We all struggle to generate enough revenue to keep our cities in the black while keeping sims happy. I have discovered that if used properly, tollbooths can be a highly effective source of revenue and on many of my cities where I have implemented a border tollbooth plan I now generate revenue surpluses for my transportation networks.
First some caveats. Sims don't like tollbooths on inner city roads and if a residential or commercial zone is "pointing" two the section of road where the tollbooth is placed it will get the symbol of a little green car with a slashed red circle indicating it doesn't have access to a road. Industrial zones do not have this problem.
How I use tollbooths
I have found that the most effective place to put tollbooths are on all roads leading into and out of my cities. Since I use a standardized grid pattern for all my cities, and normally place industrial zones along the edges of my city (to reduce the pollution impact) this works really well. What I'm discovering is that a lot of the traffic along my city edges are actually just sims passing through from one adjacent city to another adjacent city. By putting tollbooths on all edge roads I'm able to hit these sims up twice for tolls and am able to make a killing with revenue. In the case of a city I was playing last night, I was generating 10,000 in net profit per month from my transportation network which had total monthly transportation revenues of around 16,000. The key with this is that your cities need to have lots of road connections between cities.
The attached screen capture from early on in my efforts with our first SimCity challenge which gives a good illustration of a good intercity road grid. You can see the post where I first posted this image at http://simcityforum.com/showpost.php...6&postcount=45