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South Carolina Demonstration Project: Black River Bridge Replacements on SC 377 in Williamsburg County
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 the 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:
This section discusses how well the SCDOT project met the specific HfL performance goals in these areas.
The performance goal for this project was to achieve work zone and worker safety during construction and reduce future crash injuries and fatalities by 20 percent (based on a preconstruction 3-year baseline). The project was successful in holding work zone-related crashes below the preconstruction crash rate of an average of 19 crashes per year over the past 6 years. SCDOT reported one vehicle crash in the work zone that resulted in a fatality. The crash was not related to construction. One other work zone crash occurred involving three passenger vehicles. Therefore, the performance goal of holding crashes below 19 crashes per year was achieved. Historical data between 2001 and 2006 is presented in table 5.
Impact on Traffic
Traditionally, this single project would have encompassed three or more contracts at different times. SCDOT used the concept of route management to combine the separate contracts into one and reduced the time highway users were impacted by more than 50 percent. Under this contract, the contractor worked on all bridges and the intersection at the same time, reducing the total construction time spent on the project. A key feature of route management was to minimize the duration of work zone slowdowns.
SCDOT reported no noticeable congestion during construction, largely because the use of staged construction allowed nearly continual use of the original two-lane route configuration. Queue length was not an issue because traffic flowed freely though the work zone.
Sound intensity (SI) and smoothness testing were conducted by personnel from the National Center for Asphalt Technology in Auburn, AL.
To get baseline measurements of the existing roadway, two areas were considered for this project. The first area, on SC 377, included the four bridges on SC 377 and five pavement sections among the bridges. The second area was along U.S. 521. Thus, this project had five pavement sections on SC 377 (S-1 through S-5), four bridge sections (B-1 through B-4), and one pavement section on U.S. 521, shown in figures 18 and 19.
Figure 18. SC 377 pavement and bridge test sections.
Figure 19. U.S. 521 pavement test section.
The test section encompassing the intersection of U.S. 521 and SC 377 was 2,000 ft long, with 1,000 ft on either side of the intersection. Both the eastbound and westbound lanes had identical surfaces, so only the eastbound lane of U.S. 521 was tested for convenience. Three repeat runs were made for both smoothness and OBSI measurements on all test sections to ensure data repeatability.
Sound Intensity Testing
SI measurements were made using the current accepted OBSI technique AASHTO TP 76-08, which includes dual vertical SI probes and an ASTM standard reference test tire (SRTT). SI testing was done before reconstruction and on the new bridges and pavement surface after project completion. SI measurements were obtained at a constant vehicle speed of 45 miles per hour (mi/h). A minimum of three runs were made in the right wheelpath with two phase-matched microphone probes simultaneously capturing noise data from the leading and trailing tire-pavement contact areas. Figure 20 shows the dual probe instrumentation and the tread pattern of the SRTT.
Figure 20. Onboard sound intensity probe and tire tread of the SRTT.
The average of the front and rear OBSI values was computed over the full length of the bridge decks and pavement to produce SI values. Raw noise data were normalized for the ambient air temperature and barometric pressure at the time of testing. The resulting pre- and postconstruction mean SI levels are A-weighted to produce the noise-frequency spectra in one-third octave bands, as shown in figure 21 for the bridges, figure 22 for the SC 377 pavement sections, and figure 23 for the U.S. 521 pavement section.
Figure 21. SC 377 bridge sections mean A-weighted sound intensity spectra.
Figure 22. SC 377 pavement sections mean A-weighted sound intensity spectra.
Figure 23. U.S. 521 pavement section mean A-weighted sound intensity spectra.
The noise frequency spectra shown in figure 21 through figure 23 are typical of an HMA pavement. The peak frequency for this type of surface is normally in the middle range of the spectrum at 800 to 1,000 hertz (Hz), followed by a steady reduction in values as frequency increases. The pre- and postconstruction mean SI levels for the pavement and bridge sections are summarized in table 6.
The goal of this HfL project was to achieve a tire-pavement noise measurement of less than 96.0 dB(A). The SI for the newly constructed pavement surfaces was 94.5 dB(A) for SC 377 and 94.7 dB(A) for U.S. 521. The average value dropped 2.1 dB(A) and both pavements meet the performance goal. However, the overall post-contruction SI for the bridges increased by 0.6 dB(A) when compared with the pre-construction values. The mean SI value of all four bridges was 98.2 dB(A), which does not met the 96.0 dB(A) goal.
Smoothness measurements from the test sections were collected using AubSurn University’s ARAN test van (figure 24). The ARAN van is equipped with a high-speed inertial profiler that is used to make profile-based smoothness measurements in each wheelpath of the pavement surface. Smoothness in each wheelpath is reported in terms of inches per mile in accordance with the International Roughness Index (IRI) approach.
Figure 24. Auburn University ARAN test van.
Smoothness measurements were collected at a constant speed of 45 mi/h. The raw smoothness data were processed to determine the average IRI for both wheelpaths.
Table 7 shows the IRI average values for the pavement and bridge sections.
The overall IRI values increased 32 percent for the SC 377 road sections, decreased 4 percent for the bridges, and increased 6 percent for the U.S. 521 pavement section. The increase in smoothness across the bridges is an improvement, but none of the newly constructed pavement surfaces meets the target IRI value of less than 48 inches per mile. The smoothness goal for bridges is extremely difficult to achieve on short-span bridges because the mean is influenced by the bump caused by expansion joints in the structures. The roadway sections, on the other hand, could have been smoother after the project was completed.
The HfL requirement for user satisfaction included a performance goal of 4-plus on a 7-point Likert scale for the following two questions:
SCDOT conducted a user satisfaction survey in which nearby residents were solicited by mail. Instead of a 7-point scale, the agency used a 5-point scale to determine the level of project satisfaction. An equivalent 5-point scale response score of 2.9 was needed to achieve the HfL satisfaction goal. A total of 51 highway users responded to the survey. The results were satisfaction scores of 4.4 for the new highway and bridges and 4.3 for the approach used to minimize disruption. Both scores exceed the equivalent goal of 2.9. Some respondents commented that the finished highway was rough in localized sections, which is evident in the measured IRI values, but the consensus was that the finished project was an improvement. The Appendix contains the complete survey results.