When first posted as such in January 1964, SR 543 started at Jct US 99 south of the Peace Arch and headed east, then north to the Pacific Highway border station. When I-5 was completed through Blaine circa 1966, SR 543 was rerouted from I-5 Exit 275 directly north via a 2 lane expressway to customs. Trucks are prohibited from using the I-5/BC 99 border crossing. The SR 543/BC 15 border crossing is open to trucks and is in fact referred to as truck customs, although it is open to cars as well. There are plans to widen SR 543 to 4 lanes from I-5 to the border. Perhaps SR 543 should be designated as an Interstate route. Yes, it has grade crossings but so does I-180 in Cheyenne, Wyoming. SR 543 is 1.09 miles long.
Before 1964, SR 543 had been the Truck Route branch of PSH 1.
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