>Specifically, to deter non-resident access to and traffic, especially high-speed traffic, near them. There are a wide number of reasons, but promoting pedestrian (and particularly child) traffic safety, deterring crime, and reducing noise pollution are among the reasons.

Not building a road would do a much better job of accomplishing these goals. The road is built for the sole purpose of allowing people easy access to their home.

>But are cul-de-sacs assumed to have that average life?

Assume they have double the life, the road taxes still don't come close to paying for it.