As you may work during the night (or during a heavy rainfall) you need to use lights. Configure them using the quick access ([F4] by default) or the "my truck" > "cabin modification" menu displayed on the right when the game is paused.
You should rather use the low beam ([F4] +  by default). But these lights do not provide too big field of view which is dangerous at high speed and on twisty roads.
Therefore use the high beam as well ([F4] + ). Such lights provide much bigger field of view. Emergency light ([F4] +  by default) is a blinking orange lamp located at the top of the truck (if you buy it) and the trailer (if it is assembled there). If you buy additional front and roof lights you can turn them on. The truck not only looks better but you see the road better as well.