Network Structure

Main Page > Technical Documentation > Network Structure

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
 * These are placed at various positions within a lane and are tied into signalized and ramp metering control logic. Detectors allow one to set points along the segment to measure specific traffic flow parameters, such as speed, volume, etc.

Node Properties

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.

Entry Node Inputs
 * Flow Rate (veh/h)
 * Fleet Type Percentages
 * Driver Type Percentages

Common Link Properties

Upstream/Downstream Node ID A link has a beginning (upstream) and ending (downstream) node. Link ID values are created by concatenating the upstream and downstream node ID values. For example, Link ID ‘12’ corresponds to a link that has upstream node ID ‘1’ and downstream node ID ‘2’. The upstream and downstream node IDs are set automatically in the various network creation user interface (UI) screens. However, those values can be overridden by the user in the 'Custom' network creation module.

Length Freeway and Two-Lane Highway modules: For tangent segments, the link length is entered in this cell. For curved links, the link length will be based on the curve radius and central angle. Consequently, the length cell for a curved link will be colored gray, indicating that it cannot be edited directly. User input of length is restricted to integer values; however, the program may replace entered values with decimal values as a consequence of curved links and/or coordinate point adjustments.

Custom network module: The length is automatically calculated for both tangent and curved links. It is based on the entered upstream and downstream node coordinate values, as well as the control point coordinate values for curved links.

Orientation Angle This input corresponds to a compass angle based on the direction of travel (see Figure below). For example, the segment angle would be set as follows for the given travel directions: In the Freeway and Two-Lane Highway modules, the orientation angle only needs to be specified for the first link. Subsequent link orientation angles are calculated by the program and are a function of the previous link orientation angle and any specified horizontal curvature. In the Custom network creation module, the orientation angle is calculated by the program, based on the entered node coordinate positions.
 * 0o for perfectly horizontal east-to-west travel
 * 180o for perfectly horizontal west-to-east travel
 * 90o for perfectly vertical south-to-north travel
 * 270o for perfectly vertical north-to-south travel.

Horizontal Curvature A link can either be straight (i.e., tangent) or curved. If it is a curved link, additional inputs are required, specifically:

Control Point X/Y Coordinates Horizontal curves are represented as a 3-point (quadratic) Bezier curve. The control point is a point in between the link start and end points that affects the amount of curvature of the link. In the freeway and two-lane highway network creation modules, these values are set to provide as close a match as possible to the alignment of the curve per the circular curve inputs of radius and central angle. In the Custom network creation module, the control point inputs are used in lieu of curve radius and central angle inputs.

Turning Direction This property specifies the general turning direction of the curve (right or left) relative to the direction of travel.

Curve Radius Horizontal curve radius is entered in units of feet.

Central Angle Horizontal curve central angle is entered in degrees.

Superelevation Horizontal curve superelevation is entered in percent.

Grade The percent incline or decline of the segment. This is measured as the elevation change over the length of the segment divided by the segment length, multiplied by 100.

Free-Flow Speed For tangent links, the free-flow speed must be explicitly specified. For curved links, the free-flow speed can be explicitly specified or it can be estimated. In the latter case, the free-flow speed is estimated according to the model discussed here. Units of entry are ft/s.

Downstream Vehicle Turning Percentages Currently, the assignment of a vehicle's turning movement (left, through, or right) at the end of its current travel link is done at the moment it exits its previous travel link. This turning movement assignment is performed by drawing a uniform random number (between 0 and 1) and comparing to the specified turning percentages. The specified turning movement percentages can be applied to all vehicle fleet types equally, or can be varied by fleet type. The latter condition is useful, for example, in situations where one of the downstream connecting links connects to a route that prohibits large trucks.

Upstream/downstream connecting link ID's
 * The upstream and downstream links that each link connects to must be specified
 * Entry links have no upstream connecting links--ID values should be set to zero
 * Exit links have no downstream connecting links--ID values should be set to zero
 * An example of how to code these connections is shown below

Upstream/downstream connecting lane ID's
 * The upstream and downstream lanes that each lane connects to must be specified
 * Lanes that have no upstream connecting lane (e.g., a left-turn bay) should have a connecting ID value of zero
 * Lanes that have no downstream connecting lane (e.g., an acceleration) should have a connecting ID value of zero
 * An example of how to code these connections is shown below

Two-lane highway-only Link Properties

Passing in Oncoming Lane The options for passing in the oncoming lane are:
 * Allowed ‘Direction 1’
 * Allowed ‘Direction 2’
 * Allowed Both Directions
 * Not Allowed

Passing/Climbing Lane Direction The options for passing/climbing lane direction are:
 * None
 * Direction 1
 * Direction 2
 * Both Directions

Passing/Climbing Lane Rule If a passing/climbing lane has been specified, one of the following driving rules can be specified:
 * Slow Vehicles Move Over
 * Fast Vehicles Move Over

Lane Data Lane Types
 * Regular
 * Turnbay Left
 * Turnbay Right
 * Acceleration
 * Deceleration
 * Auxiliary
 * Median

Lane Turn Movements
 * Through Only
 * Right Only
 * Left Only
 * Through + Right
 * Through + Left
 * Left + Right
 * Through + Right + Left

Upstream/Downstream Connecting Link/Lane IDs Example



Nodes

Connecting Link IDs

Connecting Lane IDs