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Arrow Iowa Demonstration Project: Improvements to the 24th Street–I-29/80 Interchange in Council Bluffs

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Data Acquisition and Analysis

Data on safety, traffic flow, quality, and user satisfaction were collected before, during, and after construction to determine compliance with the HfL performance goals. The primary objective of acquiring these types of data was to quantify the project performance, provide an objective basis on which to determine the feasibility of the project innovations, and 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 Iowa DOT project met the specific HfL performance goals related to these areas.


Work zone safety for the workers and traveling public during construction was improved through the use of prefabricated bridge components and the A+B bidding method. Both features were intended to accelerate construction and reduce the construction time from two construction seasons to one. A key component of this project was that traffic was not impacted over the winter, when inclement weather would increase the driving hazard through the construction zone. Consistent with this concept, the fall 2007 letting date was selected to allow early ordering of the steel girders for spring delivery and restrict traffic impact to the April–through–October construction season.

Furthermore, placing precast deck panels over interstate traffic minimized the public’s exposure to overhead construction hazards, compared to traditional cast–in–place construction in which concrete form work and casting are done directly over live interstate traffic lanes.

During construction, no worker injuries were reported, which means the Iowa DOT exceeded the HfL goal for worker safety (incident rate of less than 4.0 based on the OSHA Form 300 rate). No motorist incidents were reported in the construction zone on the 24th Street bridge or on the interstate below. In contrast, the existing interchange area had an above–average crash rate in the past. Crash data from 2001 to 2005 show the following statistics:

  • Total crashes: 146 (47 involved personal injury (fatal and nonfatal) and the remainder are assumed to be noninjury (property damage only))
  • Interchange crash rate: 146.2 crashes per hundred million vehicle–miles traveled (HMVMT)

Most crashes occurred on the ramps and at the ramp terminals and appeared to be the result of minimal storage for vehicles on the ramp. The new bridge project added more turn lane storage and storage capacity. The result is more efficient signal cycles that clear the arriving vehicles in short, efficient platoons, promoting shorter queues and less exposure to opposing traffic.

Construction Congestion

The HfL program specifies performance goals for reducing both total construction duration by 50 percent and construction impacts on traffic. Under conventional methods, the construction impact on both roads was estimated at two full construction seasons (16 months).With the use of accelerated construction techniques and contracting, the impact was reduced to one construction season. The innovations reduced congestion several ways:

  • The use of precast deck panels reduced congestion:
    • By improving traffic flow during construction and reducing motorist impact because of the shortened construction period.
    • By reducing materials deliveries such as fresh concrete and concrete forms and therefore construction traffic because the deck panels were fabricated offsite.
    • By requiring less onsite storage area.
  • A+B bidding shortened the duration of construction congestion by allowing the Iowa DOT to select the most efficient bid in terms of construction cost and duration of traffic impact.
  • Installation of an intelligent transportation system was specifically targeted at reducing construction congestion. The system had automated components to detect travel speed and queuing lengths and helped the contractor regulate traffic flow.

The longer life of the structure from the use of HPS, HPC, and flooded backfill is expected to reduce congestion because of reduced future maintenance activities. Both the reduction in total construction time and in the impacts on motorists compared to conventional construction methods for this project far exceeded the HfL performance goals.

Traffic Study

To assess the impacts of the construction project on motorists, researchers conducted a series of travel time runs to determine the additional travel time required to traverse both I–29/80 and 24th Street in the vicinity of the project. The travel time studies included exiting maneuvers from I–29/80 onto 24th Street. Studies were conducted midway through the construction schedule.

Researchers used the floating vehicle methodology to collect travel times, attempting to mimic the typical driving speed of other vehicles along the various roadway segments of the construction zone. Data were collected on weekdays during daytime hours (7 a.m. to 7 p.m.) when traffic demand was high and the work zone would have the greatest impact. Over a 2–day period, researchers collected a total of 17 travel times on I–29/80 and 34 times on 24th Street.

Overall, travel speeds along I–29/80 averaged 60 mi/h in the eastbound direction and 59 mi/h in the westbound direction. Neither exit ramp to 24th Street queued back onto the freeway mainline lanes during any of the travel time runs. Consequently, there were no measurable impacts of construction on I–29/80 traffic.

Traffic congestion data include the computed impacts on traffic exiting to 24th Street from either direction on I–29/80 and on traffic on 24th Street itself. Travel times along the 1.5–mile length of 24th Street over 2 days of data collection averaged 5.8 minutes in the northbound direction and 6.5 minutes in the southbound direction. For eastbound I–29/80 traffic exiting at 24th Street and then turning left, travel times to the northern terminus of 24th Street averaged 4.0 minutes over the 0.9–mile distance. For westbound I–29/80 traffic exiting and then turning left, the travel time for the 1.4–mile journey to the southern terminus of 24th Street averaged 4.5 minutes.

Iowa DOT officials indicated that the same traffic management plan would have been used for this project regardless of whether accelerated construction techniques were used. Therefore, the benefits to the public from accelerated construction can be computed by determining the extent to which conditions during construction increased travel times over normal nonconstruction conditions, and then determining how many fewer days of construction were required by using these accelerated construction techniques.

Unfortunately, actual travel times under normal conditions (before the start of construction) were not available for this analysis. Therefore, researchers estimated what travel times may have been on the roadway under typical conditions before construction. Chapter 15 of the 2000 Highway Capacity Manual was used to estimate an average running time of 119 seconds per mile on 24th Street.

Added to this arterial segment running time was the additional delay expected to have existed at the interchange with 24th Street. For this estimate, researchers relied on guidance developed by the Texas Transportation Institute for the Texas Department of Transportation in Recommended Ramp Design Procedures for Facilities Without Frontage Roads. Assuming a fairly well–timed and operating diamond interchange, this guidance estimates the delay through a standard diamond interchange at about 21 seconds per vehicle.

Summing the running travel time along the 1.5–mile length of 24th Street with the additional 21 seconds required to traverse through the interchange at I–29/80 yielded a total expected travel time on 24th Street of 3.3 minutes in each direction. For the I–29/80 exiting traffic, researchers estimated normal travel times of 1.9 minutes and 1.4 minutes for the eastbound–to–northbound and westbound–to–southbound maneuvers, respectively. Using these numbers, table 2
presents the per–vehicle delays estimated to have been generated by the traffic management plan used for this project. For simplicity purposes, it appears reasonable to use a 2.6–minute delay per vehicle for all movement types within the interchange (for both through and exiting traffic).

Table 2 . Per–vehicle delay estimates
  Travel time, minutes per vehicle
Movement Estimated normal travel times Travel times during construction Additional delay
24th Street northbound 3.3 5.8 2.3
24th Street southbound 3.3 6.5 3.2
I–29/80 eastbound to 24th Street northbound 1.4 4.0 2.6
I–29/80 westbound to 24th Street southbound 1.9 4.5 2.6

In the HfL application the Iowa DOT submitted, traffic volumes at the site were provided for 2004 and 2030 estimates of traffic demands north, south, and across the 24th Street bridge. For simplicity purposes, researchers used the bridge volume and extrapolated the count to 2008 values. This was estimated at 14,000 vehicles per day. Multiplying the daily traffic demands by the 2.6–minute–per–vehicle delay estimate from above, researchers calculated a total of 607 vehicle–hours of delay per day while the traffic management plan for the project was in place.


Sound Testing

Sound intensity (SI) measurements were made using the current accepted onboard sound intensity (OBSI) technique AASHTO TP 76–08, which include dual vertical sound intensity probes and an ASTM standard reference test tire (SRTT). Sound testing was done before construction and on the new bridge surface shortly after it was opened to traffic. OBSI measurements were obtained from the bridge at the posted speed limit of 35 mi/h. A minimum of three runs were made in the right wheelpath with the two phase–matched microphone probes simultaneously capturing noise data from the leading and trailing tire–pavement contact areas. Figure 17 shows the dual probe instrumentation and the tread pattern of the SRTT.

New Dual Probe Setup photo OBSI dual probe system and the SRTT

Figure 17 . OBSI dual probe system and the SRTT.

The average of the front and rear OBSI values was computed over the full length of the bridge deck to produce sound intensity values. Raw noise data were normalized for the ambient air temperature and barometric pressure at the time of testing. The resulting mean sound intensity levels were A–weighted to produce the noise–frequency spectra in one–third octave bands, as shown in figure 18. This chart shows that the new bridge surface was quieter at every band in the spectrum and particularly for the low frequencies, which means that noise from the new bridge will tend to not travel as far as noise from the old bridge.

Mean  A-weighted sound intensity frequency spectra.

Figure 18 . Mean A–weighted sound intensity frequency spectra.

Global noise levels were calculated by using logarithmic addition of the one–third octave band frequencies between 315 and 4,000 hertz (Hz). The global noise levels were 99.2 and 94.4 dB(A) for the old and new bridge, respectively. For reference, a 3.0 decibel difference in noise is considered noticeable to the human ear. The original portland cement concrete bridge deck surface had multiple patches and distresses and was 4.8 decibels louder than the newly constructed bridge surface. Moreover, the HfL target value of less than 96.0 dB(A) was met.

Smoothness Measurement

Smoothness testing was done in conjunction with noise testing using a high–speed inertial profiler integrated into the noise test vehicle. Figure 19 shows the test vehicle with the profiler positioned in line with the right rear wheel. Figure 20 graphically shows the test results.

High-speed inertial profiler mounted behind the test vehicle

Figure 19 . High–speed inertial profiler mounted behind the test vehicle.

 Mean  IRI values for the old and new bridges

Figure 20 . Mean IRI values for the old and new bridges.

The overall IRI values are 199 and 86 in/mi for pre– and postconstruction, respectively. Postconstruction IRI is more than 40 percent lower and is a direct result of quality construction. Figure 20 shows large peak values near the north end of the existing bridge, whereas the new construction has eliminated all but a few minor spikes in roughness.

The HfL goal for IRI of 48 in/mi, which reasonably can be met on long, open stretches of pavement, was not met on this project. It is extremely difficult to achieve this mean ride measurement on a short–span bridge of this type because of the influence of the bumps at each end of the structure on the mean. Nonetheless, the new construction is a vast improvement over the existing bridge.

User Satisfaction

The HfL requirement for user satisfaction included a performance goal of 4–plus on a Likert scale of 1 to 7 for the following two questions:

  • How satisfied are you with the new facility?
  • How satisfied are you with the approach the Iowa DOT used (keeping 24th Street open) to construct the new facility in terms of minimizing disruption?

The Iowa DOT conducted a stakeholder survey in which nearby residents and businesses were encouraged to complete electronic survey forms (pdf format) indicating their approval of a wide variety of issues ranging from the most effective means of communication to construction details.

Instead of a 7–point scale, a 4–point scale was used to determine the level of project satisfaction. On either scale the targeted level of satisfaction needed to be at least 57 percent to meet HfL goals. The overall response indicates that the level of satisfaction exceeded the HfL goals; 97 percent of users gave high scores to the importance of the approach used on this project and 89 percent gave good to very good marks to the way the project was carried out. The Appendix contains the complete results of the survey.

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Mary Huie
Center for Accelerating Innovation

United States Department of Transportation - Federal Highway Administration