Detection issues in CitySample

I would like to ask a question about the detection in CitySample. Just like in the picture, when I control the character to enter a vehicle parked on the roadside and drive to the middle of the road, the AI-controlled vehicle will stop regardless of whether my character is in the car or not. But if I drag a sandbox vehicle into the level and control the character to enter the vehicle and park it in the middle of the road, I observe that other AI vehicles will stop only when the character is in the car. Once my character exits the vehicle, the AI-controlled vehicle continues driving and crashes into the towed vehicle. I would like to ask what is the detection mechanism here? If I directly modify the default pawn class to be this sandbox car, it seems to be invisible in the game and no other vehicles can detect me. Please tell me why this is and how should I solve it?

Thanks in advance! Really looking forward to hearing from you!