Because it offers contractors and DOTs alike a safe and economical method in rebuilding our aging bridge infrastructure, the rental bridge market has gained a strong foothold in the highway construction arena.
The safety value is obvious – a detour or bypass bridge separates work zone traffic from construction activity. This affords additional safety not only to construction workers, but also to the motoring public. Work zone fatalities continue to be an increasing problem and the use of bypass bridging is one method to ensure a safe worksite.
An Acrow project recently completed in Colorado provides a clear example of the practical benefits of a rental bridge used as a detour during major highway construction.
As part of a larger project to widen Interstate 25, it was necessary to replace two bridges in a high-growth, densely congested corridor. While it had already been determined that a detour bridge would be necessary, the two-lane temporary structure was selected as an alternative to building a concrete detour lane that would need to be demolished at a later date in the project.
The traffic was diverted to the temporary bridge in November 2016 as the existing northbound bridge is demolished and rebuilt. It is anticipated that in May 2017 the temporary bridge will be used for southbound traffic while the existing span is demolished and rebuilt, with the original traffic configuration expected to resume in October 2017.
Use of the rental structure allows for a fixed dollar-amount to be allocated to the detour bridge, and can help ensure the project stays on or ahead of schedule, important for both contractors and government agencies.
But contractors looking to truly understand the full benefits of temporary bridging as part of their approach to DOTs and other public agencies may want to take a deeper dive into the cost-benefit analysis.
What is interesting, for example, is the fact that “economy” from the perspective of many DOTs and other government agencies at the county and municipal levels goes beyond the actual price tag of the temporary bridging. Many have started evaluating a project’s work zone “user costs,” defined as the additional costs borne by motorists and the community at large as a result of work zone activity. Using that analysis, stake holders can evaluate the economic impact of various construction methods to ensure that impacts to the public are minimized and that the most economical solution is selected.
An example of the economic savings realized by the utilization of a temporary bridge in lieu of staged construction is highlighted below. To investigate the feasibility of renting a bridge on a major highway the main factors that come into play are:
- Travel delay costs
- Vehicle operating cost (VOC)
- Maintenance delay cost/maintenance of traffic strategy (MOT)
In addition to these three basic costs there are also additional work zone user costs that come into play such as business impacts and inconvenience to the local community.
Travel delay costs are calculated by multiplying the estimated delays to personal travel, truck travel, and freight inventory caused by the work zone by a unit cost of $/hour of travel time. To correctly estimate the delay time, which is the additional travel time to traverse a work zone or detour around it, you need to look at the additional time required due to changing speeds as the driver decelerates to enter a work zone and then accelerates to normal speed after leaving the zone. Added to this delay, the reduced speed delay while traversing the work zone must be factored in plus any stopping or queuing delays encountered in the work zone. These are all in addition to any delay times required if a detour route is also required to avoid the work zone activity. An example of a detour delay as outlined in the FHWA’s (Federal Highway Administration – USA) publication Work Zone Mobility and Safety Program is:
Detour Delay Time = (detour length/detour speed)-(normal travel length/upstream speed)
Detour Delay Time = (3.0 miles/40 mph)-(2.0 miles/55 mph) = Detour Delay Time of 2.32 min/vehicle
Since traffic flow is not a constant through a 24-hour day, a series of readings must be taken during the 24-hour period to determine the average detour delay time that can be analyzed using the average daily traffic volume.
Now that the travel delay time is calculated we have to apply a monetary value to the travel time delay. The monetary value of travel time is based on the concept that time spent traveling otherwise would have been spent productively. The monetary value of travel time is the sum of:
- Dollar value of personal travel time (only passenger cars) – The hourly dollar value of road users’ personal travel time estimated based on some percentage of their wages.
- Dollar value of business travel time (only passenger cars) – Business travel time is defined as the hourly dollar value of road users’ business travel time estimated on employers’ cost of employees that includes both wages and benefits.
- Value of truck travel time (only trucks) – The hourly dollar value of truck travel time is estimated based on the compensation costs of truck drivers that includes both wages and benefits.
- Cost of freight inventory delay (trucks only) – The inventory cost is computed by multiplying the average payload of the truck with the average value of the commodities shipped by truck.
- Cost of vehicle depreciation (all vehicles) – Total vehicle depreciation costs are estimated from the average annual ownership costs of vehicles and are applied to the delay time.
The second major component requiring analysis in determining the cost of work zone delays is the vehicle operating cost (VOC). The vehicle operating costs are the expenses incurred by road users as a result of vehicle use which are mileage dependent. They do not include any fixed costs such as insurance, depreciation, storage and financing. VOC does include consumption costs such as fuel, engine oil, tire wear and repair and maintenance. The sub components of VOC are any speed change required, additional stopping, idling while queuing and any additional mileage costs.
The three basic models that are used to determine VOC in the United States are:
- National Cooperative Highway Research Program Report 133 (1972)
- Texas Research and Development Foundation Method (1982)
- Highway Economic Requirements System – State Version -20 (HERS-ST)
The most common international VOC models are available through the World Bank’s Highway Design and Maintenance Standards and the British Cost Benefit Analysis Program.
The third delay component to discuss is the maintenance delay cost associated with the maintenance of traffic strategy (MOT). Usually MOT includes all physical controls such as barriers and other devices for managing work zone safety. Recently, MOT strategies have been expanded to include strategies for addressing public information.
MOT strategies can be outlined as follows:
- Temporary traffic control strategies that include traffic control devices to maintain work zone traffic and coordination with other infrastructure elements such as utilities and railroad traffic.
- Transportation operational strategies that account for work zone safety planning, law enforcement strategies and ability to make adjustments based on changing site conditions.
- Public information or public awareness strategies that keep the local public and businesses aware of the ongoing work zone impacts.
There are a series of economic tools available to help decision makers identify and quantify the economic costs associated with long term projects. These tools are excellent resources for the owner to minimize the work zone impact on the public and local businesses. One such program is the Highway Economic Requirements System – State Version developed by the FHWA that has a variety of models for computing various components of work zone user costs that are detailed above.
The following is a case history of a project in a major metropolitan city with an ADT of over 22,000 total vehicles per day (including 2% trucks) that was analyzed as to the economic impacts using staged construction versus a temporary bypass bridge.
Vehicle Operating Costs:
For an urban project VOC (vehicle operating costs) of passenger vehicles is assumed to be $0.50/mile based on data from the American Automobile Association and the VOC for trucks is $1.65/mile. The adverse travel time was calculated to be 1.51 miles, which was the difference between the detour route and the existing route. Therefore, based on a 140-day construction schedule, the VOC was calculated to be:
PV = 21,560 vpd x 1.51 miles x $0.50/mile x 140 days = $2,278,892.00
T = 440 vpd x 1.51 miles x $1.65/mile x 140 days = $153,476.40
Total VOC Cost = $2,432,368.40
Travel Delay Costs:
Delay of detoured vehicles:
- Estimated rate of travel through detour route = 20 miles per hour
- Distance = 1.51 miles
- Estimated travel time to navigate detour route = 4 min 32 sec
Un-detoured travel time
- Estimated rate of travel through route = 30 miles per hour
- Distance = 0.49 miles
- Estimated travel time to navigate detour route = 59 sec
Cost of delay of detoured vehicles per day
- Average travel delay = 4 min 32 sec – 59 sec = 3 min 33 sec or 0.06 hours
- Value of time from reference guide of $17.21/hour
- Delay costs for detoured vehicles = $17.21/hour x 0.06hours x 22,000 vehicles = $22,717.20/day
- Total Delay Cost = $22,717.20 x 140 days = $3,180,408.00
Detour Maintenance Cost:
To facilitate detour:
- Additional concrete barrier = 500 LF s $25/LF = $12,500.00
- Temporary pavement marking = 1100 LF x $0.40/LF = $440.00
- Detour Signing = 415 sq. ft. x $12/sq.ft. = $4,980.00
Total cost to maintain detour = $17,920.00
Total Detour Costs:
Vehicle Operating Costs = $2,432,368.40
Travel Delay Costs = $3,180,408.00
Detour Maintenance Cost = $17,920.00
Total Estimated Detour Cost = $5,630,696.40
The estimated costs of the rental bridge required to eliminate the detour was established at an estimate value of $500,000.00 for 140 days. Erection and labor cost were assumed equal for the staged construction and the installation and removal of the temporary bridge so the net savings is estimated at over $5,000,000.00 by using the temporary bridge.