From My Perfect Automobile:
In many states the highway patrol carries two gallons of Coke in the trunk to remove blood from the highway after a car accident.
I doubt it's real. Does Coca-Cola work this way?
Mythbusters actually gave the blood cleaning a "Confirmed" in that episode (although said nothing about police carrying cola). And Snopes says that using Coca-Cola to clean comes from a fellow by the name of Joey Green. As Snopes says:
As for the claim that police use Coca-Cola, I think that is just an extension of the "Coke is Bad" myth that Snopes took on. Keep in mind that it's not the job of police to clean up accident scenes. Instead, they investigate the scene, and it is left to other professionals to do the clean up (such as medical personnel, since in many cases blood could be used for either evidence in a criminal investigation, or that it is a biohazard)
Considering that there is basically no information on this subject I doubt it's real (This question is #4 in a Google search for "blood dissolves in coca cola").
You might want to take a look at this article: http://science.howstuffworks.com/crime-scene-clean-up1.htm
It explains some of the equipment, which I quoted some of below.
This question sheds some light: http://answers.yahoo.com/question/index?qid=20071023213703AAWWgoq saying that water with some alkaline detergent would work better.
It is also not usually the police's job to clean up, and the fire department will often have to do that work.