Traffic Signal Timing: Green Time. CVEN 457 & 696 Lecture #18 Gene Hawkins

Transcription

1 Traffic Signal Timing: Green Time CVEN 457 & 696 Lecture #18 Gene Hawkins

2 The Problem N NBLT = 181 vph NBTR = 688* vph SBLT = 216 vph SBTR = 771* vph WB = 321* vph EB = 236* vph *see diagram for turning movements What is the appropriate green time for each phase?

3 Procedure Define volumes and lane groups Determine saturation flow for each movement Determine demand for each movement Define critical movements Select phasing arrangement Determine cycle length Allocate green times for each phase Check minimum green time for each movement

4 What are My Lane Groups? SBTR SBLT N NBLT NBTR Lane groups covered in previous lecture EB WB

5 Saturation Flow Rates N NBLT = 1810 vph NBTR = = 3638 vph SBLT = 1810 vph SBTR = = 3690 vph WB = 1616 vph EB = 1683 vph

6 Determine Demand Calculate volume to saturation flow ratio (v/s ratio) Movement volume / movement saturation flow v y i i s Where i v i = volume of lane group i, vph s i = saturation flow rate for lane group i

7 v/s Ratios Mvmt Volume Saturation v/s Ratio Flow Rate NBLT /1810=0.100 SBLT NBTR SBTR WB EB

8 Find Critical Movements at Intersection Phase duration must be long enough to satisfy demand of critical movements Function of Selected Phasing Pattern Conflicting Movements

9 Determine Phasing How to handle left turns? Function of various factors (lecture 17a) Factors to consider: Local policy, safety issues, traffic demands, efficiencies For this example, use phasing below

10 What are Conflicting Movements? EB Movements that cannot receive green at the same time SBTR SBLT SBTR WB NBTR NBLT NBTR NBTR EB Conflict Conflict Conflict OK OK

11 Evaluate Impact of Other Phasing Patterns A lower sum of critical v/s ratios will give better operations Phasing options: Leading or lagging lefts Split phasing Overlap phasing Lead/lag left phasing

12 Green Time Example Using the v/s ratios below, which phasing pattern has the lowest sum of critical v/s ratios?

13 Critical v/s for a Road The sum of critical v/s for a road will be the higher of the left turn plus opposing through v/s ratios =0.309 Critical! =0.308 New

14 Lead or Lag Critical Movements Given lead/lag phase pattern Find highest v/s ratio per phase Phases: NBLT & SBLT: or 0.119, use SBTR & NBTR: , use EB & WB: or 0.196, use Sum of critical v/s ratios = 0.524

15 Split Phasing Given split phase pattern Find highest v/s ratio per phase Phases: SBTR & SBLT: or 0.119, use NBLT & NBTR: , use EB & WB: or 0.196, use Sum of critical v/s ratios = Higher than leading or lagging lefts (0.524), not as efficient Note: Split phasing is generally not efficient unless through and left turn v/s ratios are similar Split phasing creates pedestrian phase challenges

16 Critical v/s for Overlaps Add v/s for left and opposing thru OR = OR = The NBLT+SBTH is critical (0.309)

17 Overlap Phasing Try to combine LT/TH movements Compare v/s for conflicting LT & TH (opposing) OR Phases: NBTH & SBLT: =0.308 SBTH & NBLT: =0.309, critical EB & WB: or 0.196, use 0.196, critical Sum of critical v/s ratios = Lowest v/s is lower than leading left (0.524), overlap is most efficient phasing

18 Lead/Lag Phasing Given selected phase pattern Set through v/s = LT v/s, move unused through v/s to overlap Find highest v/s ratio per phase Phases: SBTR & SBLT: SBTR & NBTR: or 0.089, use NBLT & NBTR: EB & WB: or 0.196, use Sum of critical v/s ratios = Same as overlap (this is a different type of overlap)

19 Overlap vs Lead/Lag Sum of critical v/s same for overlap and lead/lag Patterns use same approach Differences: Lead/lag associated with fixed time Overlap associated with actuated

20 How Do I Know Which Movements to Overlap? Both the high left and thru v/s must be in same direction NBLT + SBTR = = SBLT = = SBTR = = Put extra time in thru mvmt =0.020 that could have been moved but better to leave in thru Sum of critical v/s ratios = = 0.309

21 Case that Can t Be Overlapped NBLT + SBTR = = Swapped left v/s used in previous example NBLT = = SBTR = = Overlapped movements conflict with each other Cannot be overlapped

22 Final Overlap Phasing Phase 1 Phase 1a Phase 2 Phase 3 Overlap Sum of critical v/s ratios: = 0.505

23 Determine Cycle Length Webster s Method C 1.5 L 5 1 (v / s ) CRIT.MVMT Where, C = Cycle Length, sec L = Lost time, sec

24 What is Lost Time? Time at beginning and end of phase that is lost because of start-up delays and yellow interval Lost Time Generally, lost time = clearance intervals (y+ar) No lost time for overlapped phases Saturation Flow

25 Lost Time and Headway Area is total lost time at beginning of phase

26 Total Lost Time Phase 1 Phase 1a Phase 2 Phase 3 Assuming y+ar = 4 sec/phase Lost Times: Phase 1 = 4 sec Phase 1a = 0 sec Phase 2 = 4 secs Phase 3 = 4 secs Total lost time = 12 secs

27 Determine Cycle Length Total lost time = 3 ph 4sec/ph = 12 sec Lost time = 12 sec/phase (1.5)(12) 5 C sec 50 sec Some round cycle lengths to nearest 5 sec increment

28 Alternative Method for Determining Cycle Length Highway Capacity Manual C X c X where: LX c v a c s i ci Critical saturationratio 0.8 to 0.95

29 Determine Green Time as Fraction of Cycle Clearance time per phase Y+ar = 4 sec (y+ar)/c = 4/50 = Total yellow+all red fraction Y+AR = 4 sec/ph 3 ph = 12 sec (Y+AR)/C = 0.080/ph 3 ph = Or 12/50 = Total green fraction G = C-(Y+AR) = = 38 sec G/C = 38/50 = Check: = 0.760, OK

30 Graphical Representation Distribution of Phases Distribution of Final Times Distribution of Total Green and Total Yellow+AR Ph 3 Ph 2 Ph 1 OL g 3 y+ar 3 g 1 g 2 y+ar 1 OL Y+AR (Y) 0.24 G 0.76 y+ar 2 Total Green time is what we distribute in proportion to v/s ratios New

31 gi C g i Distribute Green Time to v s v s v s i Movements critical movements v s i critical movements G C G g i = effective green time for movement i v/s i = volume to saturation flow ratio for movement i v/s = sum of volume to saturation flow ratios for all movements C = cycle length G = total green time in cycle

32 g Time Calculations Phase sec,use 7.5 sec Phase1a sec,use1.5 sec Phase sec,use14.2 sec Phase sec,use14.8 sec Total Time 38.0 sec

33 Preliminary Timing Phase Diagram (g + y) =50 NEMA Ring Diagram (g + y) =50 =50

34 Check Minimum Green Requirements Green intervals cannot be shorter that minimum green requirement Minimum green varies by phase Dictated by policy / practice Check g to meet minimums Typically 15 seconds for major street through 7 seconds for lefts and minor street through Minimum g may be based on pedestrian clearance interval Pedestrian clearance time can include yellow and all-red intervals

35 Preliminary Timing NEMA Ring Diagram (g + y&ar) =50 = Check cycle length Ring 1: =50.0 Ring 2: =50.0 Do g times meet minimums? 7.5>7: Yes 14.2<15: No, increase to 15 sec 15.6>15: Yes 14.9>7: Yes =50

36 Final Timing NEMA Ring Diagram (g + y&ar) =50.8 = =50.8 If C rounded to 51 or 55, where to add time? Typically added to major through