Friction Triggers Appear to Alter The Tire Friction of *All* cars on the map

[craigiedon]

CARLA version: 0.9.13 Platform/OS: Ubuntu 21.0

Problem: After creating a friction trigger with 0.0 friction (according to the code in this part of the docs), I spawned two vehicles with BasicAgent and set them to drive to next way point. When the first car enters the trigger zone, both cars spin out of control. I expected that only the car entering the trigger zone would have its wheel friction changed, but it appears to change the friction for every vehicle, even those currently outside of the trigger zone.

I confirmed this was what was happening by running:

for actor in actor_list:
  wheel_phys = actor.get_physics_control().wheels[0]
  print(f"Tire Friction: {wheel_phys.tire_friction}")

At every tick. Which produces:

Time Step 35
Tire Friction: 3.5
Tire Friction: 3.5

Time Step 36
Tire Friction: 0.0
Tire Friction: 0.0

(The first car enters the friction trigger after time-step 35, at which point the second car is far away from the friction trigger)