What activity are you talking about, that is feasible at 40mph in an urban environment but not at e.g. 25mph? Can you not imagine a different way of conducting that activity?
There is a fundamental inequity between the operator of dangerous equipment comparing hours and years of life, and the pedestrian who suffers the consequence of that comparison. The USA auto industry is built on this inequity, which is why no one ever talks about it.
Many people meet friends and make deliveries e.g. via bicycle. Those motorized vehicles that are set aside specifically for deliveries often travel more slowly than other motorized vehicles.
But I shouldn't pick nits; you were speaking in generalities! So when I said "any transportation goal" and you disagreed, you hadn't actually thought of a particular exception to my universal statement. And you still haven't thought of one. Are your contributions to this discussion offered in good faith?
1. I want to visit a friend. He lives an hour away by car. New speed limit changes it to two hours. I lose an hour of life in the car.
2. Auto plant needs radiators. Truck delivering them takes longer to get them there. Costs $x more. Car costs more. Car buyer has to work longer to buy car. Car buyer wastes y hours of life.
Or maybe you don't visit your friend so often. Or you decide that 90 minutes on the train is better than driving, or maybe your friend gets tired of making a 2 hour trip to the city, so he moves closer.
There are lots of alternatives that don't involve you spending more time driving.
While it's true that goods will cost more to transport if it takes longer, that higher charge is amortized across many products in the truck, so is a very small portion of the finished product.
So if a radiator fits in a box 16x24x6" or 1.3ft^3 and a 40ft truck holds 2400 ft^3 (subtract 20% since it won't be a perfect fix, so 2000 ft^3, so you can fit 1500 of them in the truck.
If a truck+driver costs $100/hour, that means each radiator will cost 13 cents more.
Or, another way at looking at it -- all of the parts that make up a car aren't going to be bigger than a car (sure, some space is lost to packaging, but there's a lot of empty space in a car), and 6 - 10 cars can fit on a car carrier truck, so each car will end up costing around $25 more.
Though since we're talking about urban speed limits, and there aren't many urban car manufacturers, slow urban speed limits won't affect the price of cars.
I suppose this subthread is complete, because you have now completely agreed with my original statement: you'll have to leave earlier for your trip to visit your friend, and delivery trucks will have to allocate more time (routes, trucks, drivers, etc.) for their deliveries. Alternatively, freight trucks might make fewer mostly-empty trips. Note that these two examples clearly match the characterization I provided: both may be accomplished at lower speed, with sufficient planning. This will be the "cost" of safe driving.
There is a fundamental inequity between the operator of dangerous equipment comparing hours and years of life, and the pedestrian who suffers the consequence of that comparison. The USA auto industry is built on this inequity, which is why no one ever talks about it.