American Association of
State Highway and Transportation Officials
Special Committee on
Research and Innovation
FY2022 NCHRP PROBLEM
STATEMENT OUTLINE
Problem Number: 2023-C-14
Problem Title
Improve highway safety by developing guidance and standards for the
design of left turn pocket approach tapers on single
and multilane highways.
Background
Current design guidance and standards do not adequately address the
design of turn pocket approach tapers.
Existing standards provide turn pocket lengths without addressing the
issue of horizontal alignment for the lane shifts necessary to accommodate the
added width of left turn pockets.
Vehicles passing on through-movements to the right of left turns
traverse a horizontal alignment change that is different than the roadway
centerline alignment and should therefore be designed as such for traveler
safety.
This problem statement seeks three primary goals to address these
separate alignments:
1. Create guidance for treating approach tapers as independent alignments
for each of the two directions of travel.
2. Create guidance and standards for highways with tangent centerline
alignments – Existing design guidance typically consists of equations providing
approach taper lengths dependent upon width of shift and vehicle speed,
however, angle of deflection is not discussed.
Most importantly, acceptable maximum angle of deflection is not
addressed. Large angles of deflection
can result in erratic movements other than what is expected where vehicles
cross over the right edge line when entering an approach taper and the left
edge double yellow striping when exiting the approach taper in order to
negotiate their desired direction of travel.
(See Exhibit A for an illustration)
Exhibit A:
Caltrans Highway Design Manual Fig 405.2A – Standard Left Turn
Channelization
3. Create guidance and standards for highways with horizontal curve
centerline alignments – There is currently no existing guidance related to
approach taper shifts on horizontal curves.
Since angles of deflection are inappropriate for horizontal curves, use
of compound curves may be a more appropriate solution and should therefore be
studied. Furthermore, guidance for
superelevation is needed to address changes in curve radii between adjoining
curves, which may require different superelevation rates to address different
alignments on adjacent directions of travel.
(See Exhibit B for an illustration)
Exhibit B à Left Turn Pocket Approach Tapers on
Horizontal Curves
The lack of design guidance for the conditions described above affect
the ability of our nation’s design and traffic engineers to design safe
highways.
A thorough and complete literature search is necessary to exhaust all
possibility of prior efforts and sources of applicable guidance. Developing guidance and standards requires
field research to measure the physical characteristics and constraints of
vehicle instabilities while traversing a range of angle points on tangent
alignments and curved alignment of varying design speed. Field observations and evaluations of driver
behaviors of external vehicle movements using cameras and possibly
in-automobile as well, could provide the appropriate design guidance that
matches driver expectations and real life behaviors for the range of
situations.
Improving the design of the left turn pocket approach taper will improve
efficiency and mobility while reducing collisions. Improving transportation safety is part of
AASHTO’s Mission Statement where Safety Focused is its top Value. Safety is the top priority in the USDOT and
FHWA Strategic Plan and Mission Statement.
Turn pockets are typically the result of safety improvements for
vehicles who await gaps in traffic to make left turn movements the approach
tapers to turn pockets pose an even greater risk to overall highway safety
because all drivers passing through the approach taper are affected by the
horizontal alignment change from the centerline of the roadway. Consequently, properly designed approach
tapers increase the safety of all motorists traversing left turn pocket
approach tapers.
Literature Search Summary
A
thorough literature search has already been performed and the lack of national
and State guidance regarding left turn pocket approach taper lengths and
alignment, particularly in restricted situations and on curved alignments,
prompted the need for this problem statement.
The following literature searches have been performed:
·
The Caltrans Division of Research, Innovation, and System Information (DRISI)
contracted with its primary research consultants, CTC and Associates, who
performed a thorough national literature search. The report is available at: https://dot.ca.gov/programs/research-innovation-system-information/preliminary-investigations
·
The Caltrans senior librarian performed a
database literature search of all available Caltrans, TRB, NCHRP, and AASHTO
literature.
·
Caltrans design experts
performed in-depth reviews of AASHTO 7th Edition, Caltrans Highway
Design Manual, and various TRB/NCHRP publications.
·
During winter 2021, the DRISI and FHWA will
query all State DOTs for subject guidance and standards for supportive
information to assist with this proposed research objective.
Research Objective
The
objective of this research is to improve the safety of the traveling public by
developing guidance and standards for the nation’s engineers who design left turn pocket approach tapers on single
and multilane highways. The workplan
should include:
Task 1.
Review of Literature and
Current Practice.
Task 2.
Field Data Collection:
o Establish a data set of existing site conditions by locating existing
turn pocket approach tapers consisting of a range of approach speeds, angle
points and horizontal curvature.
o Investigate collision data associated with the data set locations for
applicability to design.
o Observational Study: Assemble and analyze video footage of vehicle paths
and movements traversing the approach tapers in the data set locations.
o Behavioral Study: Assemble and
analyze in-automobile video footage of driver behaviors while traversing the
tapers within the data set locations (This is an Optional Task).
o Analyze vehicle performance using ball bank indicator testing or similar
techniques for the data set locations.
Investigate the possibility of replacing field testing with analytical
models.
Task 3.
Identify Critical
Parameters: The objective of the task is
to identify the critical parameters that affect vehicle movement in left turn
pockets and in the adjacent through lanes, such as design vehicle, angle of
deflection, horizontal curvature, grade, lane width, shoulder width,
cross-slope, superelevation, etc.
Task 4.
Develop Framework for
Model: The objective of this task is to
develop a framework for incorporating the critical parameters identified in
Task 3.
Task 5.
Develop Workplan: The objective of the task is to develop a
workplan for calibrating the model based on the results from Tasks 2 and 3.
o Problem assessment using crash analysis
o Field Data Analysis
o Testing of analytical and simulation models
o Analysis of results
Task 6.
Prepare Interim
Report: Using the results of Tasks 1 – 5
to,
Task 7.
Develop Design
Guidance: Consideration for standards
recommendations for use in AASHTO, State, and local agency manuals and
guidance.
Task 8.
Prepare Final Report:
Urgency and Potential Benefits
The research is
needed immediately to fill a gap in current left turn pocket approach taper
design guidance, standards, and policy.
The consequences of not conducting this research will be the continued
misinterpretation of left turn pocket approach taper design, mainly the use of
angle points and curvilinear alignments that are mismatched to vehicle speeds
due to a lack of subject information and guidance. Consequently, all private and government
transportation engineers, general civil engineers, and practitioners will
benefit from filling this knowledge gap.
Implementation
Considerations and Supporters
The results of this research will be implemented by the
AASHTO Technical Committee on Geometric Design in a future edition of the
AASHTO Green Book. The California
Department of Transportation has committed to incorporating the research into
our Highway design manual and will make these changes available to other State
DOTs.
Recommended Research
Funding and Research Period
Total Funds Requested: $500,000
Research Period: 18 months
Problem Statement
Author(s)
John Roccanova, Project Delivery Coordinator, Caltrans, HQ Division of Design, (916) 275-2890, John.Roccanova@dot.ca.gov
Others Supporting the Problem Statement
The AASHTO Committee on Design has
endorsed this statement for submittal.
Antonette Clark, Chief, Office of
Standards and Procedures, Caltrans, HQ Division of Design, (916) 761-0587, Antonette.Clark@dot.ca.gov
Josue
Pluguez, Design Program Manager, FHWA California Division, (916) 498-5035, Josue.Pluguez@dot.ca.gov
Potential Panel Members
John Roccanova, Project Delivery
Coordinator, Caltrans, HQ Division of Design, (916) 275-2890, John.Roccanova@dot.ca.gov
Person Submitting the Problem Statement
Contact: Marshall Elizer, Chair,
AASHTO Technical Committee on Geometric Design
Assistant Secretary for Multimodal Development & Delivery,
Washington State Department of Transportation
310 Maple Park Avenue SE, PO Box
47351, Olympia WA 98504-7351
e-mail: ElizerM@wsdot.wa.gov