Submitter
Response for 2023-C-14
Improve
highway safety by developing guidance and standards for the design of left turn
pocket approach tapers on single and multilane highways
Comments:
FHWA Evaluation
FHWA Evaluation of C-14 Reviewed By:
Intersections Team/HSA/HRDS Comments: While better
developed and consistent guidance would be helpful since this overlaps the
MUTCD and AASHTO GB (and potentially AASHTO HSM and TRB HCM), it is
questionable whether this project could have a significant impact. Making
an explicit and statistically confident connection between turn lane geometrics
and crashes would be quite difficult, as detailed geometric information at this
level is not typically collected/inventoried or readily available.
Additionally, while determining whether a turn lane should be added has a
safety dimension (and CMFs), the length and particulars of the design are
dependent on context, field conditions, traffic demand, etc. and rarely an
explicit basis of safety. Review Date: December 17, 2021
Response to FHWA Evaluation
Comment: “While better developed and consistent guidance
would be helpful since this overlaps the MUTCD and AASHTO GB (and potentially
AASHTO HSM and TRB HCM), it is questionable whether this project could have a
significant impact.” Response: Thank you for your
comments. In California, we construct numerous standalone safety projects
specifically scoped for adding left turn pockets to address fatal and injury
collisions, many of which cannot meet AASHTO or California Highway Design
Manual standards due to a variety of constraints. Consequently, we
believe the project will have a significant impact.
Comment: “Making an explicit and statistically confident
connection between turn lane geometrics and crashes would be quite difficult,
as detailed geometric information at this level is not typically
collected/inventoried or readily available.” Response: It is agreed
that tying collision data directly to vehicle instability and driver behaviors
related to left turn pocket taper shift alignments will be difficult, however,
determining vehicle instability and driver behaviors for a range of deflection
angles will not be difficult and is one of the primary purposes of this problem
statement. There are a number of proven techniques such as ball-bank indicators,
computer modeling, video camera observations …etc,
that have been used successfully for these purposes.
Comment: “Additionally, while determining whether a turn
lane should be added has a safety dimension (and CMFs), the length and
particulars of the design are dependent on context, field conditions, traffic
demand, etc. and rarely an explicit basis of safety.”
Response: We have found in California that safety is a factor in left
turn pocket approach taper design because the taper introduces a change in
horizontal alignment for both directions of travel on conventional
highways. When left turn pockets are added into existing two-lane
highways, current guidance and standards use a deflection angle without
providing guidance for determining the maximum safe deflection angle for
constrained situations. There is a safety need to provide consistent
guidance for determining horizontal alignment maximum deflection angles for a
given design speed. Consequently, when designers introduce deflection
angles that are too large for the design speed, it introduces vehicle
instability and erratic driving behaviors, which is by definition a safety
concern. The “particulars of the design are dependent on context,
field conditions, traffic demand, etc” are the
normal project factors that limit the ability to obtain standard left turn
pocket approach taper deflection angles.
Contact Info:
John
Roccanova
California
Department of Transportation (Caltrans)
(916)
275-2890
Review
Date:
1/24/2022