I heard somewhere that currently in many places before a road diet is implemented there is the requirement for a traffic study to take place, and the studies cost tens of thousands of dollars.

Based on all the previous road diets that have been performed, and the data from them, would it be advisable for a city (or other road authority) to simply decree something like - when repaving a 4 lane road, the default is to diet it to 3 lanes unless (... list possible reasons why diets didn't work such as but not limited to extreme traffic levels)? In other words, dieting the roads is rebuttably presumed a good idea during repaving. Such a policy would not only save money, but would also kill the anti-diet excuse of well, the traffic study would cost too much.

Speaking of which, does anyone have access to a copy of the Road Diet handbook? I would suspect that it would list the pitfalls of when NOT to road diet.