Junctions introduce a node model for modeling junction phenomena in detail:
- Approach lanes
- Signals
- Maneuver conflicts
From each lane, only some turns are allowed. Thus, different lanes may have different forward stars.
Lanes have a storage capacity and vehicles queuing along one lane do not affect vehicles on other lanes unless the lane spills back.
In this sense, the lane length is relevant and determines the approach topology (see next image).
The advantage of this method is a more accurate representation of:
- Turns
- Vehicle travel times with respect to the final turn
- Signalized maneuvers (if available)
- The impact of spillback due to queues on the pocket lanes
The General Link Transmission Model (GLTM) models the network only by using unidirectional links.
TRE neglects multiple outgoing lanes of the same leg. In practice, each exit link has only one lane.
This because handling separate queues in distinct lanes, produces a marginal advantage in considering queuing on the out lanes on the final node of the link.
On the other side, entry lanes need to be distinguished; thus, distinct lanes are modeled as distinct links diverging from the leg link.
The lane length determines the point along the leg where the lane starts.
The next image shows a schematic representation of a junction. Full lines are link arcs, dashed lines are lane links.
Exit capacity computation
The capacity of approach lanes is set equal to the capacity of the running lanes along the link itself, i.e.:
Often, pocket lanes increase the number of approach lanes with respect to the number of running lanes of the link. This is typically used in urban networks to increase the storage capacity immediately upstream a traffic light.
Important: As a result, the exit capacity of a link can be greater than its entry capacity and then theoutflow of a road might be temporarily greater than its entry capacity, whenever the queues stored on the lanes are cleared at the same time.
The value of the exit capacity of a lane can be determined according to values of five elements:
- UseOutCapPrt
- OCAP is the exit capacity (see the table LTYP in (→ Assignment network)).
- CAPA is the entry capacity of a link.
- NLAN is the number of lanes of the link.
- numHeadLanes is the number (calculated from model data) of approach lanes at the junction at the head node of the link. If the head node does not have a defined junction geometry, you can assume numHeadLanes=NLAN.
Link OCAP can be set in this way:
- OCAP=numeric value (not null).
- If OCAP=null, the value of OCAP is set to CAPA, thus OCAP=CAPA.
The value of the exit capacity of the lane then depends on the boolean value of the parameter UseOutCapPrt.
Finally, the resulting exit capacity of a link is then the sum of the exit capacities of its lanes, i.e.: