State Route 120 between state Route 255 (South St. Marys Street) and Rock Street was scheduled to close Thursday night.
Officials at PennDOT said the closure would remain in effect for 24 hours a day until construction of this portion of the highway is complete. It is anticipated that it will be closed for four weeks.
The detour should have an impact on tractor-trailers coming into St. Marys. Signs erected indicate large truck traffic will only be able to access St. Marys Carbon, Keystone Powdered Metal, Gasbarre Sinterite Furnace, West Penn Power, Product Assurance and Custom Industrial.
Cars entering St. Marys on SR 120 will be able to turn left onto McGill Street. From there motorists are able to access West Mill Street.