Impacts of HOV Lane Termination Upstream of System Interchange Mohammad Rehman, PE Ogbonna Abarikwu, PE, PTOE

This paper presents the findings of a high-occupancy vehicle (HOV) lane termination case study for southbound Interstate 17 (I-17) just north of I-17/I-10 junction located at milepost 200.88 in Phoenix, Arizona.

ABSTRACT Southbound Interstate 17 (I-17) experienced significant delays upstream of I-10 system interchange in Phoenix, Arizona. A detailed traffic study was conducted to assess capacity, operational and safety deficiencies and mitigation alternatives were developed for this segment. Among others deficiencies, termination of HOV lane as a general purpose lane was determined as a contributing factor to the poor traffic operation – a condition that extended to approximately one (1) mile upstream of the system interchange. The location of termination of HOV lane relative to the system interchange resulted in last minute lane-change maneuvers creating severe weave which resulted in disproportionately high number of crashes. Among the mitigations recommended was early termination of the HOV lane two (2) miles upstream of the interchange with the intent of allowing ample time and space for drivers to change into the correct lane well before the system interchange.

The recommendations of the report including provision for early termination of the HOV lane, extension of auxiliary lane and signage enhancements were constructed in the fall of 2010. Post construction observations indicate that weave in the segment north of the interchange has been significantly reduced and the lane utilization of the “recovered” general purpose lane is well balanced. While it may be too early to determine post-construction collision rate (to be done as part of an “after” study), well streamlined traffic flow with fewer brake lights are tell-tale signs of improved operation and safety.

PROJECT OVERVIEW The Project 17 MA 200 H 2954 01D Southbound I-17 Access Operation Improvement Study was undertaken by the Arizona Department of Transportation (ADOT) to determine how this segment of southbound I-17 may be improved to enhance flow, capacity and safety. The segment of I-17 experienced significant friction upstream of the I-17/I-10 junction resulting in disproportionate number of accidents. Multiple weaving sections between Thomas Road onramp, McDowell Road off-ramp and I-17/I-10 junction (all within 3,800 feet of each other), as well as HOV lane termination within one (1) of the junction were considered to be the root cause of the friction. Figure 1 shows details of the study area. Page 1

An extensive data collection effort was conducted throughout the study area. The data collected included freeway and ramp volumes (from ADOT Freeway Management System (FMS) database) supplemented with manual counts; travel time runs along heaviest travelled movements; aerial photographs flown by ADOT Photogrammetry during peak periods (for headway and density measurement); and three-year accident data from ADOT Transportation Planning Division (TPD). Peak hour traffic volumes within the study area are shown in Figure 2.

Using field data, existing conditions including freeway segment operation, weaving sections, and ramp junctions were analyzed using Highway Capacity Manual (HCM) and Leisch methods. CORSIM micro-simulation model was developed to simulate freeway mainline, on- and offramps, interchanges, and corridor-wide operation as a system. Measures of effectiveness (MOEs) included vehicular delays, queuing, travel speed, number of lane-change, percent of demand served, emissions, and fuel consumption. The CORSIM model was calibrated to field collected data for both AM and PM peak hour conditions and a validated CORSIM model was developed for development of improvement alternatives.

A total of five (5) mitigation alternatives were developed for the study. These alternatives included a combination of signage/striping modifications, auxiliary lane extension and ramp closure. Results of alternatives evaluation indicate that Alternative 4 would yield the best overall benefits in terms of safety enhancements and improved traffic operation. Figure 3 represents a schematic depiction of the Preferred Alternative. Among other improvements that constituted the Preferred Alternative, the following two (2) relate to the HOV lane termination and signage modification for improved lane utilization: 1. Drop southbound I-17 HOV lane to north of the Indian School Road overpass. Modify HOV lane signage to reflect recommended condition. 2. Use overhead signs with two outer lanes designated for I-10 traffic and two inner lanes for I-17 traffic to transmit information about the downstream I-17/I-10 junction as early as the Indian School Road overcrossing.

ADOT constructed the recommendations of the report in the fall of 2010 as District “Minor” Project (TRACS #H7465 01D). Consistent with the results of the operational analysis predicted in the report, the freeway capacity improvements (conversion of Thomas Road to McDowell Road auxiliary lane to I-10 exit lane) and traffic signage improvements resulted in substantial operational benefits. Among other benefits, a well received revelation was the significant operational improvement upstream of the system interchange that resulted from early termination of the HOV lane. Segment of I-17 between Thomas Road off- and on-ramp exhibited tranquil flow of traffic with minimum number of lane-changes from day one. ADOT is considering an “after” study to quantify the benefits from early HOV termination and other treatments. However, due to funding constraints, schedule for the study is unknown. In the mean time, this paper presents a general overview of the impacts associated with the HOV lane termination as a general purpose lane upstream of the I-17/I-10 system interchange through a comparison of “before” and “after” field observations. Page 3

IMPACTS OF HOV LANE TERMINATION The impacts of HOV lane termination presented herein are based on before and after construction field observations upstream of the I-17/I-10 junction within the influence area of the system interchange. We believe it is possible to replicate the same benefits at locations where HOV lanes terminate as a general purpose lanes, similar to that addressed in our study. However, the following conditions apply to these findings: 1. The HOV lane termination as a general purpose lane occurs within the influence area of a system interchange where a major volume split occurs; and 2. Signage improvements for adequate lane designation may be necessary up to two (2) miles upstream of the system interchange.

Freeway Operation It may be notable to mention that there is no acceptable industry analytical method (such as Highway Capacity Manual (HCM) method) for analyzing HOV lane termination besides microsimulation techniques used in this study. Therefore, impacts of HOV lane termination on freeway operation were not determined analytically as part of the Southbound I-17 Access Operation Improvement Study.

Field observations before the improvements revealed bottlenecks resulting from a significant number of lane-change maneuvers just past the HOV termination point approximately one (1) mile upstream of the I-17/I-10 system interchange. Predominantly, drivers from the general purpose lane adjacent to the HOV lane changed lane to their left to occupy the relatively vacant and available median lane downstream of the HOV lane termination point. On the other hand, drivers from the median lane changed lanes to their right to get into the correct lane ahead of the system interchange. Using videography, it was estimated that within the first 650 feet downstream of the HOV termination point, approximately 560 lane-change maneuvers from outer lane to the median lane and 80 maneuvers from median lane towards the outer lane occurred during the weekday morning peak hour when the HOV lane was enforced. In addition, 270 lane-change maneuvers occurred per hour in the same area within the outer three (3) general purpose lanes.

CORSIM micro-simulation model was developed to simulate HOV lane termination using “restricted” HOV lane and adding the decision point for the system interchange at the HOV lane termination point to trigger the lane-change maneuvers as observed in the field. These weaving movements created a bottled that resulted in reduction of travel speed from 61 miles per hour (MPH) in the Indian School Road on-ramp to Thomas Road off-ramp segment to 51 MPH in the downstream segment of Thomas Road off-ramp to Thomas Road on-ramp. Table 2 compares travel times and speeds along southbound I-17 for individual segments and for the entire study corridor for “before” conditions (Figure 1) and “after” conditions (Preferred Alternative depicted in Figure 3). Page 6

Table 2

Freeway Travel Times and Travel Speeds Before

Segment 1 Indian School Rd On-Ramp Travel Time 2 to Thomas Rd Off-Ramp Speed Travel Time1 Thomas Rd Off-Ramp to Thomas Rd On-Ramp Speed2 Travel Time1 Thomas Rd On-Ramp to McDowell Rd Off-Ramp Speed2 1 McDowell Road Off-Ramp Travel Time to EB/WB I-10 Split Speed2 Travel Time1 Overall Speed2

After

AM

PM

AM

PM

0:32 61 0:46 51 0:19 53 0:31 52 02:08 54

0:32 62 0:42 56 0:17 58 0:30 53 02:02 57

0:33 60 0:39 60 0:16 60 0:26 62 01:55 60

0:32 61 0:38 61 0:16 60 0:26 62 01:53 61

Notes:  Reported MOEs are computed from the CORSIM output and are an average of five runs. 1. Travel times reported are in minutes:seconds. 2. Travel speeds reported are in miles per hour (mph). Source: the CK Group

Field observation of current conditions show substantial improvements along the segments of I17 upstream of the system interchange, particularly the segment between Thomas on- and offramp. This segment shows tranquil flow of traffic with minimum number of lane changes. Further upstream at the new termination point of HOV lane north of Indian School Road, lanechange maneuvers might have increased (was not studied under “before” conditions) as a result of HOV lane termination. However, field observations of current conditions reveal approximately 580 lane-change maneuvers per hour during the morning peak, compared to 910 lane-change maneuvers that occurred when the HOV lane terminated one (1) mile closer to the system interchange. Reduction in instantaneous lane-change maneuvers at the HOV lane termination has been related to the increased distance (and time) now available to both the HOV and general purpose lane drivers for making the lane-change maneuvers to get into the desired lanes upstream of the system interchange. Additionally, lane designation related signage improvements up to Indian School Road (two miles north of system interchange) also contributed towards the improved traffic operation.

Safety Spatial analysis of crash data was conducted for frequency and type of collisions to identify high crash locations. Reported crash data is graphically shown in Figure 4 and results of the crash analysis are summarized by frequency and collision type in Table 2. High frequency of rear-end collisions in the segment between Thomas Road off- and on-ramp could be attributed to the severe weaving resulting from the significant number of lane-change maneuvers (approximately 640 per hour mentioned above). Other factors such as inadequate signing and striping could result in untimely driver reaction, which may have been the cause for a large number of crashes occurring prior to the improvements. Crash analysis for post-improvement conditions will be conducted as part of the “after” study. Page 7

Table 2 Roadway Segment Indian School OnThomas Off Ramp Thomas Off – Thomas On Ramp Thomas OnMcDowell Off Ramp McDowell Off RampI-10 Ramp Total

Crash Summary Type of Collision Side Single Other Swipe Vehicle

Length (miles)

ADT

Rear End

0.23

112,000

43

10

4

3

60

0.65

102,500

393

58

31

14

496

6.80

0.27

109,000

39

7

6

1

53

1.65

0.36

106,500

94

16

16

5

131

3.51

1.51

107,294

569

91

57

23

740

4.17

Total

Crash Rate1 2.13

Notes: 1. Crash Rate per million vehicle miles (MVM) of travel = (a x 1,000,000)/(c x ADT x 365 x n) a = Number of reported crashes, c = Length of segment in miles, and n = Number of years Source: ADOT/the CK Group, Inc.

AUTHORS INFORMATION Mr. Mohammad Rehman, PE (AZ, CA) is the Pre-Design Manager at the CK Group, Inc. He has over 16 years of experience in traffic, transportation, and civil engineering design and analysis. He has distinguished himself by successfully completing various complex projects in his professional career. Mohammad is adept in roadway and major interchange preliminary design, traffic operations and maintenance of traffic (MOT), corridor feasibility and sub-area transportation infrastructure needs studies, transportation plans and downtown/ campus circulation studies. Employer: the CK Group, Inc. Address: 16448 N 40th Street, Suite A Phoenix, Arizona 85032 Phone: 602-482-5884 Email: [email protected] Mr. Ogbonna Abarikwu, PE (AZ, NV, TX, UT, CA), PTOE, is the Principal of the CK Group, Inc. He has over 26 years of experience in managing transportation planning and design projects. Over the course of his career, he has successfully led multi-disciplinary transportation system planning and design teams requiring building consensus among multiple agencies, stakeholder communities, and Federal agencies on projects with large budgets and challenging schedules. He has extensive experience in establishing and interpreting MOEs from statistical data such as those used in travel demand and micro-simulation modeling. This knowledge enables him to efficiently recommend feasible and cost-effective mitigation measures for planning and preliminary engineering studies. Employer: the CK Group, Inc. Address: 16448 N 40th Street, Suite A Phoenix, Arizona 85032 Phone: 602-482-5884 Email: [email protected]

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