Clearance Intervals

1
Clearance Intervals
NCHRP 172 Signal Timing Improvement Practices
2
Clearance Interval

• According to the ITE recommended practice

Clearance Interval Yellow All Red
All Red AR
or
or
3
Where Y yellow interval (seconds) t driver
perception-reaction time for stopping,
taken as 1 sec v approach speed (ft/sec) taken
as the 85th percentile speed or the speed
limit a deceleration rate for stopping taken as
10 ft/sec2 G percent of grade divided by 100
(positive for upgrade, negative for
downgrade)
4

• L length of the clearing vehicle,
• normally 20 feet
• W width of the intersection in feet, measured
  from the upstream stop bar to the downstream
  extended edge of pavement
• P width of the intersection (feet) measured
  from the near-side stop line to the far side of
  the farthest conflicting pedestrian crosswalk
  along an actual vehicle path

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Clearance Interval
w
P
6

• Uses a comfortable and attainable deceleration
  rate of 10 ft/sec/sec
• As opposed to the emergency rate of 15
  ft/sec/sec used earlier
• Adds one second to the calculated yellow time

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Yellow Interval

• At least half the states use the permissive
  yellow rule
• allows vehicles to enter the intersection on a
  yellow signal and to be in the intersection when
  the signal turns red

8
National MUTCD

• Specifies the length of the yellow change
  interval as
• The yellow vehicle change intervals should have
  a range of approximately 3 to 6 seconds.
  Generally, the longer intervals are appropriate
  to higher approach speeds.

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Fraley vs. the City of Flint, MI

• Tort suit in Michigan (1974)
• Courts opinion
• it is not enough that a yellow time merely be
  between 3 and 6 seconds
• The yellow interval must be designed for
  intersection-specific conditions
• truck use
• intersection geometry
• other site specific characteristics

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Yellow Interval

• ITE formula gives a yellow interval long enough
  so that a clearing driver will not be forced to
  enter the intersection on the red, which is an
  unlawful act

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Yellow Interval

• Based on equation for stopping

S vot vo2/2a
vot gives the distance traveled at initial
speed vo during braking
perception-reaction time t Vo2/2a braking
distance to a final speed v 0, from the
fundamental equation of linear
kinematics
v2 vo2 2as where v final speed (ft/sec)
vo initial speed (ft/sec) a
deceleration rate (ft/sec/sec) s distance
traveled during braking (ft)
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• If the yellow begins when a vehicle is further
  away from the intersection than the minimum
  stopping distance required
• The driver will be able to stop
• If the vehicle has distance
• Reasonable for the driver to decide to clear

13

• Minimum required yellow time will carry the
  clearing vehicle into just into the intersection
• Legally entered (permissive rule)
• Just before the red begins
• Minimum yellow time

14
Eastbound car is clearing after having barely
entered the intersection by the time the red
begins. There is no all-red interval, so
northbound car receives the green immediately No
all-red interval is used
Figure 6. Possible scenario with no all-red
clearance
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• Yellow time calculated according to the ITE
  formula will carry the clearing vehicle just into
  the intersection by the time it ends
• As shown by vehicle A in the preceding slide
• If there is no all-red interval, then oncoming
  traffic is released on a green signal (vehicle B)
• Vehicle A will not be protected

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• Driver of vehicle B has a duty to yield the
  right-of-way to vehicle A legally within the
  intersection
• Permissive rule
• However, many drivers do not know this law
• Naïve for traffic engineer to expect drivers to
  yield the ROW
• To ensure safety, use all-red intervals

17
Northbound car fails to yield ROW to car A
legally in the intersection, enters soon after
receiving the green and is struck No all-red
interval is used
Figure 6. Possible scenario with no all-red
clearance
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All-Red Interval
Signal Timing Improvement Practices NCHRP 172

• In order to time phase-change intervals for
  safety, traffic engineers sometimes need to go
  beyond the minimums implied by the rules of the
  road. An All-red clearance interval should be
  considered in some cases in addition to the
  yellow

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MUTCD

• The yellow vehicle change interval may be
  followed by a red clearance interval, of
  sufficient duration to permit traffic to clear
  the intersection before conflicting traffic
  movements are released

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TCDH

• The policy of some jurisdictions is to time the
  phase change interval to allow the outset of the
  green interval for conflicting movements without
  the intersection having been cleared

21
TCDH

• Some authorities believe that the timing of a
  phase-change interval should enable a vehicle to
  clear the intersection before the onset of the
  green for conflicting movements. The following
  equation may be used to determine the phase
  change interval. It includes a reaction time,
  deceleration element and an intersection clearing
  time

CP
Where CP is the non dilemma change period
22
TCDH

• the yellow change interval be equal to the first
  two terms of the equation and the equation
  rounded up to the next ½ second, but no less than
  3 seconds and no greater than 5 seconds. The
  remainder of the change period should consist of
  an all-red interval.

23
Eastbound car clears intersection by the time the
northbound car receives green Intersection where
an all-red interval is used
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Older Driver Highway Design Handbook
Recommendations and Guidelines

• To accommodate age differences in
  perception-reaction time, it is recommended that
  an all-red clearance interval be consistently
  implemented, with the length determined according
  to the Institute of Transportation Engineers
  (1992) expressions

25

• Where there is no pedestrian traffic, use
• Where there is the probability of pedestrian
  crossing, use the greater of
• Where there is significant pedestrian traffic or
  pedestrian signals protect the crosswalk, use

All Red r
or
All Red r
All Red r
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• According to traffic laws in Michigan, USA
• a vehicle must stop when confronted with a yellow
  light, unless such an abrupt stop would endanger
  the safety of the driver as well as others
• Law enforcement officials are reluctant to issue
  a citation for not stopping during the yellow
  interval
• Unless someone is observed to have accelerated
  through the intersection
• citation is rare
• hard to prove when contested in a court of law

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• Entering the intersection when a signal turns red
  is what most officials consider a citable offense
• Red light violation
• Violations are affected by the duration of the
  change interval of the traffic signal
• yellow interval
• all-red interval

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• When entering the intersection at the end of the
  clearance interval, motorist are exposed to the
  danger of being struck by the cross street
  traffic unless an all-red interval is present

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Uniform Vehicle Code in the State of Michigan, USA

• If the signal exhibits a steady yellow
  indication, vehicular traffic facing the signal
  shall stop before entering the nearest crosswalk
  at the intersection or at a limit line when
  marked, but if the stop cannot be made safely, a
  vehicle must be driven cautiously through the
  intersection.

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• A vehicle can enter an intersection legally, even
  a fraction before it turns red
• If it takes a vehicle two-seconds of time to
  cross, then the vehicle is under eminent danger
  of being involved in a right angle crash in the
  absence of an all red interval
• An intersection without an all red interval runs
  the risk of having right angle crashes, even if
  no one violated the red light

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Example Calculate Clearance Intervals for the
Intersection of Middlebelt Road and 5 Mile Road
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Approach Speed

• Spot speed studies were taken at each of the
  intersection approaches as follows

33
120
122
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Yellow Intervals

• Peak
• Northbound
• Y 1 46.9/(210) 3.345 sec
• Southbound
• Y 1 42.5/(210) 3.125 sec
• Eastbound
• Y 1 51.3/(210) 3.565 sec
• Westbound
• Y 1 52.8/(210) 3.64 sec
• ?Peak N-S Yellow interval use 3.5 sec
• Peak E-W Yellow Interval use 4.0 sec

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Yellow Intervals

• Off Peak
• Northbound
• Y 1 55.7/(210) 3.785 sec
• Southbound
• Y 1 52.8/(210) 3.640 sec
• Eastbound
• Y 1 61.6/(210) 4.08 sec
• Westbound
• Y 1 67.5/(210) 4.375 sec
• ?Off Peak N-S Yellow interval use 4.0 sec
• Off-Peak E-W Yellow Interval use 4.5 sec

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All-Red Intervals
All Red r

• Peak
• Northbound
• (12220)/46.9 3.0 sec
• Southbound
• (12220)/42.5 3.3 sec
• Eastbound
• (12020)/51.3 2.7 sec
• Westbound
• (12020)/52.8 2.6 sec
• ?Peak N-S All-Red interval use 3.3 sec
• Peak E-W All-Red Interval use 2.7 sec

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All-Red Intervals
All Red r

• Off Peak
• Northbound
• (12220)/55.7 2.5 sec
• Southbound
• (12220)/52.8 2.7 sec
• Eastbound
• (12020)/61.6 2.3 sec
• Westbound
• (12020)/67.5 2.1 sec
• ?Off-Peak N-S All-Red interval use 2.7 sec
• Off-Peak E-W All-red Interval use 2.3 sec

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Clearance Intervals (CI)

• Peak Period
• North-South East-West
• Y 3.5 sec Y 4.0 sec
• AR 3.3 sec AR 2.7 sec
• CI 6.8 sec CI 6.7 sec
• Off-Peak Period
• North-South East-West
• Y 4.0 sec Y 4.5 sec
• AR 2.7 sec AR 2.3 sec
• CI 6.7 sec CI 6.8 sec

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Drivers Decision

• Whether to stop or not stop at the traffic signal
  may be related to
• vehicle approach speed
• color of the traffic signal when noticed by the
  driver
• location of the vehicle with respect to the
  intersection

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Drivers Decision

• natural driver behaviors aggressive vs.
  non-aggressive
• type of vehicle
• vehicle condition
• trip purpose

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Gazis Research late 1950s
x
L
W
Clearing line
S
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Gazis Study

• Car traveling at a constant speed v0
• Location of the car is at x feet from the stop
  bar, S
• Driver has 2 options
• Must decelerate and stop before line S (stop bar)
• Must continue and go through the intersection
• Dilemma Zone

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Dilemma Zone
Gazis Study
xc
Cannot stop
Dilemma zone
Cannot go
xo
S
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Gazis Study

• ?1, ?2 time at which acceleration or
  deceleration will begin after the starting
  of the yellow interval
• a1 constant acceleration rate for crossing
  the intersection 10 ft/sec2
• a2 constant deceleration rate for stopping
  before the intersection 10 ft/sec2
• W effective width of the intersection
• L length of the car (usually 20)
• ? Length of the clearance interval (YAR)

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Gazis Study
x
L
W
Vehicle location when light turns yellow
Clearing line
S
46
Gazis Study

• If the driver is to come to a complete stop
  before entering the intersection
• (x - vo?2) ? vo2/2a2
• If the driver is to clear the intersection
  completely before the light turns red
• (x w L) - vo ?1 ? vo (? - ?1) ½ a1 (? -
  ?1)2

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Gazis Study

• Assuming a maximum deceleration rate of a2, the
  critical distance is
• Xc vo ? 1 vo2/2a2
• If x xc the car can be stopped before the
  intersection
• If x impossible to stop

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Gazis Study

• Maximum distance the car can be from the
  intersection of the yellow interval and still
  clear the intersection
• Xo vo? - (W L)

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Gazis Study

• Thus, if xo xc the driver, once past the
  critical distance xc can clear the intersection
  before the signal turns red
• If xo intersection, such that
• xo awkward position if the yellow interval begins at
  that moment
• cannot stop safely and has to attempt to go
  through the intersection

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Gazis Study

• Minimum length of the clearance interval
• ?min (xc WL)/vo OR
• ?min ?2 ½ vo/2a2

(WL)
vo
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Example
x
L 20
W 80
Vehicle location when light turns yellow,
traveling at 30 mph (44 fps)
Clearing line
S
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• For driver to stop
• (X - vo?2) ? vo2/2a2
• Assume ?2 0.4 seconds and a2 10 ft/sec2
• X (44 0.4) ? 442/ (210)
• X 17.6 ? 96.8
• X ? 114.4 ft
• If the driver sees the yellow light 115 feet
  before the stop bar, the driver can stop in this
  distance

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• For driver to clear
• (x w L) - vo ?1 ? vo (? - ?1 ) ½ a1 (? -
  ?1)2
• Assume ?1 0.2
• For ? 3.0 seconds
• (828020)- 44(0.2) ? 44( 3 0.2 ) ½ 10
  (3-0.2)2
• 173.2 feet ? 162.4 feet cannot clear
• For ? 5.0 seconds
• (828020)- 44(0.2) ? 44( 5 0.2) ½ 10 (5-0.2)2
• 173.2 feet ? 326.4 feet
• criteria satisfied
• Drivers will be able to stop or clear.

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• For ? 6.0 seconds
• (828020)- 44(0.2) ? 44(6 0.2) ½ 10 (6-0.2)2
• 173.2 feet ? 423.4 feet criteria satisfied
• Drivers will be able to stop or clear.

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Highway Capacity Software(HCS)
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Highway Capacity Software

• Based on the Highway Capacity Manual (HCM)
• Special Report 209
• Transportation Research Board (TRB), National
  Research Council (NRC)

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Ten Modules

• Freeways
• Weaving
• Ramps
• Multi-lane Highways
• Two-lane Highways
• Signalized Intersections
• Unsignalized Intersections
• Arterials
• Transit
• Pedestrians

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Signalized Intersections

• Capacity
• Defined for each lane group
• Lane group one or more lanes that accommodate
  traffic and have a common stopline
• Lane group capacity maximum rate of flow for the
  subject lane group that may pass through the
  intersection under prevailing traffic, roadway
  and signalized conditions

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• Traffic Conditions
• Approach volumes (left, through, right)
• Vehicle type
• Location of bus stops
• Pedestrian crossing flows

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• Roadway Conditions
• Number and width of lanes
• Grades
• Lane use
• Including parking lanes
• Signalized Conditions
• Signal phasing
• Signal timing
• Type of control
• Signal progression

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Level of Service (LOS) for Signalized
Intersections

• Defined in terms of delay as a measure of
• driver discomfort
• Driver frustration
• Fuel consumption
• Lost travel time

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• Delay experienced by a motorist includes many
  factors
• Signal control
• Geometrics
• Incidents

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• Total delay
• Difference between actual travel time and ideal
  travel time
• In the absence of traffic control, geometric
  delay, incidents and when there are no vehicles
  on the road
• In HCS only control delay is quantified
• initial deceleration delay
• Queue move-up time
• Stopped delay
• Final acceleration delay

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• Previous versions of HCM/HCS (1994 version or
  earlier)
• Only included stopped time delay
• Latest version includes control delay

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LOS

• LOS criteria are stated in terms of average
  control delay per vehicle
• Delay is dependent on
• Quality of progression
• Cycle length
• Green ratio
• V/c ratio for lane group
• Phasing design
• Designated by letters A - F

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LOS Criteria for Signalized Intersections
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LOS A

• Describes operations with very low control delay,
  up to 10 sec/veh
• Occurs when progression is extremely favorable
• When most cars arrive during the green
• Most vehicles do not stop at all
• Drivers can select speed and path

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LOS B

• Describes operations with control delay 10 and
  up to 20 sec/veh
• Occurs with good progression, short cycle lengths
  or both
• More vehicles stop than with LOS A
• Causing higher levels of average delay

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LOS C

• Describes operations with control delay greater
  than 20 and up to 35 sec/veh
• Fair progression, longer cycle lengths, or both
• Individual cycle failures may begin to appear at
  this level
• No. of vehicles stopping is significant
• Many still pass without stopping

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LOS D

• Describes operations with control delay 35 and
  up to 55 sec/veh
• Influence of congestion becomes more noticeable
• Longer delays result
• Unfavorable progression
• Long cycle lengths
• High v/c ratios
• Many vehicles stop
• Proportion of vehicles not stopping declines
• Individual cycle failures are noticeable

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LOS E

• Describes operations with delay 55 and up to 80
  sec/veh
• The limit of acceptable delay
• Indicate poor progression, long cycle lengths and
  high v/c ratios
• Individual cycle failures are frequent occurrences

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LOS F

• Describes operations with delay 80 sec/veh
• Considered unacceptable to most drivers
• Occurs with oversaturation
• When arrival flow rates exceed the capacity of
  the intersection
• Occurs at high v/c rations below 1.0 with many
  individual cycle failures
• Poor progression and long cycle lengths may also
  contribute

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• INPUT
• Geometric conditions
• Traffic conditions
• Signalization conditions

Operational Analysis Procedure

• VOLUME ADJUSTMENT
• Peak hour factor
• Establish lane groups
• Assign volumes to lane groups

• 3. SATURATION FLOW RATE
• Ideal saturation flow rate
• Adjustments

• CAPACITY ANALYSIS MODULE
• Compute lane group capacities
• Compute lane group v/c ratios
• Aggregate results

• LEVEL OF SERVICE MODULE
• Compute lane group delays
• Aggregate delays
• Determine levels of service