This patch allows you to increase or decrease the maximum amount of road vehicles per player in the game at any one time. This is setting can be found under the "max_roadveh" setting in opentdd.cfg, with the default being 500. It can be changed in increments of 100, from 0 up to 5,000 (which is more than enough for anyone) If you change this setting, say, to 100, when you already have 101 road vehicles in-game, then the game will give you a 'too many vehicles in game' error.

Max Road Vehicles option in Patch Settings: Vehicles