SUMO allows loading and saving simulation state since version 0.18.0.


Saving at fixed times#

To save the state at specified times during the simulation add the option --save-state.times

sumo --save-state.times TIME1,TIME2,TIME3

By default the state will be written to files named <PREFIX>_<TIME><SUFFIX> where <PREFIX> can be set via option --save-state.prefix (default state), <TIME> is the simulation time and <SUFFIX> is either one of .xml.gz or .xml as controlled by the option --save-state.suffix (default .xml.gz).


The file extension '.xml.gz' indicates gzip-compressed xml which can be used directly by all Sumo applications and can also be unzipped with standard file compression tools

When using the option --save-state.times, output files can also be specified by using the option --save-state.files

sumo --save-state.files FILE1,FILE2,FILE3

to set the file names explicitly. In particular if a plain text xml-file is desired as output (instead of the compressed format .xml.gz) the file names should be given (with suffix .xml).

Saving periodically#

Alternatively, the option --save-state.period <TIME> can be used to save states periodically. The options -save-state.prefix and --save-state.suffix apply as described above.

The option --save-state.period.keep N can be used to remove all but the last N state files.

Saving manually#

sumo-gui allows to save state files using the menu option Simulation -> Save

Saving via TraCI#

The function traci.simulation.saveState(FILENAME) can be used to trigger saving from a script.

Further options#

  • --save-state.rng: Save random number generator states
  • --save-state.transportables: Save person and container states
  • --save-state.constraints: Save rail signal constraints
  • --save-state.precision: Set decimal precision when saving states


The option --load-state <FILE> is used to initialize the simulation with the given state file. It is recommended to start the simulation with the same input files that were used when saving the state file and set option --begin to the time when the state was saved. Additional files (i.e. more vehicles) can be added without problems.

When the option --load-state.offset is set, all times loaded from the simulation state will be shifted by the given amount. This can be used to reset the simulation second to 0 after midnight and still have the loaded vehicles function properly.

The option --load-state.remove-vehicles ID1,ID2,... allows removing vehicles from a loaded state. This opens up the possibility for re-positioning selected vehicles by inserting them again with a .rou.xml file.

Loading via TraCI#

There are two ways for loading state via TraCI:

  • traci.load(['-n', '', '--load-state', FILENAME]): reloads the whole simulation (which includes reloading the network)
  • traci.simulation.loadState(FILENAME) : clears out all vehicles and persons from the network and then loads the state. (reloading the network is avoided)

The second function faster (or much faster) than the first, depending on the size of the network.


Vehicles that have not yet departed are not included in the simulation state and must be loaded from input files. When loading vehicles from an input file that are already present in the simulation state, these vehicles are automatically ignored and loading proceeds without errors.

Random number generators (RNG)#

By default, the state of the random number generators is not saved. Thus, simulations will behave differently from the original after being reloaded from a state.

When setting option --save-state.rng, the state of all random number generators will be included in the state and restored upon loading. Saving this extra state costs about 500Kb. If this overhead is too high it can be reduced by setting a lower value of --thread-rngs (default 64). The value should be no lower than the number of threads used for routing or simulation (--threads, --device.rerouting.threads**).

Know Issues#

  • flows cannot be loaded from a state file alone (the original route file must be loaded as well)
  • The internal state of the laneChangeModel is not saved
  • the internal state of the carFollowModel is not saved (not all models have this).

Older Versions of SUMO#

On versions before 0.18.0 a possible workaround is to run SUMO within a virtual machine (VM) as the state of a VM can be saved at any time. Tests with VirtualBox on Linux have shown that SUMO does not run significantly slower in a VM as directly on the host if enough hardware resources are assigned to the VM (for example if you run another program simultaneously to communicate to SUMO via TraCI you should assign enough CPU-cores to the VM).