Take a look at the great cities of the world-- think Paris, London, New York, Tokyo -- and you'll find that prosperity, livability, and opportunity are all inextricably linked with excellent rapid transit.

Why? It's simple -- rapid transit allows huge numbers of people to travel around a city in ways roads cannot, and supports the density needed for a vibrant urban economy. And if the lines are built to where the most riders are -- typically lower-income neighborhoods -- rapid transit helps address social and economic inequities by connecting people into the fabric of their cities.

But the forms of rapid transit that work best vary from city to city. And the big question for many cities that don't already have a rapid transit backbone, or who need to expand it, is how. How do you add a rapid transit network to a city built for the car -- and what technology do you use? It's a question that my home city of Toronto is grappling with right now -- as are many other cities around the world.

The principles of smart transit expansion are actually straightforward: Build it where the riders are or demonstrably will be; build it in a reserved right-of-way; and build it in places that can support future density and private investment.

For high density areas, subways can play a key role in a city's success, with a high level of ridership, a focus on two-way travel between home and work, and extensive nodal development at station points to create interesting, vibrant neighborhoods. At the other end of the spectrum, for cities in the developing world or low density cities built in a suburban form -- like Curitiba in Brazil or Ottawa -- bus rapid transit is a viable choice.

The transit confusion lies in the middle of the spectrum -- the inner suburbs with mixed development or city areas that lack density but have high passenger volumes.

The good news is that there's a straightforward solution-- light rail transit (LRT). It's one-third the cost of a subway, and, because it runs on the surface, it brings street life and vitality to the urban street, encouraging the development and rejuvenation of neighborhoods along the route.

It's also rail-based, like a subway, so there's no local pollution. And if powered by low emissions sources, an LRT network can actually fight climate change, instead of contributing to it. These are the reasons why so many cities -- from Minneapolis, to Los Angeles, to Salt Lake City-- have recently put LRT plans into action.

I recently participated in the OECD Mayors and Ministers conference in Chicago. The focus was on how to build prosperous, inclusive, low carbon cities -- and rapid transit was high on the agenda. OECD cities like Berlin and Madrid get it right: underground in the dense urban cores and light rail in the inner suburbs.

One of North America's largest cities -- Toronto -- is about to make its transit expansion choice right now. It's where I live, so I'm hoping we get it right. With incredible new LRT technologies and so many benefits to a surface-route expansion, cities like Toronto that adopt LRT networks will be reaping the rewards for decades to come.