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Fatal Crashes by Junction Type Signalized Intersections
Spring 2006
SIGNALIZED INTERSECTION
COUNTERMEASURES
NCHRP Report 500, Vol. 12
Fatal Crashes by Junction Type Signalized Intersections
Spring 2006
Manner of Collision
Comparison of Severity Signalized Intersections
Spring 2006
Fatal Signalized Intersection
Crashes by Area Type
Objectives
Reduce frequency and severity of intersection
conflicts through control and operational
improvements
Reduce frequency and severity of intersection
conflicts through geometric design
Improve sight distance at signalized intersections
Improve driver awareness of intersections and signal
control
Improve compliance with traffic control devices Signalized Intersections
Spring 2006
Objective I
Reduce frequency and severity of
intersection conflicts through
traffic control and operations
Strategy A Employ Multiphase
Signal Operations
Two-phase signals require unprotected
(permitted) left-turns
Left-turns are highest risk movement, 70-
75% intersection accidents are left-turn
related
Protected left-turn phases improve safety
dramatically Signalized Intersections
Spring 2006
Phasing for Safety
Split phase may also be used to improve
safety
Not effective for operations and capacity Signalized Intersections
Spring 2006
Problematic Left-Turn Conflicts
Opposing through traffic
Through traffic in same direction crossing
vehicular and pedestrian traffic
Website on Signal Phasing
http://www.webs1.uidaho.edu/niattproject/
http://www.dot.state.mn.us/metro/trafficen
g/dsg_crse/chap21.html#_toc429824696 Signalized Intersections
Spring 2006
Options for Left-Turn Phasing
Permitted
Protected only
Protected/permitted
Both lead and lag
Protected / Permitted Phasing
Compromise between fully protected and
permitted-only phasing
Advantages
Reduction in delay for waiting left-turns
Less time needed for protected left-turns, thus more
time for high priority movements
Safety performance of protected/permitted left-
turn phasing is not as good as protected only
Dual or triple left-turn lanes should only operate
with protected turn phases Signalized Intersections
Spring 2006
Phasing for Safety
For safety
Protected-only is best option
Guidance for selection of left-turn phasing is
in NCHRP Synthesis 225: Left-Turn
Treatments at Intersections, TRB 1996
Lead versus Lag Left-Turn Phases
Lead protected left-turn phase with green
arrow minimizes conflict with through and
left-turn vehicles
Lagging left-turn phase allows some
clearing of the queue in permitted portion
of the cycle, then less lag green time Signalized Intersections
Spring 2006
Safety of Lead / Lag
Kentucky study shows lag option with
higher crash experience
Lag: 2.07 crashes per 1000 left-turning and
opposing vehicles
Lead: 1.27 crashes per 1000 left-turning and
opposing vehicles
Safety of Left-Turn Bays with
Left-Turn Phase
California study reported;
35% reduction in total accidents when left-turn
lanes were added and left-turn phases used
15% reduction in total accidents when left-turn
lanes were added without a separate turn
phase Signalized Intersections
Spring 2006
Strategy B Split Phases
May be used at intersections that;
Are skewed
Have a large deflection angle for through
movement
Have wide medians
Are too small for simultaneous left-turns
Have left and through movements sharing a
lane
Have major imbalance in opposing left-turn
volumes
Have different profiles or divided highway Signalized Intersections
Spring 2006
Strategy C
Optimize Clearance Intervals
Clearance interval defined:
The portion of the signal cycle between end of
green phase and beginning of next green
phase
To establish clearance intervals that meet
pedestrian needs Includes yellow time and all-red time Signalized Intersections
Spring 2006
Short Clearance Intervals
Short clearance intervals contribute to
Rear-end crashes
Right-angle crashes from signal violations
Extremely short clearance intervals force
drivers to violate the red phase
Detailed discussion on yellow and all-red
intervals;
Making Intersections Safer: A Toolbox to
Reduce Red Light Running, (McGee 2003)
http://www.ite.org/library/redlight/makingint_safer.pdf
ITE Traffic Engineering Handbook
Clearance Interval
Yellow change time (sec) = speed (mph)/10
Red clearance interval = 1 or 2 sec
MUTCD yellow change interval should be 3-6 sec.
ITE Traffic Engineering Handbook yellow change
interval of 5 sec, maximum
Typical red clearance interval
§
2 sec. Typical, 6 sec
maximum Signalized Intersections
Spring 2006
Uniform Clearance Interval Length
Various studies suggest;
A uniform value of 4-5 sec. for yellow change
interval length throughout a jurisdiction
Driver expectancy would anticipate uniform
clearance intervals for a jurisdiction
ITE Handbook Clearance Interval
Calculation
where
CP = non-dilemma change period (change + clearance
intervals)
t = perception/reaction time (nominally 1 sec.)
V = approach speed (ft/sec or 1.47 x mph)
g = percent grade (+upgrade, -downgrade)
a = deceleration rate (ft/sec
2
typical 10 ft/sec
2
)
w = width of intersection, curb to curb (ft)
L = length of vehicle (ft typical 20 ft)
*Also metric m, m/sec, and m/sec
2
V
L
W
g
4
.
64
a
2
V
t
CP
+
+
±
+
= Signalized Intersections
Spring 2006
Lengthening Clearance Intervals
Clearance intervals are lost time in the
cycle
Longer clearance times may require
longer cycles to offset lost time
Longer clearance times and longer cycles
lead to red light running
Strategy D Restrict or Eliminate
Turning Maneuvers
(including Right-Turns on Red, RTOR)
Where channelization or signal retiming are
difficult: Implement to reduce crashes or risky
maneuvers;
Restrict or eliminate turning maneuvers by
channelization
Prohibit right-turns on red Signalized Intersections
Spring 2006
Restrict or Eliminate
Turning Maneuvers
Implemented by channelization or signing;
Raised concrete channelization
Flexible delineators
Signing requires periodic enforcement
Prohibit Right-Turns on Red
RTOR prohibition can reduce crashes and
their severity due to
Limited sight distance
Pedestrian interaction
Vehicles from the left and left-turns from cross
streets
RTOR prohibition can be time dependent
Effective signing is critical Signalized Intersections
Spring 2006
Strategy E
Coordinate Signal Timing
Signal coordination improves quality of
operation and safety
Vehicle platooning reduces number and
frequency of stops and maintains uniform
speeds Reduces rear-end conflicts Eliminates problems of locating gaps for left-turns
Signal coordination
Reduces red light running
Arizona study showed 6.7% decrease in
crashes
Strategy F
Employ Emergency Vehicle Preemption
Signal preemption allows emergency
vehicles to proceed through intersections
More quickly; More safely
Emergency response time is reduced by
14-50% (Callura, et al., 2001)
Study showed 70% reduction in crashes
with emergency vehicles Signalized Intersections
Spring 2006
Strategy G Improve Operation of
Pedestrian and Bicycle Facility
at Signalized Intersections
About 1/3 of pedestrian-related crashes
are within 50 ft. of an intersection. Of
these;
30% involve turning vehicles
22% involve pedestrian running across
intersection or out in front of vehicle
16% driver error
Traffic Control Improvements to
Increase Pedestrian Safety, include
Pedestrian signs, signals and markings
Crossing guards for school children
Lighted school zone crosswalks
Pedestrian-only or pedestrian-lead phase
Prohibition of RTOR
Information signs on ped push buttons
Push button functioning indicator Signalized Intersections
Spring 2006
Strategy G
Remove Unwarranted Signals
Changed conditions may no longer warrant
traffic signal, e.g., decreased volume
Unwarranted signals cause;
Excessive delays
Rerouting traffic to other roads and intersections
Higher crash rates
Violation of traffic signal control
Safety of removal of unwarranted signals
Converted to all-way stop control accidents
decreased by one/year
Converted to two-way stop control right angle
crashes increased while rear-end crashes decreased
some amount Signalized Intersections
Spring 2006
Objective II
Reduce frequency and severity of
intersection conflicts with
geo