California Code, Vehicle Code - VEH § 21060

Between the hours of 1 a.m. and 5 a.m., Sections 21650, 21660, 22502, 22504, and subdivision (h) of Section 22500 do not apply to the operation of a streetsweeper vehicle or watering vehicle, operated by a local authority, while the vehicle is actually sweeping streets or watering landscaping or vegetation within a business or residence district.  The exemption is not applicable unless the turn signal lamps at each side of the front and rear of the streetsweeper vehicle or watering vehicle are being flashed simultaneously.

This provision shall not apply when the vehicle is being driven to and from such work, nor does it relieve the driver of such a vehicle from the duty to drive with due regard for the safety of all persons using the highway or protect the driver from the consequences of an arbitrary exercise of the privilege granted.