Mass. Gen. Laws ch. 161A § 9
Notwithstanding the provisions of any general or special law to the contrary, all cities and towns of the authority shall contribute to the Massachusetts Bay Transportation Authority State and Local Assistance Fund an amount in the aggregate not less than $136,026,868 which, after fiscal year 2006, shall be adjusted each July 1 by the growth rate of the inflation index over the preceding 12 months; provided, that, after said fiscal year, in no case shall said assessment exceed 102.5 per cent of the previous year's assessment. Each municipality's share of said assessment shall equal its weighted percentage of the total population of the authority. For the purposes of this calculation, said weights shall be as follows: Boston, 18; Brookline and Cambridge, 12; the 14 cities and towns excluding Boston, Brookline and Cambridge, 9; the 51 cities and towns, 3; other served communities, 1. Population shall mean population as most recently estimated and published by the United States Census Bureau. On or before March 1 of each year, the authority shall certify to the state treasurer the amount to be assessed to each city and town as aforesaid on account of the ensuing fiscal year, and the state treasurer shall, on behalf of commonwealth, assess each such city and town on account of such year in accordance with the provisions of section 20 of chapter 59. Beginning on July 1, 2001, a city or town that is also a member of a regional transit authority or that at any time joins a regional transit authority shall have 100 per cent of the amount assessed for the operation of the regional transit authority credited against its share of the assessment made under this section; provided, however, that the amount credited shall not exceed the total amount of the assessment. The total amount of regional transit authority credits authorized herein shall be re-assessed to the 14 cities and towns and the 51 cities and towns based on the weighted percentage of said cities and towns' share of the population of said 14 and 51 cities and towns. For the purposes of this section, the words "inflation index" shall mean the per cent change in inflation as measured by the per cent change in the consumer price index for all urban consumers for the Boston metropolitan area as determined by the bureau of labor statistics of the United States department of labor.
A city or town assessed by the authority that is not receiving paratransit services for the disabled from the authority shall have 50 per cent of the amount it expended in the previous fiscal year for the operation of or membership in a local or regional paratransit service credited against its share of the assessment made under this section. The amount credited shall not exceed the total amount of the assessment; and provided further, that the amount credited shall be the most recently audited regional transit authority assessment available on January 1 of each year and shall be used to calculate the upcoming fiscal year's estimated cherry sheet assessments. The credit shall apply only to services provided to individuals eligible for paratransit services. As used in this paragraph, "paratransit services" shall mean services provided to individuals with disabilities who, as the result of a physical or mental impairment, including a vision impairment, are unable to board, ride or disembark from a vehicle in the authority's regular transportation system without the assistance of another individual, except the operator of a wheelchair lift or other boarding assistance device.
Mass. Gen. Laws ch. 161A, § 9