Your post is really vague. Where will you be traveling from--the San Juan international airport? What is "PR"? Do you mean PR-22?
From San Juan you need to take Highway 22 west to the town of Arecibo. From Highway 22 in Arecibo, there is no direct road to the Arecibo telescope. You can head south on Road 129 to Road 134 to Rd 635 east and finally south again on Road 625 which dead ends at the observatory.
Yes, it would help if I proofread my post before posting. Sorry! We'll be leaving from Arecibo and heading to Ceiba airport. What I meant to ask is if PR 22 is the quickest route to Ceiba airport from the observatory, or if there was a shorter southern route. Thx.