an extremely dense city, it's highly unlikely that there will be a grocery shop, book store, electronics retailer, Italian restaurant, furniture store, etc. less than 5km from your house. Plus all of the people who you might want to visit, your school, place of work, and so on.
* A bike is not walking. Most bikes are for one person, so if you're in a group they're not much use.
* Carrying purchases home is a pain. I should know, I did this for two years.
My favourite part of visiting Amsterdam was the excessive bike usage. Pretty much everyone has 1 or more of them, you just ride together in a group or catch a tram, even out of the city everyone would either catch a train or simply ride on the side of the highway between villages.
I do groceries by walk and it never took 2 hours. The store is not 5km away pretty much from anywhere. You have to be out of city or in the middle of night for it to be that far. Unless there is some kind of law to prevent it, stores with what you need daily pop up around cities everywhere.
In any case, 5km is still nothing on bicycle which I might take for that distance. Just to kill two birds with one stone and get a bit of exercise.