Here's a paved road (created by logging) through the dense oyamel fir and pine forest west of Mexico City where the monarchs are so thick between about 10:00 am - 3:00 pm on sunny days from November to early March that the police have imposed a 10 MPH speed limit. The butterflies use this paved road as a path through the dense surrounding forest to get to areas of moist ground (a logged dirt road clearing in the forest) where they can find drinking water. Then in the mid-afternoon the butterflies climb back 1,400 feet up the mountain to their regular overwintering cluster trees. http://www.youtube.com/watch?v=15t0HbLvkqk
Here's another video of the actual cluster trees at this same overwintering site http://www.youtube.com/watch?v=OR-4qlg7lYw This video was presumably shot in the morning and shows huge numbers of the butterflies departing downhill headed for the paved highway and watering hole (the moist dirt road) shown in the video above: