Why does the tar on roads melt in the summer sun?

The "tar" on roads is a mixture of several asphaltic and hydrocarbon compounds. These become liquid or often at higher temperatures (over 150 F). This temperature can be approached due to sunlight on a hot day and the road tar will become soft. If the supplier has adulterated the tar with more liquid oils or used the wrong compind (say roofing tar the surface may become soft or even liqufy at ambient conditions.