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Arrow I-85 Interchange Design-Build Project Using Prefabricated Bridge Elements in West Point, GA

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Economic Analysis

A key aspect of HfL demonstration projects is quantifying, as much as possible, the value of the innovations deployed. This entails comparing the benefits and costs associated with the innovative project delivery approach adopted on an HfL project with those from a more traditional delivery approach on a project of similar size and scope. The latter type of project is referred to as a baseline case and is an important component of the economic analysis.

For this economic analysis, GDOT supplied most of the cost figures for the as–built project. The assumptions for the baseline case costs were made based on discussions with GDOT staff and national literature.

Construction Time

The project had an original construction schedule of 18 months, and GDOT reported that construction was completed in 16.5 months. Traditional contracting and construction would have taken 30 months for this type of project. Therefore, the project was completed in nearly half the time needed for traditional methods. The remarkable time savings are credited to the use of D–B contracting, which incorporated incentives and disincentives, and the use of prefabricated bridge elements allowing rapid assembly of the I–85 bridge substructure. In fact, the D–B delivery method and other innovations played a major role in advancing the construction schedule 6 weeks ahead of the already–condensed 18–month construction schedule.

Construction Costs

Table 5 presents the differences in construction costs between the baseline and the as–built alternatives. All of the as–built cost estimates were provided by GDOT. Baseline cost was determined in consultation with GDOT engineering staff by (1) noting whether the itemized costs in the as–built cost table would have applied to the baseline case, (2) adjusting cost categories and costs as necessary, and (3) itemizing other costs associated with the baseline case that may not have been required for the as–built case. Therefore, the baseline cost estimate is inexact and the information presented is a most probable cost differential rather than a rigorous computation of a cost differential. Several other assumptions were made in selecting significant cost factors and determining some unit costs, as noted in table 5.

It can be estimated from table 5 that the adoption of the HfL innovations (as–built scenario) to build the I–85 interchange bridge resulted in a cost savings of $672,716 ($5,104,192 – $4,431,476) when compared with the baseline scenario.

Note: In table 5, as indicated, only the cost of the interchange bridge was considered in the cost analysis for the as–built case. The costs of the ramps and connecting roadway were not considered in the comparison

Table 5. Capital cost calculation table.
Cost Category Baseline Case As Built (D–B)
Design and Engineering1 $ 108,698 $ 110,390
Interchange Bridge Construction
Construction Inspection2
Worker Training
I–85 Interchange Bridge
Law Enforcement3
Temporary Pavement4
Total I–85 Interchange Bridge Costs5
Contract Incentives6 $ ––– $ 380,0007
Other Construction Items $ 71,440,420 $ 76,391,635
Total Cost8 $76,544,612 $81,203,111
1 Assumed to include quality assurance program costs as 1 percent of the construction cost, according to GDOT. Costs shown are from the actual contract bid and prorated for the I–85 interchange portion of the entire project.
2 Costs are prorated.
3 Cost to date for the as–built case and prorated for the baseline case.
4 Estimated cost of constructing a temporary detour lane through the work zone in both directions of I–85. See table 6 for itemized cost estimate.
5 These costs do not include the costs of the ramps and the connecting roadway that would constitute an "interchange"
6 Incentive collected for completing the entire project ahead of the planned 18–month schedule.
7 Since the incentives were tied to the overall goals of this economic development project (i.e., speedy delivery) and were not specifically related to the construction of the I–85 Interchange, they have only been included as costs in the "Other Construction Items" category and not as cost of the I–85 Interchange construction portion of the project.
8 As–built cost to date.

User Costs

Generally, three categories of user costs are used in an economic life–cycle cost analysis: vehicle operating costs (VOC), delay costs, and safety–related costs. However, considering that extensive detouring was not necessary for this project, VOC between the as–built and baseline cases was ignored. Therefore, only the differentials in the delay costs and safety costs were considered in the user cost analysis. The user cost impacts for I–85 were analyzed.

Table 6. Temporary detour pavement itemized cost estimate.
Item Quantity Cost Amount
Temporary HMA pavement 6,969 tons $90/ton $627,210
Temporary pavement base 4,435 tons $20/ton $88,700
Temporary striping (wet reflective tape) 15,840 l.f. $4/l.f. $63,360
1.5 in mill surface of two mainline lanes 14,080 yd2 $5/yd2 $70,400
1.5 in HMA overlay on two mainline lanes 1,160 tons $90/ton $104,400
Final striping (polyurea) 15,840 l.f. $0.70/l.f. $11,088
Total $956,158
Temporary lane assumed to be 2,640 ft x 12 ft, both directions of I–85.
Delay Costs

The traffic impact for the baseline case is based on maintaining each direction of I–85 traffic on the inside lane and an assumed temporary lane built on the inside shoulder (see table 5 for assumption) through about 0.5 mi (0.8 km) of work zone. This would allow the contractor to shift traffic from overhead construction activities. During this time, delivery of bridge substructure elements and all overhead cast–in–place construction would have taken place. Delay costs would have been impacted because traffic maintenance would have been in effect for the entire bridge construction duration in both directions of I–85. The following baseline information was available for I–85:

  • Based on 2005 data taken from GDOT Web–based resources, the annual average daily traffic (AADT) on I–85 is 24,900 and the truck traffic is 8 percent.
  • During construction the speed limit would be reduced from 70 to 50 mi/h (112.6 to 80.4 km/h). Reducing the speed limit through an approximate 0.5–mi (0.8–km) work zone would cause travel time to increase about 20.0 seconds or 69.2 hours per day of total vehicle time for NB and SB traffic.
  • The estimated user cost of the delay costs amounts to $1,356 a day ($40,680 a month). These costs were based on costs of $14.60 an hour per private vehicle and $77.10 an hour per commercial truck.
  • It was assumed that oversized loads on I–85 traffic would be diverted to detour roads if traditional traffic maintenance techniques were used. However, for simplicity the delay from detours is not included in the cost analysis.

Therefore, the delay cost was reduced from $1,220,400 (30 months x $40,680 a month) expected for the baseline scenario to $671,220 (16.5 months x $40,680 a month) for the as–built scenario. This reduction is based on reducing the construction schedule from 30 months (baseline construction schedule) to 16.5 months (as–built construction schedule). The net savings are therefore $549,180.

Safety Costs

As discussed earlier in this report, many crashes have occurred on this section of I–85 over the past years. Table 1 lists the number of 3–year vehicular crash rates for this section of highway as 69 total crashes. Forty–three crashes involve injuries and 26 are assumed to involve only property damage. Given the 2005 AADT of 24,900, this translates to the following injury and crash rates:

  • Injury–causing crash rate: 1.57 injuries per million vehicles traveled.
  • Noninjury crash rate: 0.95 per million vehicles traveled.

Ullman et al5 investigated the safety of work zones for various scenarios: (1) crashes during daytime and nighttime work periods when lanes were closed and work was ongoing, (2) crashes when work was ongoing but no closures were required, and (3) crashes when no work was ongoing (the work zone was inactive). They concluded that crashes increased 60 to 66 percent (an average of 63 percent) when a traffic lane was closed day or night. Given this information and considering the traffic volumes and hourly traffic variations on this highway and the expected construction schedules, table 7 presents the number of vehicles that would have passed through the work zone for the as–built and baseline projects.

5. Ullman, G.L., M.D. Finley, J.E. Bryden, R. Srinivasan, and F.M. Council, Traffic Safety Evaluation of Nighttime and Daytime Work Zones (NCHRP Report 627), National Cooperative Highway Research Program, Transportation Research Board, Washington, DC, 2008.
Table 7. Estimated total traffic on I–85 used to compute safety impacts for baseline and as–built scenarios.
  Baseline Case As-Built Case
  Northbound Southbound Northbound Southbound
Two–way AADT, vehicles/day 24,900 24,900 24,900 24,900
Directional traffic distribution factor (per GDOT data) 0.50 0.50 0.50 0.50
Fraction of AADT affected in a 24–hour period (per GDOT data) 0.63
(daytime construction assumed)
(daytime construction assumed)
(nighttime and weekend work)
(nighttime and weekend work)
Total number of construction days 450 (assumed) 450 (assumed) 120 150
Total Traffic Volume (millions)
(2–way AADT x Directional Factor x 24–hour traffic fraction *Construction days)
3.52 3.61 0.55 0.66

Table 7 shows that the total volume of traffic exposed to crash risk was much lower for the as–built case than the baseline case. The faster construction and work schedules when traffic volumes are lower (nights and weekends) and other safety measures adopted by GDOT on this project resulted in only one non–injury–causing motorist incident in the work zone on I–85.

The estimated increase in crashes for the baseline case can be computed as the product of (1) the historical crash rate for each type of crash (number of crashes per million vehicles), (2) the total volume of traffic exposed to the risk, and (3) the risk escalation factor associated with work zones (= 0.63 as discussed earlier). This is computed for the NB and SB lanes for the baseline case as follows:

  • I–85 NB lanes (baseline case)
    • Estimated personal injury–causing crashes due to work zone:

= Total traffic volume (million vehicles)* Crash rate (number/million vehicles)* risk escalation factor due to work zone

= 3.52* 1.57* 0.63 = 3.48 crashes

    • Estimated non–personal injury–causing crashes due to work zone:

= Total traffic volume (million vehicles)* Crash rate (number/million vehicles)* risk escalation factor due to work zone

= 3.52* 0.95* 0.63 = 2.10 crashes

  • I–85 NB lanes (baseline case)
    • Estimated personal injury–causing crashes due to work zone in I–85 SB lanes for the baseline case:

= Total traffic volume (million vehicles)* Crash rate (number/million vehicles)* risk escalation factor due to work zone

= 3.61* 1.57* 0.63 = 3.57 crashes

    • Estimated nonpersonal injury–causing crashes due to work zone in I–85 SB lanes for the baseline case:

= Total traffic volume (million vehicles)* Crash rate (number/million vehicles)* risk escalation factor due to work zone

= 3.61* 0.95* 0.63 = 2.16 crashes

The elevated risk noted above was monetized by assuming unit costs for the various types of historical crashes reported by GDOT from Council et al. The following mean comprehensive costs per crash for a rural highway with a posted traffic speed greater than or equal to 50 mi/h (80.4 km/h) were used in the analysis:

  • Injury–causing crash–$95,368 (injured, severity unknown, Level 5)
  • Non–injury crash–$25,735 (nature of crash unknown, Level 5)

Table 8 presents the difference in safety costs for the baseline and as–built cases. It can be computed from the table that the total expected safety costs for the baseline case would have been $781,977 ($385,925 + 396,052) as opposed to the $25,735 for the as–built case. The expected cost differential between the two scenarios is therefore $756,242, which is essentially the safety benefit of the as–built case.

Table 8. Comparison of safety costs–baseline versus as–built.
  Baseline Case As–Built Case
  Northbound Southbound Northbound Southbound
Personal injury–causing crashes
(= Crash cost ($/crash) X Number of crashes)
(= $95,368*3.48)
(= $95,368*3.57)
(No crashes)
(No crashes)
Nonpersonal injury crashes $54,044
(= $25,735*2.10)
(= $25,735*2.16)
(= $25,735*1)
(No crashes)
Total Traffic Volume (millions)
(2–way AADT x Directional Factor x 24–hour traffic
fraction* Construction days)
$385,925 $396,052 $25,735 $0

Cost Summary

Construction costs for the I–85 interchange bridge would have likely placed traditional delivery and construction methods (baseline) at $672,716 more than the as–built case. Moreover, delivering the project in only 16.5 months saved I–85 users $549,180 in delay costs and $756,242 in safety costs. Therefore, the estimated total savings from using the innovative HfL project delivery approach are $1.98 million. In other words, the innovative approach to this $4.43 million interchange project had a 45 percent cost benefit over traditional methods.

6. These costs were based on F. Council, E. Zaloshnja, T. Miller, and B. Persaud, Crash Cost Estimates by Maximum Police–Reported Injury Severity Within Selected Crash Geometries (FHWA–HRT–05–051), Federal Highway Administration, Washington, DC, October 2005.
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Mary Huie
Center for Accelerating Innovation

This page last modified on 04/04/11

United States Department of Transportation - Federal Highway Administration