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FACT SHEET
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Publication Number:  FHWA-HRT-18-016    Date:  December 2017
Publication Number: FHWA-HRT-18-016
Date: December 2017

 

Simulator Assessment of Contraflow Lanes at Signalized Intersections

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The objective of this project is to conduct a series of human factors experiments to evaluate the most effective signing and marking strategies for real-world implementations of dynamic reversible left turn lanes (DRLT) for diamond interchanges and contraflow left turn pockets (CLTP) for signalized intersections.

Introduction

Increasing traffic demand is causing more severe congestion and bottlenecks in urban environments. Constrained budgets are spurring innovations that efficiently use existing intersection footprints. From a previously sponsored research effort by the Office of Operations Research and Development (R&D), DRLT and CLT treatments were identified as two cost-effective and innovative treatments to mitigate congestion attributed to heavy left-turn (LT) volumes at diamond interchanges and signalized intersections.1

DYNAMIC REVERSIBLE LEFT TURN LANE (DRLT)

A DRLT treatment is designed for implementation at diamond interchanges experiencing heavy LT volume. DRLT treatment can reduce delay and increase capacity without requiring additional right-of-way.1 This is accomplished by converting the center lane (two back-to-back LT bays in figure 1a) into a full-length LT lane accessible by both directions of traffic (figure 1b). The use of the LT lane by each direction of traffic is allocated through signals.

Traditional diamond interchange with central back-to-back LT lane. This image shows an aerial view of a traditional diamond interchange. The image in Figure 1a is an above grade interchange. The main feature of the figure is the bridge that crosses the freeway above grade. On this bridge, there are five lanes: two lanes each direction serving through arterial traffic and an internal back-to-back left-turn lane. Arrows point to the internal back-to-back left-turn lane

Figure 1(a). Traditional diamond interchange with central back-to-back LT lane.1

Aerial view of diamond interchange with DRLT treatment. This image shows an aerial view of the proposed dynamic reversible left-turn treatment for diamond interchanges. As in Figure 1a, the depicted interchange is an above grade. The main feature of the figure is the bridge that crosses the freeway above grade. On this bridge, there are five lanes: two lanes each direction serving arterial through traffic with an internal dynamic reversible full-length left-turn lane. The reversible left-turn lane is highlighted in orange.

Figure 1(b). Aerial view of diamond interchange with DRLT treatment.1

CONTRAFLOW LEFT TURN POCKET (CLTP)

A CLTP treatment is designed for implementation at conventional signalized intersections with heavy LT volume. It makes use of the opposing through lane (OTL) when it is not in use by opposing traffic (i.e., when the cross street has priority) to provide an additional temporary LT bay, without requiring the construction of an additional lane. The CLTP lane operation begins when the leading protected LT phase ends on the cross street. After the area shaded in green in figure 2 clears of LT traffic from the cross street, a pre-signal on the up-stream end of the CLTP lane notifies drivers that they may queue in the CLTP lane. Both LT lanes discharge during the protected LT phase at the main signal. Before the end of LT phase, the pre-signal closes the CLTP lane; after a calculated discharge phase designed to clear the CLTP lane, the protected LT phase on the main approach ends to give priority to through traffic.

Signalized intersection with CLT treatment creating an additional dynamic LT bay using OTL during minor-street green phase. This image shows an aerial view of the proposed contraflow left-turn pocket lane at a signalized intersection. The contraflow left-turn pocket lanes are depicted by green boxes. Black arrows illustrate the trajectory of left-turning vehicles from the contraflow left-turn pocket lane and the regular left-turn bay into individual receiving lanes on the cross street. The red arrow shows the trajectory of left-turning vehicles from the minor approach onto the major approach. The red color represents the potential conflict between these left-turning vehicles and the vehicles queuing in the contraflow left-turn pocket lane.

Figure 2. Signalized intersection with CLT treatment creating an additional dynamic LT bay using OTL during minor street green phase.1

Problem Statement

Peak period contraflow operations have been deployed at multiple locations around the United States to mitigate congestion issues associated with temporary spikes in demand from commuter traffic. The DRLT and CLTP operations are also recommended for use during peak periods; however, the direction of flow on these short roadway segments innovatively change direction within the signal cycle. This research project is tasked with the development of signing and marking strategies that can effectively communicate the operation of the treatments to novice users in a highway driving simulator (HDS).

Previously simulated junctions were selected for testing in the HDS: Georgetown Pike (VA Route 193) at the interchange with I–495 (DRLT) and Rockville Pike (MD Route 355) at Tuckerman Lane (CLTP).1

Research Approach

The suggested signing and marking strategies for real-world implementation of the DRLT and CLTP went through two critical iterations: the first based on comments recorded during the expert panel meeting and the second based on the results of the sign lab study. During the expert panel meeting, members of Federal Highway Association Research and Development, the FHWA Manual on Uniform Traffic Control Devices (MUTCD) team, Maryland State Highway Administration, and Virginia Department of Transportation developed acceptable scenarios for signing and marking the treatments.

The sign lab study included 65 participants brought onsite to the Turner-Fairbank Highway Research Center (TFHRC) to evaluate the aforementioned innovative signing and marking strategies from an application, comprehension, and preference perspective. Ultimately, one signing and marking scenario for each treatment was selected for coding and implementation in the HDS at the TFHRC. Close cooperation with the MUTCD team throughout the entire process has ensured that the signing and marking scenarios recommended for analysis in the HDS meet guidelines established in the MUTCD and are readily implementable by interested agencies.

The HDS study is ongoing and results are expected to be reported by the Office of Safety Research and Development in an upcoming publication for 2018. Still images captured from the ongoing HDS study signing and marking configurations can be viewed in figures 3, 4, and 5 for the DRLT and figure 6 for the CLTP lane.

On-peak upstream signalized intersection (DRLT). This image shows three guide signs above a signalized intersection mounted on a cantilever structure. On the far-left lane, there is a no left turn arrow, as well as a changeable guidance sign that reads “South 495” with an arrow pointing straight through the intersection. The middle lane has a static sign displaying the same message. The far-right lane has a static sign reading “West 193” with an arrow pointing straight through the intersection.

Figure 3(a). On-peak upstream signalized intersection recommendations (DRLT).

Off-peak upstream signalized intersection (DRLT). This image shows three guide signs above a signalized intersection mounted on a cantilever structure. On the far-left lane, there is a no left turn arrow, as well as a changeable guidance sign that reads “THIS LANE CLOSED” in white text on a black background. The middle lane has a static sign that reads “South 495” with an arrow pointing straight through the intersection. The far-right lane has a static sign reading “West 193” with an arrow pointing straight through the intersection. There are also two three-lensed vertical signal heads located on the cantilever structure; one signal head is located between the left and middle lane, while the other is located between the middle and right lane.

Figure 3(b). Off-peak upstream signalized intersection recommendations (DLRT).

On-peak mid-bridge gantry recommendations (DRLT). This figure shows the recommended signing for an overhead gantry located at the center of the bridge during on-peak hours. This gantry features an overhead lane use control signal centered over each lane. Reading from left to right, there’s a red “X”, a white-on-black one-way left-turn arrow, a white-on-black one-way left-turn arrow, and a green downward pointing arrow.

Figure 4(a). On-peak mid-bridge gantry recommendations (DLRT).

Off-peak mid-bridge gantry recommendations (DRLT). This image shows the recommended signing for an overhead gantry located in the center of the bridge during Off-peak hours. This gantry features an overhead lane use control signal centered over each lane. Reading from left to right, there’s a red “X”, a red “X”, a one-way white-on-black left-turn arrow, and a green downward pointing arrow.

Figure 4(b). Off-peak mid-bridge gantry recommendations (DRLT).

On-peak downstream gantry recommendations (DRLT); during Off-peak, leftmost changeable message sign will display a white-on-black “THIS LANE CLOSED” message. This image shows the recommended signing of the downstream intersection. There are three guide signs above a signalized intersection mounted on a cantilever structure. On the far-left lane, there is a changeable guidance sign that reads “South 495” with an arrow pointing left and the word “ONLY” underneath. The middle lane has a dynamic sign displaying the same message. The far-right lane has a static sign reading “West 193” with an arrow pointing straight through the intersection.

Figure 5. On-peak downstream gantry recommendations (DRLT); during off-peak, leftmost changeable message sign will display a white-on-black “THIS LANE CLOSED” message.

On-peak (open) pre-signal recommendations. This image shows the recommended signing at the “pre-signal” location during On-peak hours when the pocket lane is open for vehicles to queue. At the median nose, there is a changeable message sign with a positive contrast W12-1 sign (i.e., two downward pointing arrows on a yellow-on-black changeable message sign). Over the contraflow left-turn pocket lane and the two adjacent lanes to the CLTP lane, there are overhead lane use control signals. On the lane to the left of the CLTP lane, there is a red “X”. Over the CLTP lane, there is a double stacked lane use control signal; one signal indicates a green downward pointing arrow while the other is a white-on-black one-way left-turn sign. On the lane to the right of the CLTP lane, there is a green downward pointing arrow.

Figure 6(a). On-peak pre-signal recommendations (CLTP).

Figure 6b. On-peak (closed) and off-peak pre-signal recommendations. This image shows the recommended signing at the “pre-signal” location during off-peak hours and during on-peak hours when the pocket lane is closed. At the median nose, there is a changeable message sign with a positive contrast R4-7b sign (i.e., a changeable message sign with white-on-black “KEEP RIGHT” and a diagonal left-pointing arrow displayed). Over the contraflow left-turn pocket lane and the two adjacent lanes to the CLTP lane, there are overhead lane use control signals. On the lane to the left of the CLTP lane, there is a red “X”. Over the CLTP lane, is a double stacked lane use control signal; one signal indicates a red “X” while the other is blank. On the lane to the right of the CLTP lane, there is a green downward pointing arrow.

Figure 6(b). Off-peak pre-signal recommendations (CLTP).

References

  1. Hale et al. (2016). Traffic Bottlenecks: Identification and Solutions. Report Number: FHWA-HRT-16-064

For more information, please contact:
Michelle.Arnold@dot.gov | Rachel.James@dot.gov
FHWA-HRT-18-016
HRDO-20/12-2017(200)E

 

 

 

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