Roads in downtown Calgary pass below bridges or Plus 15s in dozens of locations, each of them marked by a sign indicating the overhead clearance.

But ever wonder how accurate those signs actually are?

After yet another collision between a tall truck and low bridge last week, we decided to take a look for ourselves.

Using a laser measure, we scoped out the smallest distances between the roadway and the overhead obstacles at more than a dozen locations throughout downtown.

Every single site had more clearance than the sign indicated — but the margin of error varied widely from one spot to another.

The smallest variance was along Riverfront Avenue S.E., beneath the Fourth Avenue Flyover, where the measured height was just 3.5 centimetres above the posted height.

Using a laser measure at the lowest point we could find between the roadway of Riverfront Avenue S.E. and the 4th Avenue Flyover, we found the distance to be 4.535 metres. The sign indicates 4.5 metres. (Robson Fletcher/CBC)

The largest variance we found was at the Plus 15 over Stephen Avenue at Bankers Hall.

The sign there indicates 4.0 metres, while the laser measure found it to be 4.81 metres.

For those who don't love metric measurements — that's a difference of more than two-and-a-half feet.

The sign on the Plus-15 over the roadway of Stephen Avenue at Bankers Hall indicates a clearance of 4.0 metres, but a laser measure found it to be 4.812 metres. (Robson Fletcher/CBC)

Just a bit further east along Stephen Avenue, however, the Plus 15 at the Telus Convention Centre was nearly bang on.

It has a posted height of 5.0 metres and came in with a measured height of 5.08 metres.

The indicated 3.6-metre height of the Centre Street Bridge above Riverfront Avenue — where that commercial garbage truck got stuck last week — was also pretty close to the measured height: 3.68 metres.

This interactive map depicts the measured and indicated heights at each location. Click on each pin for details:

The city deliberately builds a "buffer zone" into the clearances it posts on bridges and Plus 15s, said roads spokesman Chris McGeachy.

He said the aim is typically for a buffer of about 10 centimetres but it does change depending on the age of the bridge and when the signs were installed.

But that shouldn't lull drivers into a false sense of security if their vehicles are close to the maximum height, said Andrew Barnes with the Alberta Motor Transport Association.

The semi's cargo was just a bit too tall for First Street S.E. underpass in downtown Calgary. It got stuck in April 2016. (Monty Kruger/CBC)

While he's never heard of a bridge being lower than indicated, he advised to always assume the actual height is the same as the height on the sign.

"Some of them are deadly accurate," he said.

"In my own personal vehicle, I never take it for granted that anything is higher than what it says."

6 bridge strikes in 2 years cost more than $80,000

Bridge strikes come with a significant cost to the city, McGeachy said, both in time and money.

"There's the damage to the bridge structure itself, but also there's other impacts," he said.

"When these things happen, typically the fire department responds to them and then we also have an engineer go out and assess the damage to the structure, not to mention the impacts to commuters who are trying to travel through these routes."

Six bridge strikes over the past two years caused more than $80,000 in damage, McGeachy said.

Those figures don't include strikes to Plus 15s or more minor bridge strikes that don't cause structural damage.

McGeachy said damage can vary widely depending on the incident, noting a 2006 case, in particular, in which the pedestrian bridge at 14th Street and 75th Avenue S.W. was hit so hard that a main span girder had to be replaced at a cost of more than $200,000.

He said the city does go after at-fault drivers to recoup some of those costs, where possible.

New warning systems coming

Realizing the signs aren't always enough, the city has also installed electronic warning systems at several locations in an effort to cut down on bridge strikes.

The systems involve an infrared beam set up above the road in front of a bridge or Plus 15.

When an overheight vehicle approaches, it trips the beam and triggers a warning sign that is meant to alert the driver that a collision is imminent.

A new warning sign on First Street S.E. that is meant to alert drivers when a vehicle is too tall to clear the Plus 15 walkway overhead. (City of Calgary)

Last summer, the city announced plans to add warning systems at seven more locations in the downtown core.

The first of those is now installed on First Street S.E. between Sixth and Seventh Avenues.

McGeachy said the other six should be installed before the end of the year at the following locations:

Ninth Avenue S.W., west of 11th Street

Fifth Avenue S.W. between Fifth and Sixth Streets

Centre Street South between Third and Fourth Avenues

Fourth Avenue S.E. west of Third Street

Macleod Trail S.E. between 10th and 11th Avenues

Fourth Street S.W. between 11th and 12th Avenues

The city estimates it will cost about $150,000 to install the systems and maintain them for the next three years.

The Centre Street Bridge already has a warning system in place, and McGeachy said it was functioning properly when that commercial garbage truck hit the bridge last week — it appears the driver just didn't notice.