The importance of road links
I must admit that I feel guilty for not posting for a while. The reason is simply that I’ve been swamped with work lately, finishing a report on a work project related to a previous post on Bad Locations – Bad Logistics?, dealing the cost of disruptions in freight transport. The report will hopefully spin off into a book chapter and a journal paper, or at least a conference paper, so I hope. Let alone some posts for this blog. It was when writing the literature study section of the report that I remembered the very first paper I read on transportation vulnerability. It is a paper that shows how to attach a value to the importance of road links.
The New Zealand Desert Road
Hazards and closure duration
Traffic flow and Closure Cost
In terms of closure cost, Dalziell follows the traditional approach of calculating the increase in generalized travel cost, looking at various scenarios for alternative routes, and the decrease in trips based on the travel demand elasticity in these scenarios. Interestingly, she defines the elasticity as
– (rp + rp-1 + … + r2 + r)
where r = ratio of cost of travel with all links open, to the cost of travel with one or more road links closed. This is contrary to the traditional measure of elasticity, expressed as
% change in number of trips / % change in cost of trip.
The elasticity parameter p is estimated from the predicted traffic flows versus observed flows during a road closure (with road closures apparently occurring rather frequently on this road, there is sufficient data for observed flows).
The total cost of road closures is assumed to be the sum of
- The change in vehicle operating costs
- The lost user benefit from those trips that are cancelled or suppressed
- The change in accident costs
Note here that the traffic model assumes that all travellers have perfect knowledge of the road situation at the start of their travel. This will not always be the case, and some travellers may be caught by a road closure while travelling, such that the may have passed the junction with the best available alternative and may have to go back (at a higher cost) or cancel their trip altogether (lost benefit). This means that the total costs are usually underestimated, but it is still the best estimate available.
Using Mone Carlo simulation, the cumulative frequencies of the annual road closure costs for the range of hazard and closure scenarios can then be estimated.
Cost-benefit and mitigation
A road closure will have a cost associated with it, so will a mitigation effort, e.g better snow clearing equipment. The issue is whether the benefit of mitigating will offset the cost of closure. The worth of the mitigation will depend on its ability to reduce the probability and/or cost of road closure. Thus, since mitigation efforts do affect the probability and impact of closures, it is necessary to re-calculate the scenarios with the mitigation option in place. It is not simply a matter of road closure costs versus mitigation costs, but rather a matter of expected closure costs with mitigation place versus expected closure costs without the mitigation in place, compared to the capital and maintenance costs of the mitigation option.
Conclusion
References
Dalziell, E., & Nicholson, A. (2001). Risk and Impact of Natural Hazards on a Road Network Journal of Transportation Engineering, 127 (2) DOI: 10.1061/(ASCE)0733-947X(2001)127:2(159)
Author links
- canterbury.ac.nz: Alan Nicholson
- rsrc.co.nz: Erica Seville, nee Dalziell
Related
- husdal.com: Bad locations – Bad logistics?
- husdal.com: Are roads more important than computers?
- husdal.com: The reliability and vulnerability of transportation lifelines