That seemed like a pretty good reason to have lower speed limits in pedestrian areas to me.
The goal of improving roads is to increase the throughput of traffic. If it seems as though a problem could be solved by slowing down traffic, that's like solving a problem with a circuit by throwing a resistor into it. It may solve the immediate problem, but it will also reduce the overall efficiency of the circuit by converting energy into heat rather than usable work.
They could also put up signs that say 100% of the people who drive 40 mph reach their destinations 33% faster than those who drive 30 mph. It's just a distraction from the fact that solving the problem in a better way is more expensive than just blaming drivers and letting insurance handle the fallout.
So maybe those signs are on streets where throughput is not the primary goal, and where substantial barriers would interfere with people moving from their parked cars to the sidewalk and the like.
If you look at a high-traffic street where parking is prohibited, the Magnificent Mile in Chicago, you'll notice that there are gigantic concrete planters between the cars and the regular pedestrian traffic. Retail stores protect their entrances with concrete bollards. Construction zones protect workers from traffic with segmented Jersey barriers and water-filled crash barrels. Those are actual solutions.
But then you have suburban pearl-clutchers who lobby for a 15 mph speed limit and speed bumps in their cookie-cutter subdivisions, rather than actually making their streets safer for pedestrians. Or by replacing yield signs with stop signs. Those people don't realize that "change the law" is not always equivalent to "fix the problem".
"The goal of improving roads is to increase the throughput of traffic" - that is considered an outdated goal by most planners these days. For one thing, if you increase the capacity of roads, you simply encourage more driving, and things end up congested anyway. In many cases urban planners now actively seek to decrease the throughput of traffic on mixed-use streets in urban areas.
As I mentioned before, the urban planners who are adding resistance to the system are not helping the overall efficiency. Intentionally decreasing the capacity of urban infrastructure is almost criminally stupid. It approaches the level of solving an overpopulation problem by conducting a decimation lottery every time the number of people approaches some upper limit.
Some planners are shutting down some roads to increase throughput, by encouraging greater use of more improved roads. Others are altering roads, or installing traffic circles, or adding stop signs and speed bumps, just to make vehicle traffic gratuitously more difficult. Those people are the assholes.
Congestion results from insufficient infrastructure to meet public demand. If your sidewalk is too narrow for the foot traffic, and people get jammed up and spill out onto the street, you cannot solve the problem by making it narrower and throwing caltrops onto it. And that doesn't work with cars on roads, either.