Algodoo

Now make a scene that shows a standard traffic light (here in the U.S.) with Green, Yellow, and Red lights. When the light is Green, the Sim runs. When Red, the Sim stops. When Yellow, the Sim runs at slow speed. The lights switch being ON automatically, just like they do at most intersections. Can you do that?

EDIT: BTW, this scene you made doesn't "stop the simulation from running", but rather it "prevents the simulation from running". It's a subtle but notable difference.

I only know how to make the simulation start or stop, I don't know how to change the sim speed.

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JohnG -- sim.timefactor = 1.0 (1.0 is the default speed. A value higher or lower will change how fast the scene runs. It has the same effect as the manual slider above the Run button).