In 1911, when the Sproul Road Bill was passed, a large number of Legislative Routes (LR) were assigned.

These were the primary internal numbering until the present Location Referencing System was adopted in 1987.

The establishment of the Interstate Highway System in 1959, as well as wanting to eliminate some concurrences in Pittsburgh, resulted in a small renumbering in 1961.

In the mid 1950s, the signs were modified to have "PA" instead of "Penna", with the lone exception being the mainline Pennsylvania Turnpike (which continues to use "Penna" today for both the mainline, the Northeast Extension, and the Pennsylvania Turnpike Commission logo); additionally, the numbers were made more round and the signs were made larger in order to be more legible while driving, and the keystone shape itself remained a cutout.

Most of the 1950s vintage signs were replaced with the newer rectangular cutout with the painted-on keystone by the early 1970s, though a very few remain in scattered places on non-decommissioned roads.