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Clearance Intervals

Clearance Intervals. NCHRP 172: Signal Timing Improvement Practices. v. Yellow = Y = t +. 2a ± Gg. w + L v. P + L v. P v. Clearance Interval. According to the ITE recommended practice:. Clearance Interval = Yellow + All Red. All Red = AR =. or. or. Where:

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Clearance Intervals

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  1. Clearance Intervals NCHRP 172: Signal Timing Improvement Practices

  2. v Yellow = Y = t + 2a ± Gg w + L v P + L v P v 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

  5. 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

  7. 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.”

  9. Fraley vs. the City of Flint, MI • Tort suit in Michigan (1974) • Court’s 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

  10. 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

  11. 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)

  12. 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 < the calculated stopping distance • Reasonable for the driver to decide to clear

  13. v Y = t + 2a ± Gg • 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

  15. 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

  16. 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

  18. All-Red Interval • “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” Signal Timing Improvement Practices NCHRP 172

  19. 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”

  20. 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. w + L v v = t + + 2a ± Gg 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

  24. 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. w + L v P + L v P + L v P v All Red = r = • 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: or All Red = r = All Red = r =

  26. 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

  27. 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

  28. 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

  29. 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”.

  30. 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

  31. Example: Calculate Clearance Intervals for the Intersection of Middlebelt Road and 5 Mile Road

  32. Approach Speed • Spot speed studies were taken at each of the intersection approaches as follows:

  33. 120’ 122’

  34. v Y = t + 2a ± Gg Yellow Intervals • Peak • Northbound Y = 1+ 46.9/(2*10) = 3.345 sec • Southbound Y = 1+ 42.5/(2*10) = 3.125 sec • Eastbound Y = 1+ 51.3/(2*10) = 3.565 sec • Westbound Y = 1+ 52.8/(2*10) = 3.64 sec • Peak N-S Yellow interval use 3.5 sec • Peak E-W Yellow Interval use 4.0 sec

  35. Yellow Intervals • Off Peak • Northbound Y = 1+ 55.7/(2*10) = 3.785 sec • Southbound Y = 1+ 52.8/(2*10) = 3.640 sec • Eastbound Y = 1+ 61.6/(2*10) = 4.08 sec • Westbound Y = 1+ 67.5/(2*10) = 4.375 sec • Off Peak N-S Yellow interval use 4.0 sec • Off-Peak E-W Yellow Interval use 4.5 sec

  36. w + L v All-Red Intervals All Red = r = • Peak • Northbound (122+20)/46.9 = 3.0 sec • Southbound (122+20)/42.5 =3.3 sec • Eastbound (120+20)/51.3 =2.7 sec • Westbound (120+20)/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

  37. w + L v All-Red Intervals All Red = r = • Off Peak • Northbound (122+20)/55.7 = 2.5 sec • Southbound (122+20)/52.8 =2.7 sec • Eastbound (120+20)/61.6 =2.3 sec • Westbound (120+20)/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

  38. 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

  39. Driver’s 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

  40. Driver’s Decision • natural driver behaviors: aggressive vs. non-aggressive • type of vehicle • vehicle condition • trip purpose

  41. Gazis Research late 1950’s x L W Clearing line S

  42. 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

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

  44. 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 (Y+AR)

  45. 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

  47. 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 < xc it will be uncomfortable, unsafe or impossible to stop

  48. Gazis Study • Maximum distance the car can be from the intersection of the yellow interval and still clear the intersection: Xo =vo - (W + L)

  49. 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 < xc , a driver at a distance x from the intersection, such that xo <x< xc will find him/herself in an awkward position if the yellow interval begins at that moment • cannot stop safely and has to attempt to go through the intersection

  50. Gazis Study • Minimum length of the clearance interval • min = (xc+ W+L)/vo OR • min = 2 + ½ vo/2a2*+ (W+L) vo

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