It was one of the few things mayoral candidates agreed on in the Toronto election race: co-ordinating traffic signals to let people get around more quickly.

With traffic and congestion topping residents' concerns, it was an obvious promise to make. And it was bound to resonate with drivers or cyclists who get frustrated as they start out from an intersection, only to see the next signal turn red in the distance.

"At the end of the day, it's about making the signals serve the traffic," said Steve Buckley, general manager of the city's Transportation Services department, who sat down with The Globe and Mail for a lengthy interview on the benefits and challenges of signal co-ordination.

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"People don't care if they go fast but they care [that] they're starting and stopping. So the goal is [to] get them moving smoothly and moving along."

But motorists' dream of an endless chain of green lights is likely to remain elusive. Traffic-signal co-ordination – based on the simple premise that engineers can control the flow of traffic by tinkering with signal cycle times – works. In a complex urban environment, though, there's only so much that can be done.

Among the factors that complicate matters are the size of the city's grid and streets, the growing influence of pedestrians and the impact of transit vehicles.

Mr. Buckley said that the cycle timing for many intersections is years out of date. Traffic patterns have since changed as the city grows. The city hopes to re-time 1,000 of the city's 2,500 intersections over three years.

"Our goal is trying to squeeze as much out of the system as we can, there's still opportunities out there," he said. "The system's obviously getting to a point where it's at capacity, and we need to sort of just make it function better."

Doing that, though, will run the engineers up against some challenges.

Street design and layout

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Signals on one-way streets are the easiest to co-ordinate, with traffic all moving in the same direction. It gets more challenging when you're dealing with two-way streets that have heavy flow in one direction at certain times of day. Although John Stuart Mill would appreciate the utility of building the system around the needs of the larger group of drivers, the effects on the others could become be too great. The question then becomes: Is it worth making trips in one direction much longer and less convenient in return for a small improvement in the other direction?

Heavy two-way flow in both directions is easier to time but still runs into problems when those roads cross other major roads. Consider King Street, Mr. Buckley suggested. As it crosses smaller side-streets it can take the majority of the signal cycle, perhaps 70 per cent. But then it hits Spadina or University and must settle for perhaps half of the time. Complicating matters, both streets are hitting the intersection with high volumes.

Pedestrians

Engineers once assumed that people would walk at a minimum of 1.3 metres/second, about 4.7 kilometres per hour. But as society ages, the industry standard has dropped to 1 metre/second, 3.6 kilometres per hour. The difference means that minimum signal times – designed to ensure pedestrians can cross safely – have had to be increased. The effect is greatest on wide streets but also noticeable where a major road intersects with a small cross street. With the quieter street given 33 per cent more time, traffic on the busier street must wait longer.

At the same time, the idea of what is traffic is changing. An increasing number of people are walking as their primary method of transportation, particularly in downtown Toronto, inevitably prompting the question of just whose interests should be taken into account when timing a signal. Mr. Buckley, who said that "historically" they've timed signals for cars, noted that some intersections might have three to five times as many pedestrians waiting to cross as vehicles.

Transit

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Another wrinkle is transit-signal priority. The idea is based on the egalitarian notion that transit vehicles typically carry many more people than anything else on the road and should get privileged access at intersections. The downside is that it can mess with signal timing.

Imagine a road with a streetcar line mixed with traffic. The intersections are on a regular cycle but it gets hijacked every time a streetcar rolls up. It might take a few adjusted signal cycles to get back on the rotation – but only until the next streetcar rolls up. The obvious solution of including the streetcar schedule in the signal rotation can't work unless the transit vehicles' movements become much more reliable.

The future

These challenges notwithstanding, there are ways to improve on the current system. A number of mayoral candidates gave plugs to a "smart" traffic-signal concept developed at the University of Toronto. It's one of a number of such products likely to be considered as the city moves to update its own technology.

About 330 intersections now use a system called SCOOT, an early form of adaptive signal that is ripe for replacement. It would cost about $100,000 per intersection to put in a modern system.

"To be honest with you, in this world right now if I said I have something that can help reduce congestion, folks will likely throw money at us," Mr. Buckley said. "We have this quote-unquote $6-billion [congestion] problem out there, why wouldn't we throw another $10- or $20-million at it."