Parked for the night at a Rest Area in Menomonie, WI.
( I love the different colors of 'white' light when shooting at night.)
With the cost of diesel fuel so high many companies have installed APU's (Auxiliary Power Units) on the trucks. These are usually small, 2 cylinder, lawnmower sized engined that will run a Heater, A/C, and put 120v power into the bunk for the driver. The driver can run the APU and stay warm in the winter or cool in the summer without running the truck engine. This saves the trucking companies a lot of money.
However tonight I noticed that quite a few trucks were parked with their running lights on. This is usually an indicator that they are running the truck engine and either not running the APU or do not have an APU.