Good question. The MPH traps are begin a short distance before the 1/4 mark. The second beam also trips the ET. I'm not sure of the distance between the MPH beams though, never really gave it any thought. Judging by the distance, I'd have to guess its somewhere in the 50 to 60 foot range. Have you checked the NHRA website to see if they have it defined?
------------------
Jeff Chambers
Trophy Stock #3
11.462 Seconds / 117.43 MPH
Chambers Racing Team