Federal Highway Administration crews will close Zion National Park's Zion Tunnel twice this week to conduct a traffic safety survey, park Superintendent Harold Grafe says.

The survey is designed to determine the impact of large vehicles on overall tunnel safety, Grafe said, "and to determine what size vehicles may use the tunnel without adversely effecting other users."The tunnel will be closed to all traffic from 10 p.m. Tuesday to 6 a.m. on Wednesday, and during another eight-hour stretch beginning at 10 p.m. Wednesday, Grafe said.