Arterial Traffic Analysis Some critical concepts Prepared by Philip J. Tarnoff
Agenda Fundamentals Signal timing concepts Actuated signal controllers Developing timing plans Types of signal control First generation Advanced techniques Relationship to the big picture (ITS)
Cycle Length Fact & Fiction Facts: Increased cycle length increases intersection capacity Shorter cycle lengths reduce delay Fiction: Cycle length has a significant impact on capacity Offset and cycle length are independent Longer cycle lengths always reduce congestion
Cycle Length and Capacity Effective Green Time (Sec./Hr.) 3,600 3,500 3,400 3,300 3,200 3,100 3,000 2,900 2,800 2 Phase 3 Phase 4 Phase 80 100 120 140 160 180 200 220 240 Cycle Length (Sec.)
Cycle Length and Capacity Cycle Length change 2 Phase 3 Phase 4 Phase 120 to 140 0.75% 1.16% 1.59% 140 to 160 0.56% 0.86% 1.17% 160 to 180 0.43% 0.66% 0.90% 180 to 200 0.34% 0.53% 0.71%
Webster s Formula for Optimum Cycle Length C = (1.5*L+5)/(1-Y) Where: C is cycle length in seconds L is lost time = [(no. of phases)*(lost time per phase)+(all red time)] Y = sum of the critical lane flows/(1900)
Question Using Webster s equation, calculate the cycle length for an intersection with the following characteristics: Two phases (E-W and N-S) Assume lost time on each phase = 2.5 sec. 1 lane eastbound flow = 75 vph 1 lane westbound flow = 100 vph 2 lane northbound flow = 2600 vph 2 lane southbound flow = 1500 vph 2 lane flows equally divided in each lane
Question Use Webster again for: Eight Phase controller Assume lost time on each phase = 2.5 sec. Flows Equally divided N-S two thru lanes Single lane for all other movements Use flows on next slide
Question - continued NBL = 180 VPH NBT = 892 SBL = 44 SBT = 904 EBL = 104 EBT = 168 EBR = 240 WBL = 68 WBT = 160 WBR = 52
Cycle Length & Progression (offset) Space Ideal multiple of cycle length Time
Longer Cycle Lengths can Increase Congestion Upstream throughput may exceed downstream link capacity Turning bay storage can be exceeded Increased vehicle headways with long cycle lengths (longer green times)
Space Long Cycle Length Reduces Capacity Lost (wasted) green time Time
Cycle Length and Headway Space Headway Time
The Bottom Line About Cycle Length Results for an Isolated Intersection 30 Delay (sec./veh.) 25 20 15 10 5 800 vph 1200 vph 0 20 40 60 80 100 Cycle Length (sec.)
Basic Facts About Split Allocates intersection capacity to conflicting movements Directly entered on pretimed controllers Implicitly selected for actuated controllers through: Maximum green times Minimum green times
A Correct Split is Important 60 50 Delay 40 30 20 10 900 vph 1100 vph 1300 vph 0 0 25 50 75 100 Split Error (%)
Conclusions About Split Split i = Cycle * (Volume i )/(Total Volume) Bad splits mean high delays particularly at high volumes
Offsets Can Produce Smooth Flow Offset is the time relationship between intersections Offset is only relevant for undersaturated intersections Offset is useful for higher intersection spacing than generally realized Value of offset depends on mid-block friction
Platoons Determine Offset Effectiveness Space Platoon Dispersion Time
Platoon Profiles Traffic Flow % Saturation 1 0.8 0.6 0.4 0.2 0 % Saturation 1.00 0.80 0.60 0.40 0.20 0.00 1.00 0.80 0.60 0.40 0.20 0.00 1 5 9 13 17 21 25 29 33 37 1 41 5 45 9 13 49 17 21 25 29 33 37 1 41 5 45 9 49 13 17 21 25 29 33 37 % Saturation 41 45 49 Time (sec.) Time (sec.) Time (sec.)
Midblock Friction Need for coordination (offsets) determined by separation and midblock friction High friction disperses platoon Low friction allows the platoon to remain together
Levels of Midblock Friction Friction Level Typical Facility Turning Others Low High-type suburban arterial Turning bays No driveways, parking or pedestrians Medium Low-type suburban arterial Some protected turning movements Shopping center driveways, few pedestrians, no parking High Urban CBD No protected turning Pedestrians Parking Driveways
Midblock Friction Affects Platoon Dispersion Platoon profile @ 1500 ft. low friction 1.00 0.80 0.60 0.40 0.20 0.00 1 5 9 13 17 21 25 29 33 37 41 45 49 % Saturation Time (sec.) Platoon profile @ 1500 ft. moderate friction % Saturation 1.00 0.80 0.60 0.40 0.20 0.00 1 5 9 13 17 21 25 29 33 37 41 45 49 Time (sec.) Platoon profile @ 1500 ft. High friction 1.00 0.80 0.60 0.40 0.20 0.00 1 5 9 13 17 21 25 29 33 37 41 45 49 % Saturation Time (sec.)
Conclusions About Offset Offset = Spacing - Queue Length Speed Discharge Rate Offset must take queue length into account If midblock friction is low, coordination (the use of offsets) is useful for long intersection spacing
Question Calculate the relative offset between two intersections for the following conditions: Spacing 1000 ft. Ave. speed 50 fps Ave. Queue 16 veh. Queue Discharge.5 veh./sec.
Phase Sequences Leading/lagging turns present four options for NB-SB and EB-WB movements If time-space diagrams are used, best procedure is to determine offsets for favored direction Try different phase sequences to achieve best greenband in reverse direction
Space Alternate Phase Sequences NB Left Turn SB Left Turn Time
Space Alternate Phase Sequences NB Left Turn SB Left Turn Time
Double Cycling Another tool to help progression Used where interface between different types of roadways with different cycle length requirements is needed Not a panacea Only ½ the cycles are coordinated Cycle lengths are constrained to multiples of each other
Double Cycling at Work Space Double Cycled Intersection Time
Why do I need to know all this? Signal timing optimization programs do NOT always provide the best signal timing Cannot optimize for oversaturation Do not always model mid-block sources well Do not always model platoon dispersion Do not always permit evaluation of alternative phase sequences Independent analysis is mandatory
Signal Timing Process Arterial Timing Select Cycle Length Range yes Saturated? no Select Cycle Length Range Evaluate Phasing Alternatives Use Optimization Software Consider Double Cycling Determine Offsets & Splits
Points to Ponder Longer cycle lengths are not necessarily better Incorrect splits (including max greens) can significantly increase delay Offsets are important and must take secondary flows into account Alternative phase sequences can make a big difference