Twenty-nine states and the District of Columbia have laws that regulate engine idling, but each of these states has different applications of idle reduction and various exemptions.
Some states regulate only certain vehicle types, such as school buses, state vehicles, commercial vehicles, diesel vehicles, or vehicles over a certain weight rating.
Some states regulate idling geographically, such as on school property, in business districts, or in certain counties. Other states have time limits on how long an engine may be idled, or idle limits during certain times of the year.
Details for each of the states can be found at the US Department of Energy’s Alternative Fuels Data Center Federal and State Laws and Incentives page.
Source: US Department of Energy, Alternative Fuels Data Center, Federal and State Laws and Incentives.