This section describes priority junctions (two-way yield-controlled intersections). Topics include:

• Overview of priority junctions

• Keywords

• Geometric priority junctions: Keywords

• Geometric priority junctions: Example

• Saturation-flow priority junctions: Keywords

• Saturation-flow priority junctions: Example

Overview of priority junctions

A priority-junction control is an unsignalized intersection between a major road and a minor road. It has different names in the U.K. (where "priority junctions" are common) and the U.S. (where "two- way yield-controlled intersections" are rare). In the U.S., the minor road approach (at a "T"), or approaches (at a crossroads) have "yield" signs. In the U.K., the yield signs are optional. Traffic on the minor need not stop before entering the intersection, but must give way to any major road traffic.

CUBE Voyager offers two model variants, both of which are calibrated to U.K. traffic conditions: a full empirical model based on junction geometry and a simplified models in which the user provides saturation flows which have been estimated or measured externally to CUBE Voyager.

The geometric model is developed based on the TRRL report 941, and the saturation model is developed based on a similar model utilized in the signal saturation flow model. Both models can be used to analyze the capacity, delay, queue and LOS of roundabout intersections. The low flow delays are the inverse of the capacity for both priority intersections.

When a single-lane major road is being modeled, very large capacities may be reported. This occurs when a movement, which is unconstrained, shares a lane. The capacity is chosen to give the correct ratio of volume to capacity (as calculated for the constrained movement).

References

• Kimber, R. M., Mcdonald, M., Hounsell, N. B. (1986). The prediction of saturation flows for single road junctions controlled by traffic signals. Transport and Road Research Laboratory Report RR 67.

• Semmens M. C. (1980). PICADY: a computer program to model capacities, queues and delays at major/minor junctions. Transport and Road Research Laboratory Report RL 941.

Keywords

This section describes keywords for priority junctions:

• GEOMETRIC

• SINGLELANE

GEOMETRIC

The keyword "Geometric=y", invokes modeling of layout geometry at a priority junction.

The default, "GapAcceptance=n", at a priority junction is for the user to supply saturation flows directly.

The logical-valued keyword is used to indicate that an approach consists of a single lane. It is applicable to many junction types:

SINGLELANE

• Signals

  Geometric Data - SINGLELANE may be coded. Saturation Flows - SINGLELANE may be coded.

• All-way stop-controlled intersection

  SINGLELANE may not be coded. code NUMBEROFLANES = 1.

• Two-way stop-controlled intersection

  SINGLELANE may be coded for minor road. Use FOURLANEMAJOR to describe major road.

• Priority intersection (two-way yield-controlled intersection)

  Geometric Data - SINGLELANE may be coded.

• Roundabout

  Saturation Flows - SINGLELANE may be coded.

  Empirical - SINGLELANE may not be coded.

  Gap Acceptance - SINGLELANE may not be coded.

Coding SINGLELANE = Y for an approach precludes the use of EXCLUSIVELANES, CANSHARERIGHT, or CANSHARELEFT on that approach.

At two-way stop-controlled intersections and priority junctions, a minor arm, which does not have SINGLELANE=Y explicitly coded, has two lanes.

Geometric priority junctions: Keywords

This section describes the keywords for geometric priority junctions:

• CENTRALRESERVATIONWIDTH

• CROSSING2ENTRY

• CROSSING2EXIT

• CROSSINGLENGTH

• FREEFLOWCAP

• MAJORROADWIDTH

• VISIBILITY

• WIDTH

CENTRALRESERVATIONWIDTH

CENTRALRESERVATIONWIDTH is a real-valued keyword (only) applicable to geometrically modeled priority junctions. Its value is the width, in meters or feet, of the curbed central reservation in the major road. If there is no central reservation, or the central reservation is composed of ghost islands (that is, road markings), then CENTRALRESERVATIONWIDTH should be zero (the default). Otherwise its value should be in the range from 2.2 to 5.

In the diagram below, the CENTRALRESERVATIONWIDTH is given by 1⁄2(W5 + W6).

CROSSING2ENTRY

This keyword specifies the position of a zebra crossing on an approach to a roundabout or a minor approach priority junction. Its value is the number of vehicles that may queue at the junction without impeding pedestrians who wish to cross. At roundabouts and single lane minor approaches to priority junctions, a single integer value should be supplied. However, on two-lane minor approaches to priority junctions, separate values should be supplied for each of the two lanes.

CROSSING2EXIT

This integer-valued keyword specifies the position of a zebra crossing on an exit from a roundabout or priority junction. Its value is the number of vehicles that may queue at the crossing without impeding vehicles using other exits from the junction.

CROSSINGLENGTH

This real-valued keyword allows the specification of the length of a zebra crossing that crosses an approach to a roundabout or priority junction. If it is absent then no crossing will be modeled.

FREEFLOWCAP

By default, the unmodelled movements from the major approaches of a priority junction have infinite capacity. However, this may result in too large a capacity for the approach as a whole when it must share a lane with a modelled turn. You can supply a capacity for these movements by coding FreeFlowCap=value and SingleLane=t.

MAJORROADWIDTH

MAJORROADWIDTH is only applicable to geometrically modeled priority junctions, where it is required. The presence or absence of this keyword allows the two methodologies for priority junction modeling to be distinguished.

MAJORROADWIDTH is a real valued keyword whose value is the width, in meters or feet, of the major road lane (excluding any central reservation or ghost islands) near the intersection. It is illustrated in the four diagrams below. In each case the major road width is given by the expression 1⁄2(W1 + W2 + W3 + W4).

VISIBILITY

This real-valued keyword allows the visibilities, in meters or feet, at a geometrically coded priority junction (two-way yield-controlled intersection) to be entered.

Visibilities may be coded for the minor road left and right movements and for the opposed movement from the major road. Minor road visibilities should be measured from a point ten meters before the give-way line and 1.05 meters above the road surface. The major road visibility is measured from the mid-point of the turning lane (again at height 1.05m), along the major road, towards the road center line.

WIDTH

This real-valued keyword is used to specify the lane widths (in meters or feet) for a minor road at a priority junction (two-way yield-controlled intersection).

Code widths for left and right movements on minor roads and for the opposed movement from the major road. The width for the major road’s opposed movement is the width of the lane from which vehicles on the major road turn. Coding techniques for minor roads vary with the number of lanes.

To describe a two-lane minor road:

• Do not code SINGLELANE=T.

• Code the width of the left lane in the left movement.

• Code the width of the right lane in the right movement.

To describe a one-lane minor road:

• Code SINGLELANE=T.

• Code the width of the single lane in the left movement, or the right movement, but not both.

The coded widths should be the lane’s average width during the final 25 meters of the approach before the give-way line.

Geometric priority junctions: Example

This example illustrates the coding of a three-arm priority junction, with input geometric data.

Junction,
   Node = 250,
   Type = Priority,
   Approach1 = 455,
   MajorRoadWidth = 10.9,
   CentralReservation = 1.2,
   Approach = 455,
      Movement = Right,
         Width = 2.5,
         Visibility = 170.0,
   Approach = 249,
      Movement = Left,
         Width = 2.05,
         Visibility = 130.0,
      Movement = Right,
         Width = 2.05,
         Visibility = 125.0,
   Approach = 251,
      Movement = Right,
         Width = 2.9,
         Visibility = 150.0,
   Approach = 255,
      SingleLane = y,
      Movement = Left,
         Visibility = 100.0,
      Movement = Right,
         Width = 3.1,
         Visibility = 127.0

Saturation-flow priority junctions: Keywords

This section describes the keywords for saturation-flow priority junctions:

• CANSHARELEFT

• CANSHARERIGHT

• EXCLUSIVELANES

• SATFLOWPERLANE

CANSHARELEFT

This keyword specifies that there is a shared lane to the left of the exclusive lanes for this movement (that is, the movement can share with the movement to its left). Note that this keyword does not mean "can share with left turners" unless either the movement is THROUGH or the movement is on the minor leg of a three-arm junction.

If a movement has CANSHARELEFT=T coded then there must be a movement to this movement’s left and the movement to this movement’s left must have CANSHARERIGHT=T coded.

If SINGLELANE=T then CANSHARELEFT should not be coded.

CANSHARERIGHT

This keyword specifies that there is a shared lane to the right of the exclusive lanes for this movement (that is, the movement can share with the movement to its right). Note that this keyword does not mean "can share with right turners" unless either the movement is THROUGH or the movement is on the minor leg of a three arm junction.

If a movement has CANSHARERIGHT=T coded, then there must be a movement to this movement’s right, and the movement to this movement’s right must have CANSHARELEFT=T coded.

If SINGLELANE=T then CANSHARERIGHT should not be coded.

EXCLUSIVELANES

This integer-valued keyword gives the number of lanes, on the specified approach, which are reserved for the exclusive use of vehicles making the specified movement.

If SingleLane=t then ExclusiveLanes should not be coded.

SATFLOWPERLANE

This real-valued keyword allows the specification of saturation flows in pcu (vehicles) per hour per lane.

Saturation flows at signals are the flows that would be observed if the movement had a continuous green all other movements had no flow and no green.

Saturation flow at a priority junction (two-way yield-controlled intersection) is defined similarly, except that no signal aspects are involved.

Saturation-flow priority junctions: Example

The example describes a priority junction using with a three-lane major and two-lane minor using default saturation flows per lane.

Junction,
   Node = 229,
   Approach1 = 396,
   Type = Priority,
   Approach = 228,
     Movement = Left,
       ExclusiveLanes = 1,
       CanShareRight = y,
     Movement = Through
       ExclusiveLanes = 1,
       CanShareLeft = y,
       CanShareRight = y,
     Movement = Right,
       ExclusiveLanes = 1,
       CanShareLeft = y,
   Approach = 396,
     Movement = Left,
       ExclusiveLanes = 1,
       CanShareRight = y,
     Movement = Through,
       ExclusiveLanes = 1,
       CanShareLeft = y,
       CanShareRight = y,
     Movement = Right,
       ExclusiveLanes = 1,
       CanShareLeft = y,
   Approach = 315,
     Movement = Left,
       ExclusiveLanes = 1,
       CanShareRight = y,
     Movement = Through,
       CanShareLeft = y,
       CanShareRight = y,
     Movement = Right,
       ExclusiveLanes = 1,
       CanShareLeft = y,
   Approach = 409,
     Movement = Left,
       ExclusiveLanes = 1,
       CanShareRight = y,
     Movement = Through,
       CanShareLeft = y,
       CanShareRight = y,
     Movement = Right,
       ExclusiveLanes = 1,
       CanShareLeft = y