Intelligent Roadway Information System

Road Topology

For many of the features IRIS provides, a model of the road network topology is needed. This model consists of a set of roadway nodes, or r_nodes, which describe the connection points on the road network. For instance, each entrance ramp, exit ramp, station or intersection is represented by an r_node.

Road Topology


All r_nodes with the same road and direction-of-travel are grouped into corridors. The r_nodes within a corridor are automatically sorted based on their coördinates. Corridors are linked to each other by exit and entrance r_nodes. For example an exit r_node from Northbound road A to Eastbound road B would automatically link to an entrance onto Eastbound road B from Northbound road A.


Select View ➔ System ➔ Roads menu item

Before creating r_nodes, roads must be created. With edit mode ON, enter the road name into the text box at the bottom of the form and press the Create button.

Field Description
name Unique road name, including affixes such as "St" or "Rd"
abbrev Abbreviation (up to 6 characters), used for labeling detectors
road class Freeway, arterial etc. Affects style of map segments
direction Primary direction


Select the R_Nodes tab on the client interface to create or edit r_nodes.

In the Selected Roadway Corridor frame, there is a Corridor widget containing a list of all corridors which contain at least one r_node. When a corridor is selected, the r_node list will be populated with all r_nodes on the corridor. Also, any new r_nodes created will be on the selected corridor.

Press the Add button in the Selected Roadway Corridor frame to start creating a new r_node. The mouse cursor will change to a crosshair while over the map. Select a point and the new r_node will appear there, as well as in the selected corridor list.

Once an r_node is selected, any fields can be edited. Changing the roadway or direction will move the r_node to another corridor. Other attributes are nearest cross-street, cross-street direction, and notes.

R_Node Types

Node Type Description
station Detector station or shape node
entrance Entrance ramp onto corridor
exit Exit ramp off corridor
intersection At-grade intersection with another corridor
access Network access (for traveler information)
interchange Freeway interchange (for traveler information)

R_Node Transitions

An r_node transition describes how entrance or exit nodes connect with linked nodes.

Transition Description
none No transition — non-entrance or -exit r_nodes
loop Interchange loop ramp
leg Leg ramp
slipramp Transition ramp between parallel road corridors
CD Transition to or from a collector/distributor road
HOV Bypass ramp for high occupancy vehicles
common Common section joining two corridors
flyover Flyover ramp (bridge over mainline)


The meaning of lanes depends on the r_node type. For an entrance or exit ramp, this is the number of lanes entering or exiting the corridor. For other types, it is the number of lanes on the corridor.


Lane shift is the difference (number of lanes) between the corridor reference lane and the attach side of the r_node. This value is used for lane continuity analysis within a corridor. Each corridor has a reference lane, which corresponds to the leftmost lane along the entire corridor.

Station ID

An identifier for the detectors associated with a station r_node. It is displayed on the traffic layer. It must start with the letter S, followed by a unique number.

Speed Limit

This is the posted speed limit at the r_node. The units are miles per hour. For ramp r_nodes, it may be used to indicate the advisory speed limit on the ramp. The limit is used in travel time estimation.


A pickable r_node can be used for incident locators when deploying DMS from an incident.


When rendering the traffic layer, r_nodes are rendered in two passes. The second pass consists of r_nodes which have been marked above. This prevents overpasses from being drawn incorrectly.

Attach Side

This flag indicates whether the r_node is attached to the left side of the road. This can be used to create left entrance or exit ramps.


This flag allows an r_node to be deactivated. An inactive r_node will not be included in any corridor or drawn on the traffic layer.


The r_node Detectors tab allows vehicle detectors to be created and associated with an r_node.