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Basic Principles of Intersection Signalisation. Design Manual for Traffic Signals in Ireland.

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Design Manual for Traffic Signals in Ireland

- The “Design Manual for Roads and Bridges” (DMRB) was introduced in 1992 in England and Wales, and subsequently in Scotland and Northern Ireland. A modified version, the “National Roads Authority Design Manual for Roads and Bridges” (NRA DMRB) was formally introduced for use in Ireland from 2001. The DMRB provides a comprehensive manual system which accommodates, within a set of loose-leaf volumes, all current standards, advice notes and other published documents relating to the design, assessment and operation of trunk roads (including motorways) in the United Kingdom.
- The NRA DMRB takes the DMRB and adapts it for use on national roads in Ireland through a series of implementation documents. These take the form of NRA Addenda to the individual documents contained in the DMRB and, in some cases, complete replacement NRA Standards. At present the NRA DMRB only implements the design standards contained in Volumes 1,2,4,5,6,7,8 and part of 9 of the DMRB.

Four basic mechanisms

- Discharge headways, Saturation flow
- The critical lane and time budget concept
- Effects of right turning vehicles
- Delay

Discharge Headways

- The first discharge headway is the time between the initiation of the green indication and the rear wheels of the first vehicle to cross over the stop line.
- The Nth discharge headway (N>1) is the time between the rear wheels of the N-1 th and N th vehicles crossing over the stop line.
- The headway begins to level off with 4 or 5th vehicle.
- The level headway = saturation headway

Saturation flow rate

- In a given lane, if every vehicle consumes an average of h seconds of green time, and if the signal continues to be uninterruptedly green, then Svph could enter the intersection where S is the saturation flow rate (vehicles per hour of green time per lane) given by

Lost time

- Start-up lost time:At the beginning of each green indication as the first few cars in a standing queue experience start-up delays,
- e(i) = (actual headway-h) for vehicle I calculated for all vehicles with headway>h
- green time necessary to clear N vehicles,
- The change interval lost time: It is estimated by the amount of the change interval not used by vehicles; this is generally a portion of the yellow plus all-red intervals.
- The 1994 Highway Capacity Manual (HCM) adds the two lost times together to form one lost time and put it at the beginning of an interval. Default value = 3.0 seconds per phase

Effective green time & Capacity of Lane group

- Actual green time
- Yellow + all red time
- Total lost time
- The ratio of effective green time to cycle length is ‘green ratio’
- Saturation flow rate of a lane group is the theoretical capacity of the lane group if 100% green time was available.
- Actual capacity of a lane group,

Example

- A given movement at a signalised intersection receives a 27-second green time, and 3 seconds of yellow plus all red out of a 60 second cycle. If the saturation headway is 2.14 seconds/vehicle, the start-up lost time is 2 seconds/phase and the clearance lost time is 1 second/phase, what is the capacity of the movement per lane?

Critical Lane

- This concept is used for the allocation of the 3600 seconds in the hour to lost time and to productive movement.
- The amount of time required for each signal phase is determined by the most intensely used lane which is permitted to move during the phase. All other lane movement in the phase require less time than the critical lane. The timings of any signal phase is based on the flow and lost times of the critical lane.
- Each signal phase has one and only one critical lane.
- Capacity can be maximum sum of critical lane volumes that a signal can accommodate. The max. total volume that can be handled on all critical lanes for a given time budget (within an hour),

Capacity (using critical Lane volume)

- the effect of number of phases and cycle time on Vc
- Lost time remains constant through out
- (h= 2.15s, lost time = 3s/phase)

v/c ratio and PHF

- (volume-to-capacity) V/C ratio: In signal analysis, the v/c ratio is often referred to as the "degree of saturation". The v/c ratio is the ratio of the actual or projected demand flow rate (during the peak period, usually defined as 15 minutes) in a lane group, and the capacity of the lane group.
- When analyzing an existing facility under existing conditions, a measured flow rate is compared to an estimated capacity. To be meaningful, the existing flow rates should represent arrival flows, Often, however, it is easier to count departure flows; and this is what many intersection volume studies actually document. Where departure flows are used as the basis for a capacity analysis, a resulting v/c ratio> 1.00 cannot be accepted as accurate. If the departure count is correct, then the capacity of the lane group must be equal to or greater than the observed flow. When analysis "predicts" a v/c ratio in excess of 1.00, only one interpretation is possible: capacity has been underestimated.
- The PHF is used as a volume adjustment factor. This adjustment assumes that all movements peak during the same 15-minute time-period. It is a very conservative assumption.

Consideration of v/c ratio and PHF

The cycle length equation becomes,

Effects of right-turning vehicles

- Right turns can be made from a
- Shared lane operation
- Exclusive lane operation
- Traffic signals may allow permitted or protected right turn
- Right-turning vehicles look for a gap in the opposing traffic on a permitted turning movement, which is made through a conflicting pedestrian or an opposing vehicle flow.
- Right-turning vehicles consume more effective green time than through vehicles.

Effects of right-turning vehicles

- Through Car Equivalent
- depends on the opposing
- flows, and the number of opposing lanes

Example

- Example: consider an approach with 10% RT, two lanes, permitted RT phasing, a RT equivalency factor of 5, and an ideal saturation headway of 2 sec per veh. Determine
- the equivalent saturation headway for this case,
- the saturation flow rate for approach, and
- the adjustment factor for the sat. flow rate? (adj. flow rate / sat flow rate of TH vehicles)

Calculation of cycle length

- Min. cycle length,
- Considering desired v/c ratio,
- Considering peaking within hour,
- Desirable cycle length,

Performance measures

- Delay, Queuing, Stops
- Delay most directly affects drivers’ experience.

Performance measures

- Stopped Time Delay: time a vehicle stopped waiting to pass the intersection.
- Approach Delay:
- stopped time + acceleration + deceleration
- Travel Time Delay: (actual travel time-desired travel time)
- Time-in-queue Delay: Total time from joining a queue to passing the stop line

Webster’s Delay Model

- Webster’s uniform delay (UD) formula
- Red time,
- Height of the triangle,
- Area of the triangle, (UD)
- Average delay per vehicle,

Webster’s optimum cycle length

developed based on minimization of overall delay at the intersection.

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.

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
- When exclusive right-turn phases are used, the two opposing right-turns are given the same amount of green time. This can be inefficient where the turning volumes are different. In these cases, the exclusive turning phase is split into leading and/or lagging green phases.

Splitting the exclusive RT phase

- One direction is released while the other is held. The following type of phasing is called “overlapping phases”.
- The ring diagram is used to calculate the number of phases. The number is decided by counting the phases boundaries in the ring diagram.
- 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

- The possible options of using leading or lagging phases:
- 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. But this type of design is still in use.

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.
- The dilemma zone can be eliminated either by changing the speed limit or by selecting an appropriate minimum duration for the yellow signal phase that results in xc=xo

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,

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
- 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
- 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.

Capacity Analysis

- In capacity analysis the substantive results of signalized intersection analysis are realized for first time.
- 1. Determining v/s ratios: The volume or total flow rates adjusted for the PHF and lane distribution. The total saturation flow rates for each lane group, adjusted for eight types of prevailing conditions. The capacity analysis begins by computing the ratio of these results (v/s) for each lane group.
- 2. Determining critical lane groups and the sum of critical lane v/s ratios: Critical lane groups are determined by comparing adjusted per lane flows in each lane group using a ring diagram. Critical lane groups are identified by comparing v/s ratios. The approach and methodology is the same as done by comparing lane flows. Ring diagrams are used to identify overlapping portions of the signal phase. The Σ(v/s)ci is used to calculate min practical cycle time. (shown with example in next two slides)
- But at this stage, the signal timing has already been specified. Thus, there is an iterative process at work in which signal timing affects v/s ratios, while resulting v/s ratios can be used to affect signal timing.

Capacity Analysis

- The total length of phases A and B is controlled either by the left ring (the WB through movement), or by the right ring (the combination of the WB left turn and the EB through/right-turn movement).The v/s ratio for the WB through movement, 0.45, is compared to the sum of the v/s ratios for the WB left turn and the EB through/right-turn movements, 0.20 + 0.35 = 0.55. The right ring involves the highest sum of v/s ratios, and is the critical path. Phase C is discrete. The larger v/s ratio for the two lane groups in the phase determines which is critical. In this case, the NB right-turn lane group is critical, with a v/s ratio of 0.35.
- Thus, the critical lane groups are the WB LT, the EB TH/RT, and the NB RT. As there are three critical movements (there could have been only two in this case), this signalization involves three sets of lost times in each cycle.
- The sum of the critical lane group v/s ratios, Σ(v/s)ci may also be determined. In the sample case, the sum is 0.20 + 0.35 + 0.35 = 0.90. Hence, the signal timing must allocate 0.90 of real time as effective green. Conversely, only 1 - 0.90 = 0.10 of real time is available to allocate to lost times. If Σ(v/s)ci value is greater than 1.00, it is clear that the specified geometric design and signalization are inadequate to handle the specified demand flows.

Capacity Analysis

- 3. Determining lane group capacities and v/c ratios: Individual lane group capacities can be determined from saturation flow rates and v/c ratios can then be directly computed by dividing the lane group demand flow rate by the capacity. The critical v/c ratio for the intersection may also be found.
- A value of Xc> 1.00 indicates that the cycle length, phase plan, and geometry specified are inadequate to handle critical flows in the intersection. An increase in the cycle length, a more efficient phase plan, and/or addition of lanes to critical lane groups would be necessary to remedy the situation. If Xc<= 1.00, the specified cycle length, phase plan, and geometry are adequate. If Xc>= 1.00, and Xi for one or more lane groups is greater than 1.00, the situation can be remedied by reallocation of green time within the specified cycle length and phase plan.

Capacity Analysis

- 4. Modifying signal timing based on v/s ratios: The results of capacity analysis may indicate a need to adjust the cycle length and/or reallocate green time. While this can be done on a trial-and-error basis, v/s ratios can be used as well. The following equation can be used in this purpose.
- In general, green is allocated to provide for equal Xi in all critical lane groups. To accomplish this, each Xi for a critical lane group must be set equal to Xc It is also possible to set Xi to 1.00 (or close to 1.00) for minor movements, such as a protected right turn, assigning all excess green time to through movements. This policy almost always results in very high delays to the minor movements, however.
- The design process is iterative. If a reasonable signal timing methodology is used initially, specified signal timings should not produce terribly unreasonable results. Even where totally inadequate signal timing is initially proposed, the timing can be refined with capacity analysis, and the solution reworked. Once a reasonable timing is established, small trial-and-error changes using software should result in optimal timing without extraordinary effort.

LOS

- The measure of effectiveness for signalized intersection level of service is average individual stopped-time delay. A delay value is estimated for each lane group; results are then aggregated to obtain averages for each approach, and finally to obtain an average value for the intersection as a whole.

LOS

- In order of importance, the three variables affecting delay are cycle length, green time, and v/c ratio. All of the models, assume random arrivals. The variable having the largest impact on delay values is the quality of progression. This is accounted for using an adjustment factor applied to a delay estimate for random arrivals. The importance of this is that the v/c ratio has a relatively small effect on delay compared to other variables.
- In signalized intersection analysis, a delay of over 60 seconds/vehicle (LOS F) may occur on a lane group with a v/c ratio under 1.00, even significantly so. Thus, situations arise in which the LOS is F because of high delay, but there is no evidence of a breakdown in flow. High delays occur frequently when cycle lengths are long and the green time for the subject movement is short. Exclusive right-turn phases, for example, often involve short green intervals in a long cycle length. Even if capacity is sufficient, the delays involved could exceed 60 seconds/vehicle. For signalized intersections, therefore, LOS F does not necessarily imply the "failure" of a lane group (i.e., insufficient capacity for existing or projected demand).

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