Starting Friday vehicles on Southbound Interstate Five will be directed onto the innovative “Shasta Viaduct” north of the Pit River Bridge at Lake Shasta. The unusual span required realignment to meet seismic, safety and operational standards. It’s not a typical structure because it combines an arch construction with a curve, presenting some engineering challenges that were difficult to overcome. While the work was being done, traffic had been directed to a temporary lane on the far right shoulder. One lane on the viaduct opened on Friday, and that configuration will be in effect for at least three weeks. The speed limit will be 55 miles an hour and delays of up to an hour or more should be expected during peak times. Drivers should pack extra water and snacks.