The vehicle dynamics at intersection are governed by the intersection model described in [Road Intersection Model in SUMO, Krajzewicz et al]. Of course, the model in the latest revision may deviate somewhat due to the ongoing evolution of the model. This page gives an overview over the configuration options governing the intersection model.

Internal links#

The most important configuration aspect is whether the dynamics within an intersection shall be modelled or not. This is configured while building the network or alternatively when launching the simulation. When simulating without internal links, Vehicles are still subject to right-of-way rules (waiting at traffic lights and at minor roads) but they will appear instantly on the other side of the intersection after passing the stop line. They cannot block the intersection, wait within the intersection for left turns nor collide on the intersection.

When set to true, the network will not contain internal lanes, (lanes within an intersection). Instead, vehicles will jump across the intersection. To avoid a systematic shortening of route lengths, the length of all edges is set artificially to the length between junction centers without changing their appearance. When set to false (the default), The network will contain lanes within intersections on which vehicles will drive just as on normal lanes, albeit subject to some blocking constraints.


The presence of internal links in a network can be changed by reprocessing the network with netconvert -s -o --no-internal-links BOOL or by setting the corresponding option in the netedit option dialog (F10)

When set to true, lanes within intersections are ignored. This option is not needed when the network does not contain them in the first place. Note, that route lengths will be wrong when ignoring internal lanes in a network that includes them.

Speed when approaching the intersection#

Vehicles that approach an intersection distinguish 3 cases

  • neither crossing nor merging takes place or the vehicle has right-of-way (linkState M,G or O ): vehicles drive across the intersection without slowdown
  • a zipper merge must be performed (only at junction of type type="zipper", linkState Z). Speed depends directly on neighboring traffic (braking only takes place on demand)
  • interference with other streams of traffic is possible (merging/crossing) and vehicle does not have right of way: vehicle prepares to break until reaching a specific distance (configurable for each location with connection attribute visibilityDistance). At that distance, the vehicle determines whether higher-prioritized traffic presents a conflict and then either brakes or accelerates across the intersection.

In each of these cases, the vehicle may also slow down if another vehicle is still on the intersection and in the way.

Waiting before the Intersection#

Vehicles wait with a context dependent offset before the end of their lane where it meets the intersection. The exact position depends on several factors explained below.

Lane Shape#

Usually the lane ends exactly where the intersection shape starts. One way to customize the exact position for each lane is to edit the intersection shape. An alternative method is to set a custom endpoint for an edge.

Another possibility is the set the endOffset attribute for either the <edge> or <lane> element. This will shorten the edge/lane by a set amount.


using endOffset currently does not affect junction shape so there will be a visible gap between the end of the lane and the start of the junction shape. The shape of internal lanes will be lengthened to compensate for this gap.


Vehicles from a minor road wait exactly at the end of the lane in order to minimize the distance they have to cover when a suitable gap in traffic is found. Vehicles waiting at at a traffic light wait with an offset of 1m ahead of the lane end.

Vehicle Class#

Each edge or lane may carry a stopOffset child element to specify an additional stopping offset for vehicles of certain classes. This can be used to define a bike box. The exact syntax is explained at stopOffset.

Waiting Within the Intersection#

In some situations, drivers are permitted to wait within the intersection for a gap in foe traffic. Typical cases are left-turning vehicles from the prioritized direction that wait for oncoming traffic or right-turning vehicles that have to wait for straight-going pedestrians. In SUMO this is modeled with internal junctions. An internal junction splits the internal lane in two parts. Upon reaching the intersection, drivers may enter the intersection and drive up to the end of the first part despite approaching foe vehicles. They are not permitted to enter the intersection if it is blocked by vehicles already on the intersection or if there is a red traffic light.

When building networks, netconvert automatically recognizes common cases for waiting within the intersection and creates internal junctions as necessary. At intersections which are controlled by a traffic light, internal junctions are built for every stream that has a green minor phase (dark green). Thus, loading custom traffic light plans during network building may influence the building of internal junctions.

Since version 0.25.0 the user also has the option for customizing the presence and location of internal junctions.

Speed while passing the Intersection#

All the rules for vehicle speed are applicable but there is an additional feature to model speed reductions while turning. Since SUMO version 1.0, lane speeds within the intersection are reduced in accordance to their turning radius. This means, that vehicles will reduce their speed while turning according to their individual speedFactor.

The speedLimit is computed as

speedLimit = sqrt(radius * factor)

where factor is set by --junctions.limit-turn-speed <FLOAT> (default 5.5) If factor is set to 0, the turning limit is not applied.

Additionally, the computed speed will never be higher than the average speed of source and destination lane for that particular connection.

The speed can be overruled by setting the connection speed explicitly in the XML input or via netedit.

Junction Blocking#

In most jurisdictions, drivers are forbidden to enter an intersection if the outbound road is jammed to prevent them from blocking the intersection. By default, vehicles in SUMO try to prevent blocking intersections. This is accomplished by the no-block-heuristic which prevents them from driving onto the intersection if they are likely to become stuck there. This heuristic may be disabled by modifying the simulation network ahead of the simulation.


The no-block-heuristic does not apply to geometry-like nodes (nodes without intersecting streams of traffic)


Junction blocking cannot occur when using option --no-internal-links

netconvert options for allowing drivers to drive onto an intersections#

Junction model attributes for allowing drivers to drive onto an intersection#

By setting the junction model parameter jmIgnoreKeepClearTime in a <vType>-definition, drivers of that type will ignore the no-block-heuristic after their accumulated waiting time exceeds the parameter value (in seconds).

Ignoring blocking vehicles after some time#

When vehicles in SUMO are unable to move for some time they will be teleported to resolve dead-lock. If this is not desired, sumo-option --ignore-junction-blocker <TIME> may be used to ignore vehicles which are blocking the intersection on an intersecting lane after the specified time. This can be used to model the real-life behavior of eventually finding a way around the offending vehicle that is blocking the intersection.

Customizing Behavior at Junctions#


The speed of a vehicle may be set using the speed command. When combined with speed mode command various safety-related rules can be disabled. Among them are flags for

  • overriding save speed in regard to leader vehicles, or vehicles already on the intersection
  • ignoring right-of-way rules
  • ignore red lights

Driver Parameters#

The behavior at intersections can be configured with junction model parameters. The following aspects can be affected

  • aggressiveness when merging from a unprioritized road (impatience)
  • distance-keeping to pedestrians
  • driving onto an intersection despite of #Junction_Blocking rules
  • ignoring red lights
    • speed when ignoring red lights