If they are cold they would, otherwise it's a very common stress response for ones that are not used to going outside with all the sights + smells they've never seen before.
ahh. my beardie will pancake when there are cars driving by, at work she will run around happy as a clam until the bay doors open exposing the parking lot and busy streets. she is perfectly normal until she comes into view of the open garage doors and then stops in her tracks and lays flat on the ground until either the door closes or i come grab her. when she is chilling out on my shoulder however she stays pretty light in color regardless of cars around her.