The answer to this question really depends upon the interest rate at which funds were borrowed, or if we had the capital initially, the rate of return we could have gotten on that capital if it had remained unused. (Theoretically these two rates are supposed to converge to be the same at equilibrium, but in reality this almost never happens.)
So instead let's calculate as if we are paying no interest, and then consider what would happen if we'd had to pay interest.
With no interest, we simply need to figure out how long it takes to make back $1.3 billion if you have 4000 cars per day each paying $45.
4000 cars/day * $45/car * 365 days/year = $65.7 million/year
The break-even time is just dividing these two:
$1.3 billion / $65.7 million/year = 19.8 years
So, in just under 20 years the bridge will pay for itself; built in 1997 this means it will break even quite soon, in 2017.
Now think about what would happen if we had to pay interest; that money paid in 1997 would be worth more today if we'd kept it. This means that the time to break even on the bridge will be longer if we have to pay a high interest rate.
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