 There are two lanes for automobile traffic across the top of the dam , which formerly served as the Colorado River crossing for U.S. Route 93 . In the wake of the September 11 , 2001 terrorist attacks , authorities expressed security concerns and the Hoover Dam Bypass project was expedited . Pending the completion of the bypass , restricted traffic was permitted over Hoover Dam . Some types of vehicles were inspected prior to crossing the dam while semi @-@ trailer trucks , buses carrying luggage , and enclosed @-@ box trucks over 40 ft ( 12 m ) long were not allowed on the dam at all , and were diverted to U.S. Route 95 or Nevada State Routes 163 / 68 . The four @-@ lane Hoover Dam Bypass opened on October 19 , 2010 . It includes a composite steel and concrete arch bridge , the Mike O 'Callaghan – Pat Tillman Memorial Bridge , 1 @,@ 500 ft ( 460 m ) downstream from the dam . With the opening of the bypass , through traffic is no longer allowed across Hoover Dam , dam visitors are allowed to use the existing roadway to approach from the Nevada side and cross to parking lots and other facilities on the Arizona side . 
 <a href='dey_it' title='1'><mark>There</mark></a> are two lanes for automobile traffic across the top of the dam , which formerly served as the Colorado River crossing for U.S. Route 93 . In the wake of the September 11 , 2001 terrorist attacks , authorities expressed security concerns and the Hoover Dam Bypass project was expedited . Pending the completion of the bypass , restricted traffic was permitted over Hoover Dam . Some types of vehicles <a href='uninflect' title='2'><mark>were</mark></a> inspected prior to crossing the dam while semi @-@ trailer trucks , buses carrying luggage , and enclosed @-@ box trucks over 40 ft ( 12 m ) long <a href='uninflect' title='3'><mark>were</mark></a> not allowed on <a href='None' title='4'><mark>the</mark></a><a href='negative_concord' title='5'><mark>dam</mark></a> at all , and <a href='uninflect' title='6'><mark>were</mark></a> diverted to U.S. Route 95 or Nevada State Routes 163 / 68 . The four @-@ lane Hoover Dam Bypass opened on October 19 , 2010 . It <a href='uninflect' title='7'><mark>includes</mark></a> a composite steel and concrete arch bridge , the Mike O 'Callaghan – Pat Tillman Memorial Bridge , 1 @,@ 500 ft ( 460 m ) downstream from the dam . With the opening of the bypass , through traffic is no longer allowed across Hoover Dam , dam visitors are allowed to use the existing roadway to approach from the Nevada side and cross to parking lots and other facilities on the Arizona side . 
It is 2 lanes fa automobile traffic across da top of the dam , which formerly served as the Colorado River crossing ffirU.S. Route 93 . In tdawake of the September 11 , 2001 terrorist attacks , authorities expressed security concerns and thdaoover Dam Bypass project was expedited . Pending the completion of the bypass , restricted traffic was permitted over Hoover Dam . sum types of vehicles was inspected prior to crossing thedam while semi @-@ trailer trucks , buses carrying luggage , and enclosed @-@ box trucks ova 40 ft ( 12 m ) long was not allowed on no dam at all , and was diverted to U.S. Route 95 or Nevada State Routes 163 / 68 . The 4 @-@ lane Hoover Dam Bypass opened on October 19 , 2010 . It include a composite steel and concrete arch bridge , the dae O 'Callaghan – Pat Tillman Memorial Bridge , 1 @,@ 500 ft ( 460 m ) downstream from the dam . w/ the opening of the bypass , through traffic ain't no longer allowed across Hoover Dam , dam visitors r allowed to use the existing roadway to approach from the Ndada side and cross to parking lots and other facilities on the Arizona side .