I guess I view these burros as being very similar to deer. Our state has over 10,000 vehicle/deer accidents every year. Last year we saw something like 6 human fatalities caused by deer/vehicle accidents. What used to be wild animals... deer, burros, wild turkeys, coyotes, etc... have become almost domesticated and have no fear of humans or vehicles.
My work requires that I drive many early morning hours. Just about dawn, the deer begin moving. Certainly, our deer can be as large as small cattle and should be seen by drivers. That aren't!! Due to deer moving in low-light times of the day and deciding to cross highways with no regard for vehicles they see day in and day out, there is a "thinning of the herd" by autos/trucks. Seems like this is the same thing that happened with the burros. Low/no light and grazing right beside the highway.
As far as putting up signs.... If people in our state drove slow everywhere there is a "Watch Out For Deer" sign, no one would get anywhere. I like the idea that local law enforcement set up with flashing lights when they had a small group of the burros grazing ahead. If that isn't the case, I don't know how normal driving habits would allow people to see burros/deer quite a ways ahead when it's dark. Lowering the speed limit in grazing areas is not something that I see preventing vehicle/deer accidents. Certainly, if all vehicles were moving, say, at 45 mph there would less vehicle damage in a collision. That speed, however, could still result in broken legs and internal injuries to animals causing them to be put down. And, in the dark or low light times of the day, could you see a dark-haired critter even at 45 mph in time to react when they walk out in front of your vehicle??
Yes, it's sad when any life is lost... animal or human. In this case, I guess I don't see it as preventable as long as we have wild animals and we drive automobiles/trucks on the highways.