Puerto Rico Highway 103, or PR-103, is a rural road that connects from the PR-2, in Hormigueros, to the PR-101, in Cabo Rojo. The PR-103 is the old road from Hormigueros, passing through the center of Cabo Rojo, until the PR-101 (which connects from Boquerón to Lajas). Therefore, the PR-100 is a road to replacing PR-103 because it is a faster and more comfortable road.
Map of PR-103, Hormigueros, 00660, Puerto Rico
References
Puerto Rico Highway 103 Wikipedia(Text) CC BY-SA