<sod>The roadbed in the tunnel climbs at a 6% grade, and the original lighting was insufficient to allow motorists good depth perception.  The original lighting was improved in the 1960s.{{r|BR}}<eod>
<sod>The roadbed in the tunnel climbs at a 6% grade, and the original lighting was insufficient to allow motorists good depth perception.  The original lighting was improved with the installation of [[fluorescent]] lights in the 1960s.{{r|BR}}.  Assistance in design of the new lighting was given by [[Arizona Public Service]] and the [[California Division of Highways]].{{r|TE}}<eod>