CASE STUDY 3: S.R. 826-PALMETTO EXPRESSWAY INTERCHANGE OPERATIONAL ANALYSIS REPORT, FLORIDA

NW 57th and NW 67th Avenues Eastbound Off-Ramps

Project Description

Case Example 3 documents the comparative findings of pre- and post-construction operational level of service for the eastbound (EB) off-ramps and the auxiliary lane along the SR 826-Palmetto Expressway east-west corridor for the NW 67th and NW 57th Avenues interchanges. The purpose of this 'before and after' analysis was to provide the Florida DOT with a detailed analysis that documents the improved traffic operations (density and level of service) due to the eastbound off-ramp improvements and the construction of an eastbound auxiliary lane. The eastbound interchange improvement project began on January 22, 2001 and was completed March 20, 2002. FDOT conducted the study to assist them in determining if similar ramp and auxiliary lane improvements for the westbound direction would be warranted.

The eastbound SR 826 Palmetto Expressway 'before and after' study was initiated due to the Florida DOT design team completing an Interchange Operational Analysis Report (IOAR) which was submitted to Department in December 2004 for the westbound (WB) off-ramps at the NW 67th and NW 57th Avenues interchanges. The report recommended construction of a continuous WB auxiliary lane, and widening of the off-ramps to two lanes, along with operational improvements to the intersections. The Scoping Committee requested a study of the recently improved eastbound off ramps, at NW 67th and 57th Avenues, in order to verify whether the objectives of that project have been accomplished. The recommended improvements included widening of the off-ramps and adding a continuous auxiliary lane between both interchanges. Due to the similarities in configuration and recommendations, it was decided that an evaluation of before and after conditions in the EB direction (field review and microsimulation comparisons) would be the best way to establish whether the proposed WB improvements in the IOAR (December 2004) are sound and worth pursuing.

Although the eastbound interchange improvements existed when the 'before and after' study was initiated, the microsimulation analysis was conducted to quantify the eastbound ramp improvements contribution to the SR 826 Palmetto Expressway operations. Growth in the Expressways traffic as well as not having field data prior to the construction of the eastbound interchange improvements made it difficult to assess the extent of the operational improvements by field observation alone. The comparison was conducted by applying a microsimulation software analysis to determine what the changes in the level of service would be due to interchange improvements. The interchange improvements were also evaluated by using a deterministic Highway Capacity Manual software package. A comparison was made between the final analysis of the microsimulation software, the Highway Capacity Manual procedure, and the actual condition. The EB interchange improvement project began January 22, 2001 and was completed March 20, 2002.

Figure 7: SR 826/Palmetto Expressway at NW 67th (Left) and NW 57th Avenue (Right)

The Florida DOT design team completed an Interchange Operational Analysis Report (IOAR) which was submitted to Department in December 2004 for the westbound (WB) off-ramps at the NW 67th and NW 57th Avenues interchanges. The report recommended construction of a continuous WB auxiliary lane, and widening of the off-ramps to two lanes, along with operational improvements to the intersections. The Scoping Committee requested a study of the recently improved eastbound off ramps, at NW 67th and 57th Avenues, in order to verify whether the objectives of that project have been accomplished. The recommended improvements included widening of the off-ramps and adding a continuous auxiliary lane between both interchanges. Due to the similarities in configuration and recommendations, it was decided that an evaluation of before and after conditions in the EB direction (including actual and microsimulation comparisons) would be the best way to establish whether the proposed WB improvements in the IOAR (December 2004) are sound and worth pursuing.

Description

The SR 826-Palmetto Expressway is a high-speed limited access facility with a posted speed of 55 mph. The mainline facility has a typical section consisting of six lanes divided from west of NW 67th Avenue to east of NW 57th Avenue. Travel lanes are approximately 12 feet wide with 7-foot inside shoulders and outside shoulders approximately 10 feet wide. A one-way frontage road (NW 167th Street) is located on the north and south sides of the mainline facility. The frontage road provides two lanes in each direction with a posted speed of 40 mph. Figure 8 illustrates the existing conditions geometry for both interchanges after the construction of the EB improvement project.

Figure 8: Existing Conditions Geometry (EB Improvement Project Post-Construction)

Year 2004 (Post-Construction) Existing Traffic Volumes

For the purpose of this comparative analysis, the existing year (2004) A.M. and P.M. peak hour volumes were taken directly from the SR 826-Palmetto Expressway IOAR (December 2004), referenced above.

The method used in the IOAR (December 2004) to develop the A.M. and P.M. peak hour volumes for the mainline, ramps and intersections is briefly described below. For additional details such as traffic flow patterns, origin-destination survey and historical crash data, refer to the IOAR (December 2004).

Mainline and Ramp Traffic Volumes

Year 2003 Average Annual Daily Traffic (AADT) volumes for SR 826-Palmetto Expressway mainline and ramps were obtained from the FDOT's 2003 Florida Traffic Information CD. The 2003 mainline and ramp volumes were projected to year 2004 volumes by applying a growth factor of 0.5 percent. This applied growth factor was based on the historical records from the Department's traffic counting stations. The growth rate was computed from a regression analysis using the Department's Trends Analysis Spreadsheet. The A.M. and P.M. peak hour traffic volumes were estimated by applying appropriate K-factors to the AADT.

Intersection Traffic Volumes

Turning movement and seventy-two hour continuous traffic counts were collected at the signalized intersections for both A.M. (7:00 to 9:00 A.M.) and P.M. (4:00 to 6:00 P.M.) peak hours during typical weekdays in April 2004.

Year 2000 (Pre-Construction) Traffic Volumes

A comparison of historical AADTs from the 2004 Florida Department of Transportation Traffic Data CD was done based on key stations along the corridor. Table 2 provides a summary of this comparison.

As shown in the table, the 2001 AADTs declined from the previous year AADTs, due to construction. For this reason, volumes for the year 2000 were determined to be representative of the most recent entire pre-construction year along the corridor. Also shown in Table 2 below, is the average reduction factor of 0.91 applied to the Year 2004 (Post-Construction) volumes taken from the IOAR (December 2004) to develop the Year 2000 (Pre-Construction) volumes.

Figure 9 illustrates the Year 2000 (Pre-Construction) traffic volumes that were developed for the A.M. and P.M. peak hour periods for the mainline, ramps and signalized intersections.

Figure 9: Year 2000 (Pre-Construction) A.M. and P.M. Peak Hour Traffic Volumes

Analysis and Results

Ramps, Mainline and Intersection Level of Service Analysis Comparison

For the purpose of comparing the traffic conditions before and after construction at the eastbound off-ramps at NW 67th and NW 57th Avenue, the results of a micro-simulation model for SR 826 - Palmetto Expressway IOAR (December, 2004) for A.M. and P.M. peak hours were obtained by Florida DOT. The existing geometry was reviewed and assumed for the Year 2004 (Post-Construction) conditions. The geometry and traffic volumes were revised for the Year 2000 (Pre-Construction) conditions. Listed below are revisions made to the geometry.

• NW 67th Avenue - reduced to one-lane eastbound off-ramp
• NW 57th Avenue - reduced to one-lane eastbound off-ramp
• Removal of eastbound auxiliary lane between NW 67th and NW 57th Avenues

Eastbound Ramp Merge / Diverge Level of Service Analysis Comparison

The eastbound off-ramps at the interchanges of NW 67th Avenue and NW 57th Avenue were improved from one-lane to two-lanes with a full auxiliary lane between the NW 67th Avenue interchange on-ramp and NW 57th Avenue interchange two-lane off-ramp.

Ramps were analyzed utilizing the ramp module of the Highway Capacity Manual and other relevant methodology on ramps from the Highway Capacity Manual. The hourly volumes were converted to peak flow rates by applying truck factors, peak hour factors (PHF), driver population parameter, and passenger car equivalents as described in Highway Capacity Manual. Table 3 summarizes results of the ramps merge/diverge analyses.

As seen in Table 3 and in the Highway Capacity Manual methodology output, a comparison of pre and post construction conditions indicate improvements in the level of service, even when considering that the traffic in the segment has grown almost 10% between 2000 and 2004.

Mainline Level of Service

Levels of service analyses from the micro-simulation model and the Highway Capacity Manual software were conducted for the mainline for Year 2000 (Pre-Construction) and Year 2004 (Post-Construction). Tables 4 and 5 summarize the comparative results for A.M. and P.M. peak hour freeway mainline in the eastbound direction for pre-construction and post-construction conditions.

Micro-simulation Results

The A.M. peak hour results from the model do not indicate significant differences or marked improvements in the post construction condition, with the exception of west of the N.W. 67th Avenue off-ramp. The eastbound mainline segment between the NW 67th Avenue on-ramp and the NW 57th Avenue off-ramp indicated to be within the acceptable level of service standard for pre-construction at LOS C but for post-construction the level of service dropped below the standard level of service to LOS E. This is primarily due to the traffic increases that have occurred since the construction took place, which have caused congestion at the intersections and the mainline, congestion which then spills back into the ramps under study.

Highway Capacity Manual Software Results

Given the shortcomings of the microsimulation model, which produced spillbacks that did not allow the proper isolated evaluation of the auxiliary lane, the freeway module of the Highway Capacity Manual software was chosen as an alternative tool to better analyze and isolate conditions relevant to the auxiliary lane project. Table 5 provides the summary of the analysis. Based on the Highway Capacity Manual software freeway analysis, a comparison of 'before and after' conditions indicates generally unchanged levels of service and decreases in density along key sections of the mainline, even considering that the traffic in the section has grown almost 10% since the eastbound auxiliary lane was completed.

Conclusions and Recommendations

The purpose of this analysis was to evaluate how much the construction of the expanded freeway eastbound ramps and auxiliary lanes contributed to improving the freeway's operations. As expected the microsimulation model accurately represented the existing freeway operations and properly forecasted spillbacks from downstream. By replicating the spillbacks, however, the analysis of the auxiliary lanes as an isolated improvement was not possible to distinguish. Under these circumstances the HCM software was applied in order to evaluate the auxiliary lane as an isolated improvement in order to determine the benefits of the improvement. The HCM comparative analysis indicated that the eastbound improvements yielded moderate improvements in the freeway and ramp operations. Therefore a recommendation was made to construct similar improvements in the westbound direction.

Extensions and Guidance

Level of Effort

Case Study 3's geographic scope is relatively limited to several interchanges and the supporting frontage roads and arterial system feeding into the interchange areas. Due to the limited roadway system that was under study, the required amount of time to input the network and traffic data for the microsimulation model was modest. If the study area was wider in scope then the setup time would be significantly higher. For the deterministic HCM model, the setup time was minimal. HCM models are relatively simplistic in their data and network requirements and require minimal labor for their application.

Traffic Analysis Software Selection

An aspect of any traffic analysis study is the selection of the proper software package that best meets the objectives of the study. In Case Study 3, the objective of the project was to identify how well individual roadway improvements improved the freeway traffic operations. Many times as was the case in the Palmetto Interchange Analysis, spillback traffic as shown in the microsimulation model masked the operational improvements of the expanded ramps and auxiliary lane. By using the deterministic HCM model, the spot improvements due to the expanded ramps could be evaluated without being masked by the system deficiencies. An important lesson from Case Study 3 is for traffic engineers to select the best package that will fulfill the specific objectives of the study. When evaluating specific spot improvements such as individual ramps or limited merge areas simpler traffic analysis tools such as the HCM models may be more appropriate and effective than microsimulation models.

Modeling Process

An important lesson to learn in this case study is that microsimulation models can be used and be quite useful for conducting before and after studies even though a formal before study was not completed prior to the construction of a transportation improvement.

Data Development

Typical applications of traffic analysis software require existing as well as future traffic such as traffic volumes and turning movements. In this case study rather than forecasting the future, the model was used to recreate the traffic operational conditions prior to the completion of the improved ramps and auxiliary lanes. Traffic data for the Year 2000 prior to the construction project was developed based on traffic reduction factors established by mainline freeway historical counts. The case study demonstrated that through the development of traffic reduction factors sufficient traffic information can be developed for practical application to microsimulation models.

Measure of Effectiveness Issue

In the conduct of this study two separate software packages were applied; microsimulation and a deterministic HCM model procedure. Each program has their unique procedures for determining the level of service and measure of effectiveness. As demonstrated in this study, if the HCM procedure of Level of Service definition as used in the deterministic HCM model will be the standard, then the microsimulation results must be converted into an equivalent measure of effectiveness as defined in the HCM. For this case study all microsimulation results were converted into density and the equivalent Level of Service to match the results of the HCM procedure. Whenever a microsimulation model is used it is imperative that an HCM equivalence be established as was done in this study when using HCM Levels of Service.