Introduction to Transport

Introduction to Transport Lecture 4:Signal Timing

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 • number of phases, 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 LT 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

Right turn protection • Permitted phasing should be provided for the following conditions: • The RT demand flow within the peak hour falls within the “permitted” portion of the figure below, and • The sight distance for RT vehicles not restricted, and • Fewer than 8 RT accidents have occurred within the last 3 years at any one approach with permitted-only phasing.

Right turn protection • Fully protected phasing is recommended when any TWO of the following criteria are met: • LT flow rate is greater than 320 veh/h • Opposing flow rate is greater than 1,100 veh/h • Opposing speed limit is greater than or equal to 45 mph • There are two or more LT lanes

Right turn protection • Fully protected phasing is also recommended when any ONE of the following criteria are met: • There are 3 opposing lanes, and the opposing speed is 45 mph or greater • RT flow rate is greater than 320 vph, and the percent of heavy vehicles exceeds 2.5% • The opposing flow rate exceeds 1100 vph, and the percent of RT exceeds 2.5% • 7 or more RT accidents have occurred within 3 years under compound phasing (must have a compound phasing now) • The average stopped delay to RT traffic is acceptable for fully protected phasing, and the engineer judges that additional RT accidents would occur under the compound phasing option.

Splitting the exclusive RT phase • With different traffic volumes in two opposing direction, same amount of protected RT phasing can be inefficient This RT phase may be inefficient if only one direction has a lot of RTs.

Splitting the exclusive RT phase • One direction is released while the other is held • This type of phasing has a overlapping phase • This plan is a 3 phase signal Leading green for EB RT Overlapping phase Lagging green for WB RT compound phase if RTs are permitted.

Leading or lagging green phase • A leading green may be used without a lagging green (T-intersection or one-way street) • The correct phase plan i.e. the number of phases can be found out from the ring diagram • No. of phases is critical as that specifies the set of lost times to be calculated • A compound phasing can be created by allowing permitted RTs in the overlapping phase. This is specially useful when road geometry do not allow for an exclusive RT lane. • Leading and lagging phase controllers are no longer manufactured by NEMA.

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

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 all-red interval. This part is the length of the yellow interval.

Pedestrian requirements • Safety dictates some minimum assured crossing times for pedestrians. This in turn impacts vehicular traffic • Minimum pedestrian crossing times (pedestrian green) • Diagram for Gp calculation,

Vehicle vs. pedestrian requirements • The minimum pedestrian green imposes a constraint on the minimum red for the movement(s) being crossed.

alternatively, the minimum green for the movement(s) moving in the same direction as the pedestrian is: • As the traffic increases, normally, vehicular demand requires a larger minimum cycle length. In low traffic demand, pedestrian traffic dictates the min cycle length

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, Vcousing 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

Note that the procedure recommended does not yield a unique result, nor does it allow for development of an initial signal timing entirely by algorithm. The traffic engineer must apply some judgment in the process, and should be aware of applicable local or regional policies that may affect the process. The resulting signal timing is only the INITIAL TIMING. • 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. Finding actual green interval values (Gi): Gi = gi – Yi + tLI

Introduction to Transport