|FHWA > HfL > Projects > Vermont Demonstration Project: Route 2 – East Montpelier Bridge Reconstruction > Data Acquisition and Analysis|
Vermont Demonstration Project: Route 2 – East Montpelier Bridge Reconstruction
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
This section discusses how well the Caltrans demonstration project met the HfL performance goals related to these areas.
There were no contractor injuries or incidents during construction. Reasons cited for this success include:
Furthermore, with the replacement structure’s width of 44 ft, which is about 24 ft more than the old structure, the travel environment should be safer and minimize the “near misses” and broken side view mirrors that adjacent property owners complained about.
Preconstruction Traffic Study
Attempting to mimic the typical driving speed of other vehicles along the various roadway segments, the floating vehicle methodology was used to collect travel times prior to construction. The goal of the traffic data collection was to determine the preconstruction trip time and queues through the project’s influence area. This information was planned to be used as a benchmark to compare against similar data collected during construction.
Trips were made in each direction of travel between the first work zone warning sign (see Figure 30) and the “end construction” sign past the bridge. For the eastbound direction, the odometer was reset to 0.0 and the stopwatch was started at the intersection of VT 14 and US 2 (see Figure 31). At the first work zone warning sign approaching the bridge, the stopwatch time (to the nearest second) and the odometer reading (to the nearest 1/10th mile) were recorded accordingly. The stopwatch and odometer readings were recorded again at the end construction sign on the other side of the bridge. The final stopwatch and odometer readings were recorded upon reaching the VT 214 and US 2 intersection. The process was repeated in the westbound direction beginning at the VT 214 and US 2 intersection and ending at the VT 14 and US 2 intersection. A minimum of five travel time runs were performed in each direction.
Figure 30. First work zone warning.
Figure 31. FIntersection of VT 14 and US 2.
The data collection exercise was performed during the two peak traffic hours of noon and 4-5 PM. The first travel time data was recorded on Sunday (May 10, 2009) and was repeated on Monday (May 11, 2009).
Reducing the speed limit from 50 mph to 35 mph proportionally increased the trip time. However, based on the trip time study undertaken prior to the initiation of construction activities, traffic flowed freely and no noticeable back-ups were reported. As a result, queue lengths for vehicles approaching and traveling through the detour were nonexistent.
During Construction Traffic Study
Considering that there were no queues prior to construction, even with the reduced work zone speed limits and the fact that an on-site two-way temporary bridge was built in close proximity to the project (see Figure 17), no significant congestion related delays were experienced on this project during construction.
Sound intensity and smoothness test data were collected before and after construction. Comparing these results provides a measure of the quality of the finished bridge.
Sound Intensity Testing
OBSI test method was used to collect tire-pavement sound intensity (SI) measurements from the existing bridge to establish a baseline for comparison after construction was complete.
SI measurements were made using the currently accepted OBSI technique, AASHTO TP 76-10, which includes dual vertical SI and an ASTM recommended Standard Reference Test Tire (SRTT). Multiple runs were made at 35 mph in the right wheelpath. The SI probes simultaneously captured data from the leading and trailing tire-pavement contact areas. Figure 32 shows the dual probe instrumentation and the tread pattern of the SRTT.
Figure 32. OBSI dual probe system and the SRTT.
The average of the front and rear SI values was computed for the bridge to produce mean SI values. Raw data were normalized for the ambient air temperature and barometric pressure at the time of testing. The resulting mean SI levels were A-weighted to produce the noise-frequency spectra in one-third octave bands, as shown in Figure 33. The sound generated from the tire-pavement interaction was lower across the full range of frequencies.
Figure 33. OBSI dual probe system and the SRTT.
Global SI levels were calculated using logarithmic addition of the one-third octave band frequencies across the spectra. The mean value before construction was 99.0 dB(A), and the value for the new bridge was 96.4 dB(A). While not meeting the HfL goal of 96.0 dB(A), the new bridge was noticeably quieter than the old bridge.
Smoothness data were collected in conjunction with OBSI testing utilizing a high-speed inertial profiler integrated with the OBSI test vehicle. Figure 34 is an image of the test vehicle showing the profiler positioned in-line with the right rear wheel. Multiple test runs were performed in each wheel path in each direction. The eastbound and westbound test runs were averaged to produce a single IRI value with units of in/mile.
Figure 34. High-speed inertial profiler mounted behind the test vehicle.
Figure 35 graphically presents the mean IRI values at 20-ft intervals for the old and new bridge and approach pavement. For reference, the bridge location is shaded in the figure. Overall, the increased smoothness of the new construction decreased the IRI values for the bridge and surrounding pavement. The mean IRI value for the bridge decreased from 126 in/mi to 78 in/mi. While not satisfying the HfL goal of 48 in/mi, this represents a noticeable improvement in smoothness.
Figure 35. Mean IRI values.
VTrans conducted a postconstruction survey on user satisfaction at project completion. The agency distributed the survey to stopped traffic at the site when it was performing live load testing on the bridge as part of its research on integral abutments. The survey questions shown in Figure 36 were on prepaid postcards that were to be mailed by the respondents. Approximately 500 cards were handed out, and responses were received from 129 driving passenger vehicles and 6 from those driving heavy trucks. The results are summarized in Table 1.
Figure 36. User survey form.
In reviewing the data, it seems most users were satisfied with the movement of traffic through the construction site. The lower score from the truckers perhaps reflects the maneuvering the truckers needed to negotiate the detour curves. The scores from both groups were relatively lower for project construction time but still scored 83 percent favorable response. The delays attributed to stainless steel delivery and re-pouring of a portion of the deck, which extended the project duration, perhaps contributed to these ratings.
Both groups gave the highest score for overall satisfaction with the new bridge compared to the old bridge, with the combined score of 4.8 out of a possible 5.0, or 83 percent, perhaps because the new bridge is smoother and wider and offers a sense of safer travel. The response to the questions exceeded the HfL goal of 4-plus on a Likert scale of 1–7 (in other words, 57 percent or more participants showing favorable response).