Why are these cars driving so slowly?

They had a green light at the top of the hill and usually they'd accelerate downhill. Today they were going almost at walking speed. Why?

They were looking at these:

(image intentionally very small)

What are these creatures? Why are they in a park in the middle of the city? One of them attacked me!  (you'll see the video if the video is good, I haven't seen it myself yet). That and more in my next post. Beg for it!

Yes, a cliffhanger! :-D

Edit: the video isn't too bad. Here's a screenshot, showing the attacking reindeer about to get out of control. Good thing the handlers told people to get the kids out of the way before they untied the reindeer. There were several kids and toddlers on strollers admiring the reindeers.