Assume 1000
necessary crossings per day, and about 500 unnecessary crossing per day. Without bridge the closest route requires alternative with
extra driving of 30
miles/trip.
Therefore, daily cost = 1000 trips x 30 mi/trip x $0.15/mi =
$4500.
If necessary crossings are assumed to be
undertaken 20 days/month, the monthly cost
would be $4500 x 20 = $90,000.
P = $90,000 (SPPW i=0.06, n=25)
= $90,000 (0.233)
= $20,970 = negative effect
Still not enough to offset the cost difference, but the inconvenience,
an intangible negative impact of not having
the bridge for a month, might be more than enough
to shift the preference to the steel alloy bridge.