is it legal in PA for police to have a construction sign up at a dui check point?

Upheld under state and federal Constitution. Commonwealth v. Yastrop, 768 A.2d 318 (Pa. 2001); Commonwealth v. Blouse, 611 A.2d 1177 (Pa. 1992); Commonwealth v. Tarbert, 535 A.2d 1035 (Pa. 1987); Commonwealth v. Fioretti, 538 A.2d 570 (Pa. Super. 1988); Commonwealth v. Myrtetus, 580 A.2d 42 (Pa. Super. 1990). Under Commonwealth v. Pacek, 691 A.2d 466 (Pa. Super. 1997), a checkpoint does not have to provide a legal means of avoidance. Checkpoint must be located in area where DUI is prevalent. Commonwealth v. Blee, 695 A.2d 802 (Pa. Super. 1997). Legal U-turn in advance of checkpoint does not justify a stop. Commonwealth v. Scavello, 703 A.2d 36 (PA. Super. 1997). A checkpoint conducted at a toll booth was held illegal because it was not conducted in accordance with state Supreme Court guidelines. Commonwealth v. Yashinski, 723 A.2d 104 (Pa. Super. 1988).

http://www.superior.court.state.pa.us/Opinions/a16025.pdf  : which states

Although "roadblocks" are legal in Pennsylvania they must be

conducted according to the specifications set forth in our Supreme Court's

Commonwealth v. Tarbert, supra, and Commonwealth v.

Blouse

http://www.superior.court.state.pa.us/opinions/s12018_02.pdf  gives you those guidelines.

So the answer to your question is if it meets the guidelines above and the construction sign was already up, then probably legal.  But if you had such a stop and have been charged, contact a criminal defense attorney to review your case.  Try your state's bar or www.attorneypages.com or county bar associations.