According to the article I read, the NH people had no clue if of how to determine the cause of the reduction in emissions in the 'test' areas - whether due to changes in the usage (speed) alone, or an element of a change in what vehicles are used, or the amount of vehicles.

Typical government / civil service guff - no way of determining success, so either plough on ahead regardless (whilst touting a 'success' the 'new policy' that may have made a negligible difference, if any) or drop it because the cost of finding out would more than outweigh any potential benefits, and would require some brain power to be used.

Rather like 'smart' motorways or LTNs, I think.