Arterial Traffic Analysis Some critical concepts. Prepared by Philip J. Tarnoff

Transcription

1 Arterial Traffic Analysis Some critical concepts Prepared by Philip J. Tarnoff

2 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)

3 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

4 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 Cycle Length (Sec.)

5 Cycle Length and Capacity Cycle Length change 2 Phase 3 Phase 4 Phase 120 to % 1.16% 1.59% 140 to % 0.86% 1.17% 160 to % 0.66% 0.90% 180 to % 0.53% 0.71%

6 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)

7 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

8 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

9 Question - continued NBL = 180 VPH NBT = 892 SBL = 44 SBT = 904 EBL = 104 EBT = 168 EBR = 240 WBL = 68 WBT = 160 WBR = 52

10 Cycle Length & Progression (offset) Space Ideal multiple of cycle length Time

11 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)

12 Space Long Cycle Length Reduces Capacity Lost (wasted) green time Time

13 Cycle Length and Headway Space Headway Time

14 The Bottom Line About Cycle Length Results for an Isolated Intersection 30 Delay (sec./veh.) vph 1200 vph Cycle Length (sec.)

15 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

16 A Correct Split is Important Delay vph 1100 vph 1300 vph Split Error (%)

17 Conclusions About Split Split i = Cycle * (Volume i )/(Total Volume) Bad splits mean high delays particularly at high volumes

18 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

19 Platoons Determine Offset Effectiveness Space Platoon Dispersion Time

20 Platoon Profiles Traffic Flow % Saturation % Saturation % Saturation Time (sec.) Time (sec.) Time (sec.)

21 Midblock Friction Need for coordination (offsets) determined by separation and midblock friction High friction disperses platoon Low friction allows the platoon to remain together

22 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

23 Midblock Friction Affects Platoon Dispersion Platoon 1500 ft. low friction % Saturation Time (sec.) Platoon 1500 ft. moderate friction % Saturation Time (sec.) Platoon 1500 ft. High friction % Saturation Time (sec.)

24 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

25 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.

26 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

27 Space Alternate Phase Sequences NB Left Turn SB Left Turn Time

28 Space Alternate Phase Sequences NB Left Turn SB Left Turn Time

29 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

30 Double Cycling at Work Space Double Cycled Intersection Time

31 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

32 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

33 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