So it is a long road, and twice consolidated now. City Transportation Fund --> Basic Municipal Transportation Grant --> MSI
http://municipalaffairs.alberta.ca/msi-funding-allocations-eligibility
"Basic Municipal Transportation Grant (BMTG) funding is based on municipal status, with Calgary and Edmonton receiving funding based on litres of taxable road-use gasoline and diesel fuel sold in the province; the remaining cities and urban service areas receive funding based on a combination of population and length of primary highways. Towns, villages, summer villages, improvement districts and the Townsite of Redwood Meadows receive funding based on population; and rural municipalities and Metis settlements receive funding based on a formula that takes into account kilometres of open road, population, equalized assessment, and terrain."
The BMTG is a consolidation of 4 grants, joinged together in April 1, 2011 from the following components:
City Transportation Fund
Basic Capital Grant
Streets Improvement Program
Rural Transportation Grant
Provincial Highway Maintenance
https://www.transportation.alberta.ca/images/BMTG_Guidelines_November_2013.pdf "The Minister and the City agree that the amount of funding provided will be based on 5 cents per litre and volume of taxable road-use gasoline and diesel fuel sold within the province of Alberta, as confirmed by Treasury Board and Finance. "
Working backwards it isn't pure population, as Edmonton is over allocated if that was the case. I remember it being described as sales within city limits, but that isn't written down anywhere I can find.