Discount Signs are small signs that are just slightly taller than your average car height. They are red and have UPGRADES emblazoned across them with Dak's face on their left side.
As you drive, you will notice these signs on the side of the roads, and a red dot will pop up on the map. All you need to do is hit the sign and it is collected.
When you have driven the full length of a road, any signs you passed will be highlighted on the map as a red dot. You can go in to the map interface and set a waypoint to the sign. When you hit the sign the red dot will become a grayed out dot.