Ariz. Rev. Stat. § 48-5309
A. The authority shall develop a twenty year regional transportation plan that is subject to approval by the qualified electors of the county and financed by a transportation excise tax approved pursuant to section 42-6106 and bonds issued pursuant to article 2 of this chapter. The regional transportation plan:
3. May, among other things:
E. For the purposes of this section:
2. "Substantial change" means a change that, based on data in the transportation improvement program developed pursuant to section 48-5304, paragraph 3, results in one or more of the following conditions:
(a) A present worth of estimated expenditures required to complete all elements of the regional transportation plan that exceeds the greater of:
(b) An estimated cost to complete one or more elements of the regional transportation plan that exceeds the expenditure limitations of the plan as adjusted by the GDP price deflator as defined in section 41-563 by the following or greater percentages: