Ins. Code, § 1175

Such insurers may invest in bonds of any permanent road division in this state, or any district organized under the laws of this state, when such bonds are legal investments for savings banks of this state, or have been certified as legal investments for savings banks pursuant to Division 10 (commencing with Section 20000) of the Water Code, or when the statutes or laws providing for the issuance of such bonds provide that such bonds shall be entitled to the same force or value or use as bonds issued by any municipality, or such law specifically states that such bonds shall be legal investments for either savings banks, insurance companies, all trust funds, state school funds or any funds which may be invested in bonds of cities, counties, cities and counties, school districts, or municipalities in the state, or when such bonds have been investigated and approved by a commission or board now or hereafter authorized by law to conduct such investigation and give such approval when such law specifies that upon that approval the bonds are legal investments for insurers, or which the commissioner approves in writing as legal for investment of the funds of insurers. The commissioner in determining whether to approve any bonds as legal investments which do not otherwise qualify as such pursuant to any part of this code, shall, at the expense of any insurer requesting approval, make an adequate independent investigation of such bonds and the security therefor. A copy of the data secured in such investigation and the resulting opinion of the commissioner shall be furnished to the insurer.