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Minnesota Demonstration Project: Reconstruction of Trunk Highway 36 in North St. Paul
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 demonstrate that the innovations can be used to do the following:
Achieve a safer work environment for the traveling public and workers.
Table 1 shows vehicular crashes from 2005 to 2007 at the intersection of TH 36 and McKnight Road. Crashes resulted in 20 injuries and no fatalities at the busiest intersection within the project limits during the 3-year study period before construction. This is a significant number of crashes. A key aspect of this project was to increase safety for all roadway users during and after reconstruction.
Data supplied by Mn/DOT
Full closure created a safe work environment by eliminating conflicts between motorists and construction workers and equipment. Full closure also helped condense construction time to one season instead of the two seasons that would have been required under conventional staged construction. This minimized the safety risk to motorists, who would have had to negotiate complicated lane changes during hazardous winter driving conditions. By closing TH 36, Mn/DOT eliminated the need to stage traffic next to 20-ft embankment cuts.
No motorist incidents occurred and no worker injuries were reported during the construction project, which means Mn/DOT exceeded the HfL goal for reducing incident rates and worker safety (incident rate of less than 4.0 based on the OSHA 300 rate).
Facility safety after construction of the new design features will be greatly improved by the following:
In summary, the newly reconstructed design features have achieved a safer environment by separating pedestrian and vehicular traffic and eliminating traffic conflicts associated with at-grade intersections. The future safety record is anticipated to be considerably better than before reconstruction. In fact, crashes with injuries at TH 36 and McKnight Road for the first year (2008) after construction dropped 40 percent from an annual average of 6.7 to four, and no fatalities were reported. Table 2 shows the breakdown of the reported crashes after TH 36 was reopened.
Data supplied by Mn/DOT.
Full closure in conjunction with the locked incentive date gave the contractor an incentive to “get in and get out” and was instrumental in reducing construction time and associated traffic congestion on TH 36. Conventional staged construction methods would have interrupted traffic on TH 36 for at least 20 months. The total impact to traffic flow on TH 36 was shortened by nearly 68 percent to 6.5 months (4 months of full closure and 2.5 months of single-lane traffic). After that, bridgework continued with all four lanes of TH 36 open to traffic.
Pavement Test Site
Test data were collected from both eastbound and westbound directions of TH 36 for about 2.2 mi (3.5 km) from White Bear Avenue to TH 120 (Century Avenue). The existing pavement was a weathered, dense-graded asphalt surface with transverse cracking. Figure 12 shows the existing surface texture and a typical section of pavement. Figure 13 shows the newly constructed pavement.
Figure 12 . Existing TH 36 HMA pavement.
Figure 13 . Newly constructed TH 36 HMA pavement.
Sound Intensity Testing
Mn/DOT does not use the onboard sound intensity (OBSI) test method on any projects. However, this method was used to measure tire-pavement sound intensity (SI) on the existing and newly constructed pavements of this project for comparison.
SI measurements were made using the current accepted OBSI technique, AASHTO TP 76-08, which includes dual vertical sound intensity probes and an ASTM standard reference test tire (SRTT). Sound testing was done before construction and on the new pavement surface shortly after it was opened to traffic. OBSI measurements were obtained from the right wheelpath of the outside lane in both directions at 45 miles per hour (mi/h) (72.4 kilometers per hour (km/h)). A minimum of three runs were made with the two phase-matched microphone probes simultaneously capturing noise data from the leading and trailing tire-pavement contact areas. Figure 14 shows the dual probe instrumentation and the tread pattern of the SRTT.
Figure 14 . OBSI dual probe system and the SRTT.
The average of the front and rear OBSI values was computed over the full length of the pavement sampled to produce SI values. Raw noise data were normalized for the ambient air temperature and barometric pressure at the time of testing. The resulting mean SI levels are A-weighted to produce the sound intensity frequency spectra in one-third octave bands, as shown in figure 15 for both the pre- and postconstruction pavement surface. The figure shows that the new pavement surface has slightly higher decibel levels above 800 hertz (Hz).
Figure 15 . Mean A-weighted sound intensity frequency spectra.
SI levels were calculated using logarithmic addition of the one-third octave band frequencies across the spectra. The SI levels were 96.4 and 97.0 dB(A) for the pre- and post construction pavements, respectively. The old and new pavements were very similar. However, the SI level from the newly constructed pavement is 1.0 dB(A) higher than the HfL goal of 96.0 dB(A) or less.
Smoothness measurements were collected by the Auburn University Automatic Road Analyzer (ARAN) van (figure 16). The ARAN is a high-speed inertial profiler able to perform smoothness measurements of the pavement surface in both wheelpaths. Smoothness is reported in inches per mile as recommended by the International Roughness Index (IRI) approach and consists of a mathematical assessment of the section profile aimed at quantifying the quality of the ride in a passenger car. The ARAN van performed three runs in each direction of the outside lane at a speed of 45 mi/h (72.4 km/h).
Figure 16 . Auburn University ARAN van.
The overall IRI values were 58 and 50 inches per mile for pre- and postconstruction, respectively. Post construction IRI is close to the HfL goal of HfL goal of 48 inches per mile or less.
The HfL requirement for user satisfaction includes a performance goal of 4-plus on a Likert scale of 1 to 7 (54 percent or more showing favorable response) for the following two questions:
Mn/DOT market research staff commissioned a survey4 to determine how Mn/DOT did on the project. A total of 400 residents, 100 businesses, and 400 commuters familiar with the project were surveyed by telephone. Residents and businesses were selected from the area surrounding the project. Commuters who made at least four or more round trips per week through the TH 36 corridor were selected from the surrounding communities. The survey was conducted in August 2008 during the final stages of the project and long after the full closure was completed.
Results of a preconstruction survey indicated an even divide among residents and commuters on the use of either a quick in-and-out approach by implementing a full closure or the traditional construction method of partial closures over 2 years. Before construction, the most opposition came from businesses, with seven out of 10 not in favor of full closure.
The Mn/DOT study found that toward the end of the project, the overall response was overwhelmingly positive (nine out of 10 residents and through commuters) about the way the project was executed. Eighty-three percent of local businesses had the same opinion. This means the goal of achieving high satisfaction with the approach to reconstruct the corridor was exceeded.
Speed of project completion was cited as the most common reason for the positive feelings. Improved traffic flow was also given as a reason for the favorable response, as was a general lack of inconvenience or disruption. Figure 17 shows the top reasons, both positive and negative, those surveyed gave for why they feel the way they do about the project.
Figure 17 . Top reasons why people feel the way they do about the project. (Source: 2008 Community Construction Evaluation: TH36 Post-Closure, Mn/DOT).
The study did not address user satisfaction with the new facility compared with its previous condition. However, nearly half of those surveyed indicated there was nothing the DOT could have done better, suggesting that overall satisfaction with the newly reconstructed corridor was high. Comments to the contrary suggest removing additional stoplights (assumed outside of the project limits), completing Margaret Bridge (assumed during the full closure), and improving detour signage.
News media (print, TV, and radio), meetings, Mn/DOT’s Web site, and local events all played a role in keeping the public informed during the project. In fact, the post-construction survey report indicated that the effectiveness of communications was captured by the survey results:
At least 76 percent of respondents in each segment [residents, commuters, and businesses] said that before construction began on TH 36, they were aware that a decision was made to close the road completely during construction rather than leaving a lane open for traffic.
Most also said that before construction began, they were aware the project would continue even after the road was reopened, with through commuters the least likely of the segments to indicate awareness of this aspect, at 65 percent, and the other segments closer to three in four.
Mn/DOT engaged the local businesses of North St. Paul with workshops such as “Open for Business—Surviving and Thriving During Construction.” Workshops provided the opportunity for Mn/DOT to explain anticipated traffic patterns as construction evolved and how to tailor advertising during this time. Community involvement included special events sponsored by Mn/DOT to celebrate milestones, such as groundbreaking on the first day of the full closure and opening of the pedestrian bridge. Three public open houses were held at city hall to provide an official forum to discuss the project with DOT personnel.
Throughout the project, Mn/DOT kept the community informed of reconstruction progress with weekly updates from the project engineer and photographs posted on the project Web site. The updates were also sent by e-mail directly to anyone interested in the project. Mn/DOT's efforts to keep the public up to date on construction activities helped the project succeed.
4 2008 Community Construction Evaluation: TH36 Post-Closure. Prepared by Readex Research for Mn/DOT Market Research.