Roadblock is a slur used to describe Mexicans in a derogatory way. Called this because some have a tendency to travel slow, pacing one another on the freeway not allowing anyone else to pass. Mostly a Texas term, though it may be heard in any other of the 'border' states.
The word Roadblock is a racial slur that has been used historically to degrade and dehumanize mexicans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to mexicans. If you are unsure about whether or not it is appropriate to use the word Roadblock, it is best to err on the side of caution and avoid using it in any situation.