Q: Why does it take six months to replace a bridge on a two-lane, paved road?
A: Because there is no incentive for the winner of the bid to do it any faster.
It used to take US stamping plants the better part of a week to change over the dies on a major stamping line. Because of the slow change-over, the "economically viable run" measured in the tens-of-thousands of hoods or fenders or roofs.
Then the Harbour group started documenting how quickly the Japanese companies could change over a line.
It now takes all of a minute to change the presses. The ram (the part that comes down) lowers the punch onto their respective dies. Pins are pulled hydraulically. The die/punch sets are pushed out of position by the new sets that are sitting on roller tables and exactly the right height. The new set rolls in. The old sets roll onto another set of tables on the other side of the press-line. Hydraulics shoot pins to lock punches and dies and the press starts cycling.
Just throwing some numbers on the table. AAA estimates it costs about 82 cents a mile to drive 10k per year. The IRS allowance is 56 cents. Splitting the difference is 69 cents a mile.
Let's assume a typical detour is additional three miles. That is $2070 per thousand vehicles.
Now assume that the detour adds seven minutes of time. Further assume that every vehicle is single occupancy and the time is valued at the current Federal minimum wage of $7.25/hour. That is an additional 84 cents per driver or $840 per thousand.
This ignores secondary effects like incremental traffic congestion along detours and the impact it has on those drivers.
This math would be greatly enhanced with real data but a cost-to-society of $3000/thousand vehicles is defensible. If that cost savings were split between "society" and the contractors replacing/repairing the bridge, then the contractor would receive a bonus of $1500/per day on a bridge that saw 1000 vehicles per day.
I know I have readers with a wealth of experience. If it were a priority, is there any reason why it would take more than seven calendar-days to replace a bridge...assuming the steel and concrete and workers and equipment showed up when needed? Would they even break a sweat?