Tutorial

1. Overview

The goal of the Trolley Mod is to create a simple, effective means of setting up trolley problems in the CARLA simulator. These can be used to record human decisions when confronted with ethical dilemmas on the road, which later could teach the AI of a self-driving car what is the socially accepted decision when confronted with a similar situation.

In the case of the mod, the trolley problems are set up such that there are two victims or group of victims on each side of the road in CARLA’s Unreal Engine 4 (UE4) maps. When playing the Trolley Mod, the car will accelerate automatically, and the user can only swerve left or right. Thus, the human playing the simulation has no choice but to hit something. Upon conclusion of the collision, the choices that the driver made are recorded into a text file for later analysis. That way, raw data will exist for uses such as teaching an AI how to make decisions, compiling sociological information, etc. From this point forward, each individual Trolley problem will be called a scenario. A sequence of scenarios is what constitutes a Trolley Mod simulation. The following text will demonstrate an example implementation of a Trolley Mod simulation to give the user an idea on where to begin.

2. How to set up a Trolley Mod simulation

First, it is important to understand how the Trolley Mod spawns object models into a level in the Unreal Engine 4. It takes an input text file named TestVictims.txt, which contains a list of tuples; the syntax for these tuples are discussed in greater detail later. Each tuple specifies a victim—such as a pedestrian, a car, or some other object—and provides any other ancillary data that describes the object. This text is parsed so the there is a reference to a UE4 Actor class in memory. The remaining text is converted into whatever data type is appropriate—such as integers—to represent the ancillary data. Other functions then take this parsed information and spawns the desired Actor into the level opened in the UE4 editor. For more information on navigating the editor, view this link.

There are three types of objects that can be spawned automatically with the Trolley Mod: pedestrians, vehicles, and prop objects. These correspond to the Walker, the CarlaWheeledVehicle, and StaticActor classes respectively. The first two are native to the CARLA plugin, while StaticActor is a product of the Trolley Mod. The mod adjusts these classes so each class instance stores new information pertaining to it. All three classes share two variables:

1. TestNum: an integer that tracks what scenario is being run.
2. GroupMemberNames: a delineated string that lumps together the names of all victims in the scenario that the class instance is a part of. It is formatted in the form of Victim1^Victim2^Victim3...

Walker Actors have additional information:

1. Age: an integer.
2. Gender: a string.
3. GroupNumber: an integer that tracks on the map if a Walker is part of a group. If it equals 0 or isn’t included in an input string, the Walker is considered solitary. Otherwise, all Walkers that share the same nonzero GroupNumber is considered to be part of the same group. For example, if three Walkers on the map share GroupNumber = 1, then all three of those Walkers are considered part of the same group. An important note: proximity has nothing to do with whether or not the script interprets a Walker as part of a group—only the GroupNumber.
4. GroupSize: an integer that simply records the size of the group the Walker resides in. If it isn’t part of a group, the size is 1.
5. SpecialTrait: a UE4 Name that stores a user-specified trait that makes the Walker unique, such as being pregnant, disabled, etc. It is stored using the Tags array native to all UE4 Actors; this page is a good reference for basic information of Tags. It can be left blank if desired.

The input text file should be in the following format: TestNumber|Victim_1|Victim_2

Note that there are no spaces whatsoever in the text. For each victim specifically, the following format is used:

1. Pedestrians (Walker classes): ClassName,Gender,Age,GroupNumber,SpecialTrait
   • If the Walker has no special trait: ClassName,Gender,Age
   • If the Walker has a special trait: ClassName,Gender,Age,0,SpecialTrait
   • To spawn a Walker group, chain them together in the string using “^” like so: ClassName,Gender,Age,GroupNumber,SpecialTrait^ClassName,Gender,Age,GroupNumber,SpecialTrait. There is no limit to the number of Walkers that can be spawned in a cluster using this method. However, to ensure all of these Walkers are treated as a group, make sure they share the same GroupNumber, otherwise the output data will be affected!
2. Vehicles (CarlaWheeledVehicle classes): ClassName
3. Objects (StaticActor class): ClassName,StaticMesh
   • ClassName is "StaticActor" for all cases.
   • StaticMesh affects the kind of object that will be spawned in. To elaborate, the StaticActor class is a child of the AStaticMeshActor class, which contains a UStaticMesh component. StaticMesh specifies the appearance and physics that the StaticActor will gain. For example, to spawn in a traffic cone the input text may be StaticActor,Prop_ConstructionCone. Or, if a trash can is needed, StaticActor,Prop_Trashcan. See the “StaticMeshes list” page for the compilation of currently stored static meshes.