For free roam, you can enable AI controlled vehicles by spawning a new vehicle, adding the AI control app/UI, changing the AI controller to "random", and then swapping back to the other vehicle. Have fun!!
Cool. You are able to add AI cars in freeroam. You add another car (so you have two or more) and you have to add the onscreen 'AI Control' App. With your CPU I'd recommend having 3 or less cars at a time. Other than that enjoy the game!
Another thing that may help is reducing your resolution, i had a Notebook way slower than yours and i used various way to play this game, you should be able to play on gridmap with that pc, enjoy the game and good permanence on the forums
But please notice: The more AI Players you enable, the more powerful your System needs to be, especially on detailed maps.
Thanks guys, yes i've stuck to gridmaps and 2 AI cars and it runs pretty well, I had some fun last night!. It;s better now on the more detailed scenes too but I only use my car (no AI). Alas when I get some funds I will get a better PC for this game but for now I'm still enjoying the heck out of it!