Name one place where a roundabout held up traffic worse than a lit intersection or 4 way stop. They only cause issues because people don't read things like signs and see where they are going. The next time you're at a light in traffic, count how long you're stopped and then see how long you're stopped at a roundabout. They've saved millions of gallons in fuel.
Per the money grab: the faster you go, the worse your gas mileage gets (10-20% from 55-75 MPH), and more often you have to pay for fuel. Much of that money goes to things like taxes and oil refineries, etc., and some to the actual gas station that you stopped at. So great for government and oil companies...winning?
Add in that the faster you go, the more you have to slow down, use your brakes, wear and tear on other parts including your tires, and it's more often you have to spend money on those kinds of things. So passively good for mechanics too.
But I assume you knew all that already.