Annex 4 Guidelines for Priority Signaling for an HCBRT System

Terminology on Priority Signaling

The following terminology is used to describe HCBRT signal priority:

  • Passive priority: Traffic control strategies are adjusted to suit the bus schedule and other operational characteristics (such as average dwell time at stations) along the BRT route and possibly also those on cross-streets, using a combination of fixed-time and schedule-based plans. The signals are primarily synchronized for the flow of transit vehicles. In practice, passive priority is rarely used except at bus only streets or intersections close to bus origin points where bus schedules are more likely to be adhered to, whilst the remainder of the corridor has an active priority system. Generally, passive priority is cheaper to implement, but has less potential to improve bus operation.
  • Active priority: Each bus is detected on its approach to an intersection and the signals are then changed, usually by extending the current BRT green phase (green extension) or truncating the red time (early green) before the BRT phase. If active priority is implemented by the latter, passengers in the bus sometimes do not notice that active priority is operating at the intersection, as the bus may still have to wait for some time (albeit shortened) instead of immediately passing through. Green extension can be more effective than early green because it saves red time and is also more visible to passengers. Turning buses are detected from the previous intersection so a phase can be called for the turn. Active systems are usually associated with real time control strategies and schedule or headway-based control strategies.
  • Unconditional priority: Priority is given whenever a bus detector requests it from signals.
  • Conditional priority: Includes variables that may limit priority given, such as bus occupancy, queue length and time since last priority was granted. This form of priority is sometimes called priority with constraints.
  • Direct priority: The method of providing priority to a particular bus when it has reached or is very close to an intersection.
  • Indirect priority: The method of providing priority by clearing up congested intersections ahead of time so buses can eventually travel through with little or no congestion.
  • Phase Insertion: A special green phase is injected into the normal phase sequence while all other phases are stopped.
  • Green extension: The green time for the BRT movement is extended when the transit vehicle approaches an intersection.
  • Red truncation: If the BRT vehicle arrives at an intersection during the beginning or middle of a red phase, the red phase is truncated (by shortening the green time for or omitting the preceding phases), facilitating an early return to the BRT movement phase, subject to control limitations, e.g., minimum phase duration and pedestrian phase operations should not be violated by any type of signal priority procedures, to allow the bus to go through as soon as practicable. Generally, green extension and red truncation are available together within the control environment, but not applied at the same time.
  • Compensation: Green time is allocated to a non-priority phase that was truncated/ omitted to make up for lost time.
  • Synchronization: Timing groups of traffic signals along an arterial to provide for the smooth movement of traffic with minimal stops. Synchronization can be in one direction only or in both directions. When a signal is operated in synchronization mode, it is constrained by the local cycle and its “zero point”, which defines the relationship between adjacent intersections. At this point during the cycle, the coordinated phase must be in operation. Thus phases can only be adjusted around this constraint. Essentially, the phases are adjusted to provide transit priority while remaining coordinated beyond the current cycle, such that the signal can always return to its synchronization at this point. Consequently, any adjustment is limited by the synchronization. The zero point, also known as the yield point or reference phase, may be shifted for one cycle but should allow the signal to remain in relative synchronization between intersections.
  • Phase suppression: One or more non-priority phases with low demand may be omitted from the normal phase sequence.
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Control Strategies

There are generally four kinds of control strategy for implementing BRT signal priority, in increasing degree of sophistication:

  • Fixed-time: Fixed-time control applies a pre-designed signal control plan based on the historic average conditions in an area. Fixed-time control does not receive constantly (real-time) updated information; the best control (based on historic observations) scheme is applied to the area regardless of current actual conditions.
  • Real-time: Rely on constantly updated (real-time) information to make decisions regarding priority. A real-time signal control model is more flexible to current changing conditions, hence is generally more effective than a fixed-time control model.
  • Schedule-based: Priority is awarded based on the bus schedule. If the bus is running late (beyond a pre-decided threshold; e.g. 3 minutes) then it receives priority through intersections. Schedule-based control is aimed at increasing reliability.
  • Headway-based: Priority is awarded based on the headway between buses. In this control strategy the aim is to avoid buses bunching up. The traffic control system tracks the location of buses (each with a unique identification) along the BRT corridor and compares every few seconds, the passage times to the defined table of headways for that road segment, which resides in traffic servers. Priority is only granted to buses whose headway since the previous bus is greater than the defined headway. The headway management system also drives the bus arrival sign at BRT stations. Headway-based control is more effective in reducing passenger waiting times at bus stations and improving reliability when services operate with a high frequency.

The relationship between BRT priority concepts and control strategies is shown in the table below:

Table 1 Relationship between BRT Priority Concepts and Control Strategies

Priority Concepts Control Strategy
Passive Priority
  • Fixed-time
Active Priority
  • Real-time (unconditional or conditional)
  • Schedule-based
  • Headway-based
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Effect of BRT Priority on Other Traffic

The effect of BRT priority on non-transit traffic tends to vary depending on the level of congestion and direction of travel. Although no system has the perfect priority strategy for non-transit traffic, some elements of a priority system to alleviate impacts on non-transit traffic operations are discussed below.

Real-time control strategy is preferable and has far more potential to reduce delays to non-transit traffic. If the system uses a software program to calculate signal timing plans and then selects which plan best suits the current traffic conditions, the UTCC can adjust the signal timings in the event of increased congestion, accidents, or other unforeseen events.

Constraints on maximum and minimum green times can reduce delays to non-transit vehicles at the intersection level. For example, the software could place maximum limits on any green extension. Even if the intersection is saturated with buses requesting priority, the system would only grant this maximum amount of time.

Other measures to reduce impact on non-transit traffic under congested traffic conditions whilst helping HCBRT vehicles include:

  • Compensation to cross road traffic after the BRT vehicle has cleared the intersection. Although real-time signal control strategies can automatically adjust the allocation of green times according to traffic demand/congestion levels, the control plan usually has little flexibility during peak periods. Positive intervention to compensate green time to cross-road traffic is required if this measure is to be implemented during such periods.
  • Give priority to HCBRT vehicles before the morning traffic peak even if priority cannot be implemented during the peak itself. This would assist the dispatch of BRT vehicles from depots and increase the number of BRT vehicles available to meet the morning peak hour demand.
  • Disallow or limit the priority given to BRT vehicles at congested intersections.
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