Note: Who Framed Roger Rabbit is set in Los Angeles, California, some time after World War II.
Post-WWII Los Angeles looked significantly different from the LA of today. Besides being considerably smaller, early 20th century LA was criss-crossed with a comprehensive electrified trolley system. In the years after the war, the trolley system was slowly dismantled as the freeways, now synonymous with LA, were built up.
According to rumour, General Motors, through its American City Lines and Pacific City Lines affiliates, bought up trolley systems in cities across America for the sole purpose of destroying them, thereby depriving the American public of transportation alternatives to the car. Although there is no real evidence of conspiracy (GM was convicted of conspiring with others in the automotive industry to monopolize the sale of supplies to local transportation companies), this meme forms the subtext of "Who Framed Roger Rabbit?"
Marvin Acme is killed because he was going to turn ownership of Toon Town over the the toons themselves. This would foil the plans of the mysterious company Cloverleaf, which wanted to purchase Toon Town so that it could raze it in order to build a freeway, paving the way for for LA's modern freeway system and spelling the end of cheap, clean public mass transit.
Judge Doom quotes, courtesy of imdb.com:
Judge Doom: A construction plan of epic proportions. We're calling it a freeway.
Eddie Valiant: Freeway? What the hell's a freeway?
Judge Doom: Eight lanes of shimmering cement running from here to Pasadena. Smooth, safe, fast! Traffic jams will be a thing of the past.
Judge Doom: You lack vision, but I see a place where people get on and off the freeway. On and off, off and on all day, all night! Soon, where Toon Town once stood will be a string of gas stations, inexpensive motels, restaurants that serve rapidly prepared food. Tire salons, automobile dealerships and wonderful, wonderful billboards reaching as far as the eye can see! My God, it'll be beautiful!