A pretty straightforward question. What prompted me to ask was that, in the course of preparing this post about the early Interstate bridges of Idaho (https://www.aaroads.com/forum/index.php?topic=26007.msg2572240#msg2572240), I discovered that thrie beam bridge guardrails are extremely common there. Many states augment or replace earlier, unsatisfactory bridge guardrails with thrie beams, but I found this apparently original thrie beam installation (https://goo.gl/maps/ixpEt89Q1RWsyqWh7) on an otherwise-normal 1971 bridge. Other nearby bridges built in 1971 sport normal guardrails (https://goo.gl/maps/37jCwEuJ2dy94HCVA). Now I'm wondering how many of the zillion thrie beam bridge guardrails in the state are also original equipment and even older.
:hmmm: