California Code, Public Utilities Code - PUC § 1791

Each witness who appears by order of the commission or a commissioner shall receive for his attendance the same fees and mileage allowed by law to a witness in civil cases, which shall be paid by the party at whose request the witness is subpenaed.  When any witness who has not been required to attend at the request of any party is subpenaed by the commission, his fees and mileage shall be paid from the funds appropriated for the use of the commission in the same manner as other expenses of the commission are paid.  Any witness subpenaed except one whose fees and mileage may be paid from the funds of the commission, may, at the time of service, demand the fee to which he is entitled for travel to and from the place at which he is required to appear, and one day's attendance.  If such witness demands such fees at the time of service, and they are not at that time paid or tendered, he shall not be required to appear.  All fees or mileage to which any witness is entitled under the provisions of this section may be collected by action therefor instituted by the person to whom such fees are payable.  No witness furnished with free transportation shall receive mileage for the distance he may have traveled on such free transportation.