In America driving at 6 km/h (3.7mph) in a parking lot would induce road rage in other drivers. Trying to make roads with a 6 km/h speed limit would be political suicide for any politician that recommended it.
That's literally a slow walking pace. Kids running would be breaking the speed limit. Bikes would always be breaking the limit. The speedo in my car isn't even accurate at speeds that low.
It's not about a hard speed limit, it's about driving at walking pace. It's not for thoroughfares, it's for cul-de-sacs where people live and children play. No car has any business going fast there.
Bikes aren't a problem; they're small and nimble and don't kill people when they hit them.
You seem to be intentionally missing the point here. It's about safety for playing children. Cars are a bigger issue than bikes. Especially when the bikes are being used by playing children.
In driving school I learned that I should put the car in first gear, not touch the gas and keep my foot ready to hit the brake when I enter such a street. That is, I'm supposed to go as slow as reasonably possible and be prepared to stop on a dime if a child materializes in front of the car.
It's really not that bad. Keep in mind that you typically will only go this slow for the last 15-60s of your drive. Before that you will likely go faster than on a typical drive in the US. European speed limits and especially German ones have a lot more range then US speed limits. To be that makes for a more deliberate and fun driving experience. It also turns out to be safer if you look at fatalities per mile traveled, even including the Autobahn which seems to be unfathomable to most Americans.
It's not my place to speculate, but I'm going to do it anyway: I expect you can get away with simply just driving as slowly as you can, assuming that isn't ridiculously faster than the 7km/h that is suggested. (My old automatic car (1.4L) would probably creep at about the right pace. My manual car (3.0L) isn't super happy to pull the car along while idling in 1st gear, but it's fine at ~1000rpm and that's ~8km/h.)
(This does make me wonder. I typically drive in car parks in 1st gear at <2000rpm, so max ~16km/h or ~10mph. But, you know... maybe I'm going too quickly?)
fluid-coupling automatics do have an advantage here, as they can essentially creep at any speed without causing undue stress to any of the vehicle's components, although you would need to use the brakes to go really slow.
when driving through complicated areas with lots of pedestrians (eg, parking lots), I usually travel at the lowest speed in 1st gear that puts the engine in a happy RPM range (as low as ~1200 in my car, maybe 1500 up a hill). this is definitely faster than a human can comfortably walk for a long time. if that's sufficiently slow, I guess I don't have any objections.
