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Highways for LIFE

Arrow Iowa Demonstration Project: Accelerated Bridge Construction on US 6 over Keg Creek

Data Acquisition and Analysis

Data on safety, traffic flow, quality, and user satisfaction before, during, and after construction were collected to determine if this project met the HfL performance goals. The primary objective of acquiring these types of data was to quantify project performance and provide an objective basis from which to determine the feasibility of the project innovations and to demonstrate that the innovations can be used to do the following:

  • Achieve a safer work environment for the traveling public and workers.
  • Reduce construction time and minimize traffic interruptions.
  • Produce a high-quality project and gain user satisfaction.

This section discusses how well the Caltrans demonstration project met the HfL performance goals related to these areas.


The project included the HfL performance goal of achieving a work zone crash rate equal to or less than the existing conditions. Current crash data for the project location indicate that there was one crash involving minor injuries within the last 3 years. During this project, no crashes occurred, satisfying the HfL goal. Work zone safety was ensured by completely closing the bridge to traffic, accelerated construction, and use of prefabricated bridge components. Accelerated construction methods, including the use of prefabricated bridge components, made the brief traffic detour feasible.

The project included the performance goal of achieving an incident rate for worker injuries less than 4.0 based on the OSHA 300 rate. Not only did closing the bridge to traffic help to achieve this goal, but precasting the bridge system on ground level eliminated the need for workers to spend most of their time exposed to falling hazards, as would have been required with traditional cast-in-place construction methods. No work-related injuries occurred during construction, resulting in an OSHA Form 300 score of 0.0.

Crash data provided by Iowa DOT's Safety Analysis, Visualization, and Exploration Resource (SAVER) shows a single crash in the 3 years prior to the start of this project. The complete crash history is included in the appendix. Due to the low crash rate at the site, the goal of a 20 percent reduction was not directly applicable. However, the increased durability of the driving surface created by the precast deck components and the use of flooded backfill should reduce the potential for vehicles to lose control while driving over a degraded portion of the deck or settled pavement at the end of the bridge. Moreover, it is expected that constructing the bridge to the current standard bridge width will decrease potential for future crashes due to the greater horizontal distance to the barrier rail.

Construction Congestion

The combination of accelerated construction techniques reduced the duration that highway users were impacted by more than 50 percent. Estimated construction time requiring the bridge to be closed would have been 6 months under non-accelerated construction. The actual impact on traffic lasted only 16 days.

The first stage of construction took place with the roadway remaining open to traffic. Modular components were constructed off site, and the drilled shaft foundation construction was completed adjacent to the existing bridge. Given that this stage was completed while the bridge remained fully open to traffic, the trip time across the bridge was not increased and met the HfL goal of less than a 10 percent increase in trip time during construction as compared to the average preconstruction time. On occasion, brief interruptions to traffic did occur for supply and equipment deliveries, but these interruptions did not result in significant traffic implications.

The second stage of construction occurred during the bridge closure while traffic was detoured. During the closure, the detour eliminated traffic queuing and congestion at the construction site. The modular bridge components were installed on site in a highly accelerated fashion. This innovative technique reduced the traffic detour duration by more than 90 percent. The shorter duration of closure relates to the significant reduction of total trip time as compared to the estimated duration for more traditional construction methods.

Travel Time

Data were collected utilizing the floating vehicle methodology in an effort to match the driving speeds of other vehicles along the 21-mile detour route in both directions. (Note that the effective detour was only 12 miles, taking into account the 9 miles motorist would have traveled if the bridge was open [21 mi - 9 mi = 12 mi]). For continuity, the 21-mile detour was evaluated for this travel time study and later in this report for the user cost analysis.

Researchers collected data during the bridge closure on October 27, 2011, and after the bridge was open to traffic on November 22, 2011. On these visits, researchers collected data on weekdays during daylight hours (7:00 am to 5:00 pm) when traffic demand was relatively high and the detour would have the greatest impact on travel time. In general, the traffic flow along the rural detour route was light and flowed freely without backups or congestion at or above the posted speed limit.

Figure 22 shows the detour route and identifies travel time nodes used in the data collection process. Table 1 identifies the cumulative distance along the route and the average time on each segment when the bridge was closed. No significant travel time peaks were noticed, so the data from both directions along the detour were averaged together.

When the bridge was open, the most direct route between node 1 at the intersection of US 6 and 300th Street and node 4 at the intersection of US 6 and I-80 was 9 miles along US 6 and took an average 10.08 minutes (0.17 hr) to travel. The average travel time during closure using the 21-mile detour between nodes 1 and 4 was 24.67 minutes (0.41 hr), or more than twice the time without the closure. The cost associated with the additional time to traverse the detour route is presented later in this report.

Figure 22. Map. Detour route with travel time nodes (source: Google Maps).

Figure 22. Map. Detour route with travel time nodes (source: Google Maps).

Table 1. Detour route travel times.
Node Description Cumulative Distance (mi) Average Cumulative Travel Time (min)
1 US 6 & 300th St. 0 0
2 300th St. & State Hwy 92 4 5.43
3 State Hwy 92 & I-29 16 17.76
4 I-80 & US 6 21 24.51
4 I-80 & US 6 0 0
3 State Hwy 92 & I-29 5 6.21
2 300th St. & State Hwy 92 17 19.39
1 US 6 & 300th St. 21 24.84
Combined Average Cumulative Travel Time 24.67

Evaluating trip time for the HfL goal is comparable by considering the work zone across the bridge would have been closed and traffic detoured for either 16 days or for 6 months (as estimated for traditional construction methods). Because the ABC approach reduced the days motorists spent traveling the detour, trip time was reduced and met the HfL goal of less than 10 percent increase in trip time compared to traditional construction.

The Iowa DOT received comments from their user satisfaction survey (discussed later in this report) stating that several crashes occurred on State Highway 92, (refer to question #11 in the appendix) during the detour period. The 2011annual crash total on State Highway 92 was slightly less than the average annual crash total over the past 3 years. Crash data provided by Iowa DOT's SAVER program recorded 34 crashes in 2011 whereas the highway averaged 36 annual crashes from 2008 to 2010 indicating the additional detour traffic on the highway did not increase the number of overall crashes on an annual basis.


Pavement Test Site

Researchers collected sound intensity and smoothness test data from both eastbound and westbound directions of US 6 across the bridge before construction. Comparing these data to the test results after construction provides a measure of the quality of the finished bridge.

Sound Intensity Testing

Presently, Iowa DOT does not use the OBSI test method on any projects. However, this method was used to collect tire-pavement SI measurements from the existing and newly constructed bridges for comparison.

Researchers recorded SI measurements using the current accepted OBSI technique described in American Association of State Highway and Transportation Officials (AASHTO) TP 76-10, which includes dual vertical sound intensity probes and an ASTM-recommended Standard Reference Test Tire (SRTT). SI data collection occurred prior to construction and on the new bridge surfaces shortly after opening to traffic. The SI measurements were recorded and analyzed using an onboard computer and data collection system. Researchers made a minimum of three runs in the right wheelpath of the project. The two microphone probes simultaneously captured noise data from the leading and trailing tire/pavement contact areas. Figure 23 shows the dual probe instrumentation and the tread pattern of the SRTT.

Figure 23. Photo. OBSI dual probe system and the SRTT.

Figure 23. Photo. OBSI dual probe system and the SRTT.

The average of the front and rear OBSI values from both lane directions was computed to produce the global SI level. Raw noise data were normalized for the ambient air temperature and barometric pressure at the time of testing. The resulting mean SI level was A-weighted to produce the SI frequency spectra in one-third octave bands, as shown in Figure 24.

Figure 24. Graph. Mean A-weighted SI frequency spectra before and after construction.

Figure 24. Graph. Mean A-weighted SI frequency spectra before and after construction.

SI levels were calculated using logarithmic addition of the one-third octave band frequencies across the spectra. The SI level increased 3.2 dB(A) from 98.0 DB(A) before construction to 101.2 dB(A) after construction. The new bridge did not meet the HfL goal of 96.0 dB(A) or less. The new texture of the bridge surface—while aiding traction and increasing safety—is prone to increasing noise.

Smoothness Measurement

Smoothness data collection occurred in conjunction with the SI runs utilizing a high-speed inertial profiler integrated into the noise test vehicle. The profile data collected with this equipment provide IRI values, with lower values indicating a higher quality ride. Figure 25 is an image of the test vehicle showing the profiler positioned in-line with the right rear wheel. Figure 26 graphically presents the IRI values at 20-ft intervals for the existing bridge surfaces and shows most of the postconstruction values plotted lower than the preconstruction values.

The increased smoothness of the newly constructed bridge resulted in a reduction in IRI value from 222 in/mi to 179 in/mi. While not meeting the HfL goal of less than 48 in/mi after construction, the new bridge surface is an improvement.

Figure 25. Photo. High-speed inertial profiler mounted behind the test vehicle.

Figure 25. Photo. High-speed inertial profiler mounted behind the test vehicle.

Figure 26. Graph. Mean IRI values computed at 20-ft intervals before and after construction.

Figure 26. Graph. Mean IRI values computed at 20-ft intervals before and after construction.

User Satisfaction

The HfL requirement for user satisfaction includes a performance goal of 4 or more on a Likert scale from 1 to 7 (in other words, 57 percent or more participants showing favorable response) for the following two questions:

  • How satisfied is the user with the new facility compared with its previous condition?
  • How satisfied is the user with the approach (ABC) used to construct the new facility in terms of minimizing disruption?

As an alternative to the HfL questions, the Iowa DOT posed 12 questions to roadway users, asking them to rate their responses or to select an answer category. The following questions were asked:

  1. Rate your level of use of the US 6 Keg Creek Bridge before construction.
  2. How would you best describe yourself?
  3. Rate how important you believe each approach is and how well it was carried out on this project (approaches include closing US 6 during construction, providing signage for commuters in the area, condensing closure to two weeks, using stronger materials to extend bridge life, using prefabricated components to speed construction, using multiple methods (message signs, radio, texts, etc.) to advise motorists of construction and alternative routes).
  4. Knowing it would mean traffic delays and driving through construction areas, how important was it to close US 6 during construction?
  5. How satisfied are you with the way the Iowa DOT kept you informed about the construction work?
  6. What means of communication do you use most often to learn about traffic issues/roads information?
  7. What are the best methods to keep you informed regarding traffic issues/road construction?
  8. Rate the following aspects of the new US 6 Keg Creek Bridge and surrounding areas as compared to its previous condition (aspects included lane width, visibility, signage, lighting, aesthetics).
  9. Rate the level of inconvenience you experienced as a user of the US 6 Keg Creek Bridge.
  10. Did construction on the US 6 Keg Creek Bridge deter you from visiting businesses along US 6?
  11. Were there any safety issues/concerns raised during the US 6 Keg Creek Bridge project?
  12. Rate the importance in regard to designing and scheduling projects. Answer options were: keeping a road/bridge open-allow restricted traffic, closing a road/bridge-no traffic and reducing cost and time, extending the life of the road/bridge-more initial cost, reducing future maintenance costs-more initial cost, reduce time to complete a project through the use of incentives to contractors, reduce time to complete a project through design/material selection, creation of alternative routes while project is underway, use of multiple methods (technology) to inform the public of work zone conditions.

Overall, the responses to the questions were favorable and met or exceeded the HfL performance goal. Most roadway users (7 out of 9) responding to question 8 indicated the lane width and visibility of the new bridge was better or much better than the old bridge under, which closely addresses the HfL question of how satisfied the user is with the new facility. Ten of 12 respondents to question 4 indicated that it was important to close the bridge, knowing it would mean traffic delays. This implies a favorable response to the HfL question gauging how satisfied the user is with the approach used to construct the new facility in terms of minimizing disruption. The complete results of the survey are contained in the appendix.

More Information



Mary Huie
Highways for LIFE

Updated: 12/21/2012

United States Department of Transportation - Federal Highway Administration