Transportation Engineering Lecture 9: Traffic Signal

Signal Timing Designs Development of a phase plan and sequence Timing of yellow and all-red intervals for each phase. Determination of cycle length. Green time distribution. Checking pedestrian crossing requirements. Safety (conflict avoidance) and the quality of service are the most important factors in designing signals. The process is not exact, nor is there often a single “right” design and timing for a traffic control signal.

Development of a phase plan No analytical approach available The issue of right turn protection is important number of phases, safety, efficiency, delay Higher the number of phases, bigger is the lost time in an hour offsetting the increase of saturation flow due to protected movements The phase plan must be consistent with intersection geometry, lane use assignments, volume speeds and pedestrian crossing requirements Should be consistent with standards of HCM & MUTCD Must be consistent/practical

Phase diagram and ring diagram

Phase diagram and ring diagram A “ring” of a controller generally controls one set of signal faces. Thus, while a phase involving two opposing through movements would be shown in one block of a phase diagram, each movement would be separately shown in a ring diagram.

Right turn protection If vRT (Volume of right turning vehicles)<100 vph protection is rarely used vRT ≥ 250 to 300 vph protection is almost always used Between these bounds, the provision of RT protection must consider opposing volumes and number of lanes, accident experience, system signal constraints, etc. Two general guidelines: vRT ≥ 200 vph vRT *(vO / No) ≥ 50,000 (Cross product rule) [vO: Opposing flow volume; NO: Opposing no. of lanes] Protected+permitted phase is used when full protection leads to undesirably long cycle length

Intergreen/Change/Clearance Period Intergreen consists of either yellow or (yellow + all-red) periods. It alerts motorists regarding the change from green to red light. When yellow light appears, drivers at a distance longer than their stopping distance will be able to stop comfortably; those who are nearer to the stop line than their safe stopping distance will accelerate and clear the intersection. For the case of stopping: xc is the minimum comfortable stopping distance. Any shorter, it would be uncomfortable, unsafe, or impossible.

Intergreen/Change/Clearance Period The intergreen time is, (x+W+L)/v x: safe stopping distance L: vehicle length v: Vehicle legal speed

Example: Dilemma Zone

Intergreen/Change/Clearance Period For a particular site, the relative magnitudes of the two critical distances xc, xo determine whether a vehicle can or cannot safely execute either or both manoeuvres. (fig. a-c) In the fig. a, xc≤xo , the driver can execute manoeuvre no matter where the vehicle is located at the onset of yellow. where xc> xo (fig. c) , a dilemma zone of (xc- xo) exists: a vehicle approaching the intersection at the legal speed limit can execute neither stop nor go safely, legally, and comfortably if it happens to be located within the dilemma zone at the onset of yellow.

Dilemma zone When a vehicle is in dilemma zone, cannot stop or cannot finish crossing The dilemma zone can be eliminated either by changing the speed limit which in certain locations may be undesirable or by selecting an appropriate minimum duration for the yellow signal phase that results in xc=xo This part is the length of the yellow interval. This part is the length of the all-red interval.

Example: Dilemma Zone A driver travelling at the speed limit of 30 mph was cited for crossing an intersection on red. He claimed that he was innocent because the duration of the amber display was improper and, consequently, a dilemma zone existed at that location. Using the following data, determine whether the driver’s claim was correct. Amber duration = 4.5 s Perception reaction time = 1.5 s Comfortable deceleration = 10 ft/s2 Car length = 15 ft Intersection width = 50 ft (1 mile = 5280 feet) There is a dilemma zone of 29.8 ft. Otherwise, it is unknown whether the vehicle was within the dilemma zone at the onset of amber of whether the driver was speeding cannot be proven.

Pedestrian requirements Safety dictates some minimum assured crossing times for pedestrians. This in turn impacts vehicular traffic Minimum pedestrian crossing times (pedestrian green) In case of metric system, walking speed is 4 km/hr The minimum pedestrian green imposes a constraint on the minimum red for the movement(s) being crossed.

Signal Timing Designs Development of a phase plan and sequence (check for right-turning protection) Convert all left-turning and right-turning volumes to through car equivalents Determination of cycle length. Timing of yellow and all-red intervals for each phase. Green time distribution. Checking pedestrian crossing requirements. Safety (conflict avoidance) and the quality of service are the most important factors in designing signals. The process is not exact, nor is there often a single “right” design and timing for a traffic control signal.

The methodology for establishing an initial signal timing is as follows: Develop a reasonable signal phase plan in accordance with the principles discussed so far. DO NOT include any compound phasing in the preliminary signal timing. Consider a protected right-turn phase for any right-turning movement, has a right-turning volume in excess of 200 vph, has a cross-product of the right-turn volume (in vph) and the opposing through volume per lane (in vphpl) in excess of 50,000. Other criteria based on local policies may be applied, and several phase plans may be tested.

Convert all left-turning and right-turning volumes to through car equivalents (tcu's) using Tables 1 and 2.

Establish a reasonable phase plan using the principles discussed so far. Determine the actual sum of critical lane volumes, Vco using this plan. Use volumes in tcu's for this purpose. Check the sum of critical lane volumes in tcu's for reasonableness. Make any adjustments necessary. Using following equation, determine the desirable cycle length based on a desired vlc (0.85-0.90) ratio and the PHF

Timing of yellow and all-red intervals for each phase Once the cycle length is determined, the available effective green time in the cycle must be divided (split) among the various signal phases in proportion to Vci/Vc. Timing of yellow and all-red intervals for each phase Check for Pedestrian crossing Finding actual green interval values (Gi): Gi = gi – Yi + tLI

Example: Signal Timing Recommend an appropriate signal timing for the intersection shown in the figure. The PHF is 0.92, and moderate numbers of pedestrians are present. The intersection should operate at 0.90 of its capacity during the worst 15-minutes of the peak hour, for which volumes are shown in the figure.

Solution The problem is designed for right hand drive. The solution is done accordingly. STEP 1 The first consideration is whether or not protected left-turn phases are needed for any approach.

STEP 2 All volumes should now be converted to equivalent "through car units" or tcu's.

In Phase A, the NB and SB LT movements are given protection In Phase A, the NB and SB LT movements are given protection. Since each of these movements operates out of a separate lane, the per lane volume is equal to the total volume for each movement. The larger of 2 volumes, or 263 tcus/lane/hr, is the critical volume for this phase. In Phase B, TH and RT movements from NB and SB approaches have the green. Both of these combined movements have 2 lanes each. For the NB approach, 944/2 = 472; for the SB approach, 1,031/2 = 516. The critical volume for this phase is SB volume. In Phase C, 4 sets of movements are given the green. The EB and WB LT movements have exclusive lanes; their per lane volume is equal to the movement volume. The EB and WB TH and RT movements share 2-lane approaches. For EB approach, 702/2 = 351 tcu/lane/hr; for the WB approach, 566/2 = 283 tcu/lane/hr. The critical volume for this phase is the highest of the 4 volumes moving during the phase (WB LT movement 375 tcu/lane/hr). The sum of critical lane volumes for this signal phases, therefore, 263 + 516 + 375 = 1155 tcu/hr

STEP 3 At this point, a suitable phase plan must be developed, and the critical lane volumes for each phase must be identified. A ring diagram showing the proposed phase plan, together with lane volumes moving in each phase is shown. The lane volumes given in the diagram are on per lane basis. The critical lane volumes are underlined by red.

STEP 4 The desired cycle length can now be computed, The min. cycle time, 65.7 sec. Assuming that a pre-timed signal is in use, use 70s as the cycle length, as timing dials are commonly available in 5- second increments.

STEP 5 The allocation of available effective green time in proportion to the critical lane volumes for each phase. The effective green time is the cycle time minus the lost time in the cycle, or 70­(3)(3)= 61 seconds.

STEP 6 These effective green would have to be converted to actual green. This would involve determining appropriate values for the yellow and all-red intervals in each and the relationship: g = G + Y - tL and Y = y + r Considering 12-ft lanes in each direction, Intersection width = 60 ft Comfortable deceleration = 10 ft/s2 Car length = 15 ft Ymin = 4.9s (considering 30mph speed limit) Hence, actual green time, G = g - Y + tL GA = 15.8s; GB = 29.2s; GC = 21.7s;

STEP 7 Pedestrian crossing times should also be checked to insure that these green times allow for safe crossing, if a deficiency for pedestrians is noted, the cycle would normally be increased, and the green time reallocated in the same proportion as this solution. Min. pedestrian crossing time, =5.5(moderate) + 60/4 = 20.5s Phase A, 15.8+4.9 = 20.7s>20.5s Hence, safety requirements are met.