Signal timing

1. Signal timing CTC-340

2. Key Elements • Development of safe and effective phase plan and sequence • Determination of vehicle signal needs • Timing of yellow and all red clearance intervals • Determination of the sum of the critical lane volumes • Determination of lost time per phase and per cycle • Determination of an appropriate cycle length • Allocation of effective green to various phases

3. Key Elements • Determination of ped needs • Determine the minimum time for ped green • Check to see if veh greens met minimum ped needs • If ped needs unmet -> adjust veh green times or install actuators to ensure ped safety • Usually not a single “right” answer

4. Treatment of LT • 2 guidelines for whether or not a particular LT requires protected or partially protected phase • vLT >= 200 vph • vLT *(vo/No) >= 50,000 • vo= opposing TH movement flow rate • No = number of opposing TH lanes • Protection rarely given when vLT < 2 veh/cycle • LT that sneak thru intersection after opposing traffic (sneakers – can average 2/cycle)

5. Treatment of LT • Usually provide LT lanes in both directions • Keeps opposing traffic in lines • When should permitted LT phasing be used • F18.1 • Unrestricted sight distance for LT veh • Fewer than 8 accidents involving LT veh in 3 years at any one approach

6. Treatment of LT • Guidance for choosing between protected and compound • Must meet 2 of 3 • LT flow rate > 320 vph • Opposing flow rate >1100 vph • 2 or more LT lanes • Fully protected recommended if any one of these is met • 3 opposing traffic lanes with opposing spd >45 mph • LT flow rate >320 vph & HV>2.5%

7. Treatment of LT • Opposing flow rate >1100 vph & HV > 2.5% • 7 or more LT accidents within 3 years under compound phasing • Average stopped delay to LT traffic is acceptable for fully protected phasing and engineer judges that additional accidents would occur under compound phasing • May need to ban LT traffic entirely (Arterial

8. General Considerations in Signal Phasing • 1 Phasing can be used to minimize accident risks by separating competing movements • Additional phases add delay and need to be weighed against gains in safety • Increasing phases increases lost time but also increases LT sat flow rates • All phasing must be implemented in accordance with MUTCD and have signage, markings, and signal hardware • Phase plan must be consistent with intersection geometry, lane use assignments, volumes and spds, and ped crossings

9. Phase and Ring Diagrams • F 18.2 • Phase diagram – shows all movements in a given phase within a single block of the diagram • Ring diagram – shows which movements are controlled by which ring of the controller movements • Rings generally control 1 set of signal faces

10. Phase and Ring Diagrams • F18.3 – simple 2 phase explain ring and phase diagrams • F18.4 Exclusive LT phasing • F18.5 Lead / lag LT green • F 18.6 Leading Green LT • F 18.7 8 phase Actuated • F 18.8 Exclusive Ped Phase

11. Right Turn Phasing • Protected RT only when high Ped # • Compound RT with protected LT on opposing streets • RTOR • Use unless 1 of following • Restricted sight distance fro RT • High speed conflicting TH traffic • High flow rate of conflicting TH traffic • High ped flow in crosswalk

12. Determining Veh signal req’t • Change Interval • MUTCD does not require Y and AR but does recommend them • MUTCD does prohibit Y to mark transition from R to G • Time for a vehicle 1 stopping distance away to enter the intersection under Y (front tires cross stop bar)

13. Determining Veh signal req’t • y = t + 1.47S85/(2a +(64.4*0.01G)) • t = reaction time (s) • S85= 85th percentile spd of approaching traffic or spd lmt • a = decel in ft/s^2 • G = grade, %

14. Determining Veh signal req’t • Clearance time • Provides sufficient time for veh that has just entered intersection to cross intersection and clear back bumper past curb line before G • ar = (w+L)/1.47S15 • S15= 15th percentile spd of approaching traffic or spd lmt • If some peds then • ar = max((w+L)/1.47S15, P/ 1.47S15) • If significant peds then • ar =(P+L)/ 1.47S15 • P = distance from departure stop line to far side of farthest conflicting crosswalk, ft

15. Determining Veh signal req’t • S85 = Save + 5 • S15 = Save - 5

16. Lost times • Start up lost time l1 = 2.0 s/phase • e = 2 s/phase • l2 = Y-e • Y = y+ar • tL = l1 +l2 • L = Sum(tLi)

17. Critical Volumes • Convert all volumes to equivalent thru vehicle units • T 18.1 , T18.2 show equivalents for LT ,RT traffic volumes • Use to find path thru intersection for each phase which has highest volume • Determine critical volume for each phase

18. Desired Cycle Length • Cdes = L/(1-(Vc/(1615*PHF*(v/c)))) • 1615 vphg = sat flow assuming typical conditions of lane width, HV, Grade, parking, peds, buses, area type, lane util • L = total lost time per cycle • (v/c) = v/c ratio desired for critical volumes • Want to be between 0.85 – 0.95

19. Splitting green • gTOT = C-L • gi = gTOT *(Vci/V) • Gi = gi -Yi +tLi • Do not do very short phases – confuse drivers

20. Ped Signal Req’ts • Gp = 3.2 +(L/Sp) +2.7*(Nped/WE) • For WE > 10 ft • Gp = 3.2 +(L/Sp) +0.27*(Nped) • For WE <= 10 ft • L = length of crosswalk ft • Sp = average walking speed ft/s • Npeds = # of peds crossing per phase in a single crosswalk • WE = width of crosswalk • 3.2 = min start up time for peds

21. Ped Signal Req’ts • WALKmin = 3.2 + 2.7(Nped/WE) • For WE > 10 ft • WALKmin = 3.2 + 0.27*Nped • For WE<= 10ft • Flashing Don’t walk = L/Sp • Gp <= G + Y • Either G +Y must be greater or a ped actuator must be provided