---
title: Automatic Routing
---

# Introduction

Routing dynamically in the running simulation may be adequate in the
following situations:

- there is not enough time / computing power to wait for the [dynamic
  user equilibrium](Dynamic_User_Assignment.md#iterative_assignment_dynamic_user_equilibrium)
- changes to the net occur while the simulation is running
- vehicles need to adapt their route while running

In this case [sumo](../sumo.md) may be used directly for routing
with either routes or trip files (or a mix) as input.

This routing approach works by giving some or all vehicles the
capability to re-compute their route periodically. This routing takes
into account the current and recent state of traffic in the network and
thus adapts to jams and other changes in the network.

The options listed below allow configuring which of the vehicles shall
be equipped, how often rerouting decisions shall be made and how the
estimation of travel times is computed from current and recent
knowledge. 

Further information on [routing is available](../Simulation/Routing.md).

# Options

The options related to this routing are:

| Option                                          | default        | Description                                                                               |
|-------------------------------------------------|----------------|-------------------------------------------------------------------------------------------|
| **--device.rerouting.probability** {{DT_FLOAT}} | -1             | The probability for a vehicle to have a routing device (-1 is equivalent to 0 here)       |
| **--device.rerouting.explicit** {{DT_STR}}      |                | Assign a device to named vehicles                                                         |
| **--device.rerouting.deterministic**            | false          | The devices are set deterministic using a fraction of 1000 (with the defined **probability**) |
| **--device.rerouting.period** {{DT_STR}}        | 0              | The period with which the vehicle shall be rerouted                                       |
| **--device.rerouting.pre-period** {{DT_STR}}    | 60             | The rerouting period before insertion/depart                                              |
| **--device.rerouting.adaptation-interval** {{DT_INT}} | 1              | The interval for updating the edge weights.                                               |
| **--device.rerouting.adaptation-weight** {{DT_FLOAT}} | 0.0 (disabled) | The weight of prior edge weights for exponential averaging from [0, 1].                   |
| **--device.rerouting.adaptation-steps** {{DT_INT}}    | 180            | The number of adaptation steps for averaging (enable for values > 0).                     |
| **--device.rerouting.with-taz**                  | false          | Use [traffic assignment zones (TAZ/districts)](../Demand/Importing_O/D_Matrices.md#describing_the_taz) as routing end points                        |
| **--device.rerouting.init-with-loaded-weights**  | false          | Use option **--weight-files** for initializing the edge weights at simulation start           |
| **--device.rerouting.mode**  | 0          | configure handling of [temporary obstructions](../Simulation/Routing.md#handling_of_temporary_obstructions)    |



Please note that if a vehicle gets a routing device only rerouting *before insertion* is active by default.
In order to activate periodic rerouting en route set **--device.rerouting.period**.

# Edge weights

If the routing is enabled for any vehicle, the average travel times in
the net are collected for all edges. If a vehicle needs to be routed
(either because it gets inserted or because a repeated route choice was
enabled via the ".period" option) it chooses the fastest route to its
destination edge (or district) according to the present edge weights
(travel speeds and hence travel times). The update of the edge weights
does not simply overwrite the old value but is computed as a weighted
average as described below. Since updating the weights of all edges in
each simulation step means a major slowdown for the simulation this
interval may be altered using the ".adaptation-interval" option.

!!! note
    these weights are also used when using [TraCI](../TraCI/index.md) functions *vehicle.rerouteTraveltime* and *vehicle.changeTarget* if the routing mode was set to *ROUTING_MODE_AGGREGATED* using *vehicle.setRoutingMode*. Likewise, the function *simulation.findRoute* can be switched to using these weights by setting the argument routingMode=*ROUTING_MODE_AGGREGATED*.

## Adapting by exponential average

By setting the option **--device.rerouting.adaptation-weight** {{DT_FLOAT}} the travel speed of each edge is computed as

```
FLOAT * priorValue + (1 - FLOAT) * currentMeanSpeed
```

This averaging takes place with the period set by **--device.rerouting.adaptation-interval**.

## Adapting by moving average

By setting the option **--device.rerouting.adaptation-steps** {{DT_INT}} the travel speed of each edge is computed as the
average of the INT latest meanSpeed values. This averaging takes place
with the period set by **--device.rerouting.adaptation-interval**.

!!! note
    speed values in a time interval going back as far as the multiple of **adaptation-interval** and **adaptation-steps** take part in the averaging.

## Investigating edge weights

To understand the evolution of edge weights during the simulation it can
be helpful to look at the values in detail:

- used road coloring *by routing device assumed speed* in
  [sumo-gui](../sumo-gui.md#changing_the_appearancevisualisation_of_the_simulation)
- use option **--device.rerouting.output** {{DT_FILE}} to get the raw numbers
- use [TraCI function
  ''vehicle.getParameter("device.rerouting.edge:EDGEID")](../TraCI/Vehicle_Value_Retrieval.md#supported_device_parameters)
  to retrieve the averaged traveltime value for individual edges

# Incomplete trips and flows

All vehicles which are created using a trip as input (or a flow with
"from" and "to" attributes) get automatically routed at insertion
without the need to instantiate the device for them explicitly. Whenever
an error occurs on routing because no route can be found which includes
all mandatory edges ("from", "to", and all stop edges in the correct
order) and is connected (also respecting the vehicle class permissions)
this is a fatal error and stops the simulation. This can be switched off
by using **--ignore-route-errors** which will leave the route untouched in the error case. If the
vehicle did not have a route yet (because it was defined using a trip)
and cannot find one and **--ignore-route-errors** is used, it will not be inserted.

Alternatively to using attributes "from" and "to" it is also possible to
use "fromTaz" and "toTaz", if a [district file is
loaded](../Demand/Importing_O/D_Matrices.md#describing_the_taz).

# Alternative Declaration via Parameters

Another way for defining the set of vehicles that are equipped with a
rerouting device is via [generic
parameters](../Definition_of_Vehicles,_Vehicle_Types,_and_Routes.md#devices).

The following parameters are supported as child elements of a `<vType>`, `<vehicle>`, `<trip>` or `<flow>`:

- device.rerouting.period
- device.rerouting.pre-period
- device.rerouting.probability
- device.rerouting.deterministic
- device.rerouting.mode
- has.rerouting.device


# TraCI

The device can be accessed using the TraCI function
*vehicle.getParameter* and *vehicle.setParameter*:

**getParameter**

- device.rerouting.period (returns individual rerouting period in
  seconds)
- device.rerouting.edge:EDGE_ID (returns assumed travel time for
  rerouting where EDGE_ID is the id of a network edge)
- has.rerouting.device (returns "true" or "false" depending on whether
  the device is equipped)

**setParameter**

- device.rerouting.period (double literal, set rerouting period in
  seconds)
- device.rerouting.edge:EDGE_ID (double literal, set assumed travel
  time for rerouting for all vehicles (where EDGE_ID is the id if a
  network edge). This value is overwritten at the next update interval
  (--device.rerouting.adaptation-interval).
- has.rerouting.device ("true"): can be used to dynamically enable
  automatic rerouting

# Improvement Thresholds

By default, periodic rerouting will change a vehicles route if the new route is percieved to reduce the travelTime compared to the old route by any amount.
Let `oldTime` be the estimated travelTime of the currentRoute and `newTime` the estimated time for the fastest route:

- When setting option **--device.rerouting.threshold.factor FLOAT**, the route will be changed only if `oldTime / newTime > FLOAT`.
- When setting option **--device.rerouting.threshold.constant FLOAT**, the route will be changed only if `oldTime - newTime > FLOAT`.

When both options are set, both conditions must be met.

# Randomness

When setting the option **--weights.random-factor** {{DT_FLOAT}}, edge weights for routing are dynamically
distorted by a random factor drawn uniformly from \[1,{{DT_FLOAT}}\]. This
randomization is performed every time an edge weight is used so the same
vehicle could select a different route when rerouted on different time
steps. The route generated this way may be longer than the fastest route
by the given factor (in the worst case). This randomness is a good way
to achieve a reasonable route distribution in grid networks an in other
cases where many similar route alternatives are available.

# Parallelization

Routing in the simulation can be parallelized. This is particular
helpful when periodic rerouting is active as it may consume large
amounts of processing time. To perform routing in parallel, set the
option **--device.rerouting.threads** {{DT_INT}} to the number of parallel threads desired.

It may also help to set the option **--device.rerouting.synchronize** which ensures that all vehicles are
rerouted at the same time (by default the rerouting period is aligned
with vehicle insertion time).
