Cities across Canada are reporting stagnation and even declines in public transit ridership and officials candidly admit they aren't exactly sure what's going on.

Halifax, Montreal, Ottawa, Toronto, Saskatoon, Calgary and Vancouver are among the cities to report a levelling-off of ridership. The Toronto Transit Commission – which, like many other transit systems, had been on a steady ridership climb for years – recently reported that 2015 numbers fell short of expectations and 2016 may show a year-over-year decline.

The commission is warning of a potential $30-million budget shortfall.

The challenging ridership numbers come at an unprecedented moment for public transit in Canada. Cities are trying to cover the operating costs of existing transit systems at the same time as they rush to prepare ambitious expansion plans to capture the billions now on offer from federal and provincial infrastructure programs.

The federal government has said it will take a hands-off approach to doling out its infrastructure cash, transferring it to cities based on ridership and largely leaving it up to cities and provinces to decide on priority projects.

While the federal government is now willing to cover up to 50 per cent of the cost to build new transit lines and extensions, it will ultimately be up to municipalities to produce reasonable ridership forecasts or risk having to cover the operating shortfall for years to come.

"The overall trend we're seeing in Canada and in the U.S. is ridership is stagnating or [showing] modest growth. That's the trend," said Patrick Leclerc, president and CEO of the Canadian Urban Transit Association, which is made up of transit operators from across the country. The association recently held its annual general meeting in Halifax, where ridership issues were discussed.

"The growth is not as strong as it was about five or six years ago. The last decade was major growth. Now it's slowing down. We are doing the analysis to understand what is happening in each region," he said.

Limited data on the reasons for the shift mean transit officials are left to speculate as to potential causes. The TTC's analysis concluded that the slowing economy and employment were the main factors, as well as a recent fare increase.

Other potential factors raised by Canadian municipalities include lower gas prices, the rise of Uber and other ride-sharing services, more people walking and cycling to work and the possibility that more riders aren't paying as streetcars and buses allow passengers to board rear doors with the expectation that they will tap their transit cards.

The general manager of OC Transpo, the City of Ottawa's transit system, recently told the city's transit commission that no one really knows the answer.

"Canada-wide, everyone is down," said John Manconi earlier this month. "There's all kinds of theories out there. We hear elasticity. We hear pricing. We hear this. We hear that. I think the best guesstimate anybody can give is a combination of things."

Mr. Manconi said U.S. cities are reporting similar trends and that the turning point in the data occurred after 2012.

"It appears that, post-2012, everyone started to slide and it appears to be a combination of things. But nobody can pinpoint that it's exactly this or that that has caused ridership to do what it's doing," he said. OC Transpo is promising to release an "aggressive, comprehensive" review of the situation in the coming weeks.

Bruce McCuaig, the CEO of Metrolinx, the Ontario agency responsible for the GO Transit commuter bus and rail system in the Greater Toronto and Hamilton Area, said he believes lower gas prices and slower economic growth are the main factors behind softer ridership numbers. He's convinced though that the province's ambitious, multi-billion dollar regional express-rail plan will grow ridership to 127 million per year by 2029, up from 65.7 million in 2015.

"We still feel very strongly that as we provide more service, that what we've experienced in the past will continue to occur here, which is we'll open ourselves up to new markets and we'll be successful in capturing those markets," he said.

Mr. McCuaig and Metrolinx are also responsible for the $456-million Union Pearson Express rail line linking Toronto's downtown to Pearson airport. Initial ridership numbers fell far short of expectations, forcing the agency to slash fares. Promises that the line would quickly become self-financing have been shelved, leaving taxpayers on the hook for a permanent annual subsidy.

Mr. McCuaig said forecasting ridership on that line was a challenge because it is not like GO Transit's other lines that focus on commuters. However, the fact that ridership has more than doubled since fares were cut in March shows the importance of marketing and choosing the right price.

"Price, of course, matters and you need to make sure that you price the service appropriately," he said. "When you price something at what I would consider to be a traditional transit fare, governments should expect that there's going to be a need to provide a subsidy for those kind of services."

McMaster University geography and earth sciences professor Chris Higgins, who specializes in the study of rapid transit systems, said cities need to carefully weigh the long term cost of expanded service.

"You've got to do the right things," he said.

In addition to issues such as the economy and gas prices that are being raised by transit agencies, Dr. Higgins said he also suspects the demographic impact of his own generation – the millennials – may be a factor.

While many have observed that millennials have been less interested in cars, they may also be moving to the suburbs and driving more as they start to form families. Even if they stay close to transit, he said they may be working from home more or scaling back their hours as they raise young children.

"All these types of factors can combine in a blender, really, and manifest themselves in lower ridership," he said. "Demographics are behind a lot of these things and tend to be forgotten."

Montreal (Société de transport de Montreal)

The trend: Steady ridership growth plateaued in 2013 and ridership slipped in 2015.

The cause: The STM said the decline reflects a national trend, as well as a weak economy, the growing popularity of other transportation options, lower gas prices and a harsh winter that affected service. Suburban regional rail lines have also seen declines in 2015.

The plan: Montreal is looking to expand its Blue line subway and the Caisse de dépôt et placement du Québec recently proposed a $5.5-billion light rail transit line that would require federal and provincial funding.

Ottawa (OC Transpo)

The trend: Ridership peaked in 2011 and has gradually declined since.

The cause: Public service cuts were the original explanation, but OC Transpo general manager John Manconi recently said transit agencies across the country are struggling to explain what's happening.

"I think the best guesstimate anybody can give is a combination of things," he said.

The plan : The City of Ottawa is in the midst of a major conversion of its Bus Rapid Transit into a light rail line by 2018, including a tunnel under the downtown. The city is working with Ottawa and the province on the details of a second phase to extend the line.

Toronto (Toronto Transit Commission)

The trend: After a decade of strong growth, TTC ridership came in below target for 2015 and could potentially show an overall decline in 2016.

The cause: A TTC report says GDP and employment growth for Toronto have failed to meet expectations, which leads to lower transit use. Other potential factors cited by the TTC include recent fare increases, people riding streetcars without paying and low gas prices. TTC officials have also said its system is sometimes too crowded, which hurts growth.

The plan: The TTC said it is considering a freeze on further service additions until it can sort out whether the ridership trends are a temporary blip or a permanent shift. Construction is under way on an 8.6 km subway extension from Downsview Station to Vaughan Metropolitan Station that is scheduled to open in late 2017.

Greater Toronto Area (GO Transit)

The trend: The provincially owned regional commuter service is at the heart of a major transit expansion planned for the Greater Toronto and Hamilton Area. It has had steady growth over the past decade but ridership only increased slightly in 2015, falling short of forecasts.

The cause: GO Transit says its growth is limited by the fact that commuters want more frequent service and faster travel times. Metrolinx CEO Bruce McCuaig said the economy and gas prices are the main short-term factors affecting ridership.

The plan: Known as Regional Express Rail, the plan to electrify and increase service between urban centres is expected to boost ridership dramatically. Metrolinx expects 127 million riders a year by about 2029, up from 65.7 million in 2015. The same agency is responsible for the UP Express transit line from Union Station to Pearson airport that launched last year and failed to live up to ridership projections, requiring a major cut in fares to boost riders.

Calgary (Calgary Transit)

The trend: Calgary's transit system is regarded as a model internationally, but ridership dipped in 2015 and officials have warned that the early 2016 data shows a further drop of nearly five per cent.

The cause: Officials say the decline appears to be entirely due to the economic slowdown in Alberta caused by the drop in commodity prices. Officials noted that a similar ridership decline occurred during the 2008 recession.

The plan: Calgary's light rail system launched in 1981 and the city has put forward a 30-year plan to expand the network with short, medium and longer term projects.

Vancouver's Translink

The trend: Translink saw a drop in ridership in 2013 but recovered with a two-per-cent gain in 2015.

The cause: Metro Vancouver mayors pushed hard for a regional plan to dramatically expand transit service but have failed to find a way to pay for it. A proposed new sales tax was defeated last year in a referendum.

The plan: Cities and the province are now looking for more detail on federal funding and are debating how development fees around new transit stations could fund new lines.