6 Network Structure

6.1 Overview

A SwashSim network is composed of the following components:

  • Nodes (entry, connector, exit)

    • Entry
      • Vehicles enter the network at entry nodes
      • ID numbering: Entry node numbering starts at 701 and increments by 1 with each successive entry node added to the network
    • Exit
      • Vehicles exit the network at entry nodes
      • ID numbering: Exit node numbering starts at 801 and increments by 1 with each successive exit node added to the network
    • Ramp connector
      • Used to identify a link as an on- or off-ramp connecting to a freeway mainline segment
      • ID numbering: Ramp connector node numbering starts at 601 and increments by 1 with each successive ramp connector node added to the network
    • Connector
      • Connector nodes are used in all other link connection circumstances

  • Links (entry, connector, exit)

    • Each link is bounded by an upstream node and downstream node
    • ID numbering: Concatenation of upstream and downstream node ID’s, in direction of travel

  • The upstream and downstream links that each link connects to must be specified (entry links have no upstream connecting links and exit links have no downstream connecting links)

  • Lanes

    • ID numbering
      • From outside to inside (lane 1 is outer most lane)
    • Types
      • Regular, Left Turn Bay, Etc.

  • The upstream and downstream lanes that each lane connects to must be specified

  • Control Points

    • These are placed at various positions within a lane and are tied into signalized, unsignalized, and ramp metering control logic.

  • Detectors

    • Detectors allow one to set points along a link to measure specific traffic flow parameters, such as speed, volume, etc. Detectors are also used with actuated signal and ramp metering control.

6.2 Node Properties

6.2.1 X/Y Coordinates

Each coordinate has an X and Y position. The units are feet. However, the absolute coordinate values are irrelevant–only the relative coordinate positions between nodes are relevant to the internal SwashSim calculations. In the Freeway and Two-Lane Highway modules, these coordinate values are automatically created (as a function of link length, orientation, and curvature). In the Custom network creation module, these coordinate values must be explicitly specified.

6.2.2 Entry Node Inputs

  • Flow Rate (veh/h)
  • Fleet Type Percentages
  • Driver Type Percentages

6.5 Lane Data

6.5.0.1 Lane Types

  • Regular - A lane that always extends the full length of a link. Left, through, and right turn movements can be assigned to this lane type.
  • Turn Bay Left - A left turn bay can be set to a length shorter than the full length of the link. The turn bay lane will always terminate at the downstream end of link.
  • Turn Bay Right - A right turn bay can be set to a length shorter than the full length of the link. The turn bay lane will always terminate at the downstream end of link.
  • Acceleration - An acceleration lane can be set to a length shorter than the full length of the link. Such a lane will always start at the upstream end of link. A distinguishing feature of an acceleration lane is that when a vehicle enters such a lane, from an on-ramp for example, the vehicle will immediately be tagged as needing to make a Mandatory Lane Changing and will look to move into the adjacent lane as soon as it identifies a suitable gap. For a regular lane that does not connect to a lane on the downstream link, a vehicle may make a Discretionary Lane Changing into the adjacent lane well upstream of the end of the lane, but it will not get tagged to make a mandatory lane change until it gets close to the point where it would have to begin slowing to a stop to avoid running off the end of the lane if it is not able to move to the adjacent lane.
  • Deceleration - An deceleration lane can be set to a length shorter than the full length of the link. Such a lane will always terminate at the downstream end of link. Similar to an acceleration lane, a vehicle destined for a deceleration lane, to access an off-ramp for example, will look to move into the deceleration as soon as it identifies a suitable gap.
  • Auxiliary - An auxiliary lane is a continuous lane connecting a freeway on-ramp to an off-ramp, a configuration often seen in ‘weaving’ sections. This lane type essentially combines the characteristics of an acceleration lane and a deceleration lane.
  • Passing Lane - Used with the ‘Two-Lane Highway Passing Lane’ link type. This lane has additional properties that can be defined that affect the passing, or not-passing, behavior of vehicles within this link.
  • Passing Oncoming Lane - Used with the ‘Two-Lane Highway Basic’ link type. When specifying this link type, the lane used for passing in the oncoming direction will be automatically set up by the program.
  • Median - Currently, the only option for this ‘lane’ type is a raised median–it cannot be used for vehicle travel; thus, it is ignored in all vehicle movement logic. In the animation, it will have a green background color. A median lane cannot be used on entry or exit links.

It should be noted that certain vehicle movement behavior can be obtained with more than one lane type. In some cases, however, the lane type can help bypass certain portions of the vehicle movement logic and potentially reduce the simulation time. As one example, a regular lane could be set up to allow only left-turn movements. However, for a vehicle making a through movement to the next link, such a lane would still be checked as a possibility for discretionary lane changes. Once the allowed movement (i.e., left only) of the lane is checked, it would be ruled out for a lane change candidate. But the additional step of checking the allowed movements for the lane can be skipped altogether once the initial check of the lane type (i.e., left turn bay) is made.

Another note regarding lanes designated for only a turning movement–a turn bay can also be created with a separate link, adjacent to another link that allows other movements. In this situation, vehicles will only be able to enter the turn bay at the beginning the link. This would be appropriate for situations in which there is a median or another separation method, such as ‘candlestick’ style traffic cones or a low-profile curb, separating the turn bay from the adjacent lanes. If the turn bay lane type is part of a link with other lanes, vehicles will be able to move into the turn bay lane at any point along its full length.

6.5.0.2 Lane Turn Movements

  • Through Only
  • Right Only
  • Left Only
  • Through + Right
  • Through + Left
  • Left + Right
  • Through + Right + Left

6.5.1 Lane Objects

Detector and traffic control point objects can be placed within individual lanes of a link. Information about how to insert detectors and control points can be found in the following three chapters.

6.5.1.1 Detectors

Detectors are objects that can be placed in individual lanes of a link to record various measures of passing vehicles, as indicated in the Performance Measures. Detector objects are point detectors and generally measure vehicle operational attributes, such as speed, instantaneously, at the time the front edge of a vehicle crosses the leading (upstream) edge of a detector.

Detectors do have a length property so they can also mimic the operation of a physical inductance loop detector (ILD). The default length is 6 ft but can be changed as necessary. The length is used for the purpose of replicating the measurement of lane occupancy. See the Performance Measures chapter for more information about the lane occupancy measurement. The time a detector is occupied by a vehicle is counted from the time the front edge of a vehicle passes the leading (upstream) edge of the detector until the rear edge of the vehicle passes the downstream edge of the detector.

A single physical ILD cannot directly measure vehicle speed because the length of any given vehicle crossing the detector is not known. Thus, to measure speed, two detectors are placed closely together (far enough apart to avoid electrical interference between the two, but close enough such that any change in vehicle speed between the two detectors will be negligible. With this setup, the speed is calculated as the distance between the leading edges of the two detectors (usually about 6 feet) divided by the difference in actuation times of the respective detectors. This approach is not need in simulation since the simulation engine keeps track of all vehicle properties for every simulation time step.

Available detector functions:

  • Unassigned - use this function when the detector will only used for recording traffic stream measurements
  • Actuated Signal - use this function for registering vehicle calls with an actuated signal controller
  • Ramp Meter Mainline - use this function for coordinating freeway mainline traffic performance measurements with a traffic-responsive ramp metering control algorithm
  • Ramp Meter Presence - use this function for identifying when a vehicle is waiting at a ramp meter signal
  • Ramp Meter Passage - use this function for identifying when a vehicle has passed beyond a ramp meter signal
  • Ramp Meter Queue Intermediate - use this function for coordinating on-ramp intermediate queue spillback measurements with a traffic-responsive ramp metering control algorithm
  • Ramp Meter Queue Advance - use this function for coordinating on-ramp advance (i.e., start of ramp) queue spillback measurements with a traffic-responsive ramp metering control algorithm

6.5.1.2 Traffic Control Points

Traffic control points provide a mechanism to facilitate interaction between vehicles and traffic control devices. For example, they are utilized at a signalized intersection to “communicate” the displayed signal indication to a specific lane on an approach link. Control points are placed within lanes of a link. Generally, they are placed at the location where you want the vehicle to respond to it. In the case of a signalized intersection or approach controlled by a stop sign, this position would correspond to the stop bar location.

Available control point types:

  • Signalized
  • Ramp meter
  • Stop
  • Yield

6.6 Upstream/Downstream Connecting Link/Lane IDs Example

Sample Network

Figure 6.2: Sample Network

6.6.0.1 Nodes

Node ID

Coordinates

Connected Links

X

Y

701

100

100

7011

1

400

100

7011, 12

2

700

100

12

3

700

88

34

4

1000

88

34, 48

5

1000

100

59

6

1000

112

67

7

1100

200

67, 7801

8

1100

0

48, 8803

9

1200

100

59, 9802

801

1100

350

7801

802

1400

100

9802

803

1100

-150

8803

6.6.0.3 Connecting Lane IDs

Link ID

Lane ID

Upstream Connecting Lane ID

Downstream Connecting Lane ID

Left

Through

Right

Left

Through

Right

7011

1

0

0

0

0

1

0

2

0

0

0

0

2

0

12

1

0

1

0

0

1

0

2

0

2

0

0

2

0

34

1

0

1

0

0

1

1

2

0

2

0

0

2

0

3

0

0

0

1

0

0

67

1

0

1

0

0

1

0

59

1

0

1

0

0

1

0

2

0

2

0

0

2

0

48

1

0

3

0

0

1

0

7801

1

0

1

0

0

0

0

9802

1

0

1

0

0

0

0

2

0

2

0

0

0

0

8803

1

0

1

0

0

0

0