Assessing the value of the negative effects of closing the trestle bridge: Method 2- the Òopportunity costÓ of not having the bridge for 1 month 25 years from now.
  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.