The taxable property within any county shall not at any time be obligated for all joint highway district purposes, in any amount in excess of the following limitations:
(a) In counties having a total assessed value of all taxable property therein of one hundred million dollars ($100,000,000) or less, not more than 3 per cent thereof.
(b) In counties having a total assessed value of all taxable property therein of more than one hundred million dollars ($100,000,000), not more than 2 per cent thereof.