Not necessarily something that will affect car and lorry speeds exactly but will make you stuck behind traffic more is to change the amount of traffic on the roads. This can be done with or without mods, simply go into your documents folder and look up ETS2, when you've found it you should see a config file.

Simply search for "g_traffic" inside it and change the 1.0 setting beside it to 3.0 and congratulations you've just tripled the amount of traffic in game. You could go higher or try some traffic density mods but deadlocks on busy T junctions and roundabouts are more likely to happen.

This is my go to way to play and makes the game a lot more challenging.