Cal. Sts. & Hy. Code § 92.5
The department shall install, in all tunnels and underpasses on state highways within metropolitan areas where the tunnel or underpass is of sufficient length to impede radio reception, wires, or other devices which will sustain reception of broadcasts by radios in vehicles traveling through the tunnels and underpasses.
The Legislature declares that the installation of such devices in state highway tunnels and underpasses is an improvement of highways within the meaning of Section 1 of Article XIX of the California Constitution since it will assist law enforcement officers in their patrol duties by enabling them to receive essential radio calls and also will increase the enjoyment of the people in the use of the state highways.
Ca. Sts. and High. Code § 92.5