Facts vs Fiction: Triggering an Advanced Green

Traffic and weather are by far the leading conversation topics around office water coolers. People can complain about the weather all they want, they won’t influence what Mother Nature has in store for the next day. When it comes to traffic, however, everybody’s an expert, voicing their frustration over social media and offering turn-key solutions that clueless roadway operators would be nuts not to consider. At autoTRADER.ca, we’re experts on cars; we travel the same roads as you and face the same traffic challenges on our journeys. But we also have an editorial tool to review the myths that surround the daily commute: a traffic engineer.

The Myth

Recently, our editors came across a blog whose author concluded that drivers must stop on cut lines – those circular or diamond-shaped lines cut into the asphalt near traffic lights – in order to get an advance green from a turning lane. According to the blogger’s experience, the cut lines were set so far back from the stop line at the traffic lights that the turning phase would only come on if one or two cars pulled up behind him. Otherwise, he would stand there and wait, and wait, for a phase that never comes. So the blogger’s advice is to stop on the cut lines, even if they are three car lengths away from the stop bar.

So, can you trigger an advanced green by stopping on cut lines?

The Facts

The only detail the blogger got right is what the cut lines represent: loops. Induction loops, also known as detection loops, are the oldest and simplest way to detect a vehicle at an intersection – but they are so 20th century. Buried loops are just wire, and they “see” cars through the magnetic induction caused by a metallic mass moving over the wire, thanks to a signal amp located in the traffic control box. Visible cut lines are a tell-tale sign of an installation done after the pavement was laid down, but at other intersections the loop may be completely hidden, if wires were laid during the paving job.

The rest of the blog entry, unfortunately, is pure myth – speculation based on a single and very specific case. The generalizations simply aren’t borne out in the real world.

Here’s how a vehicle-actuated advanced green is typically configured:
• if you have only one loop, it’s located at the stop line (about 1 m behind it)
• this loop is programmed for “presence”, to “see” if a vehicle is there, needing the advanced green (or some other phase)
• this first loop may also extend an advanced-green phase by a second or more for each vehicle that crosses over it, up to a programmed maximum
• an optional second loop is set back by a few car lengths (18 m in some cities), programmed for “extension”, to extend the phase, as above, for each vehicle that arrives (so it doesn’t get a red in its face once the first car clears the stop line)

Behind the Myth

So in our blogger’s case, if there’s only a set-back loop, it should also be programmed for presence. A setup like the one described (far from the stop line) is typically used where traffic is faster (posted above 50 or 60 km/h), so the phase will be called as the vehicle approaches the stop line; or where the intersection is fully actuated (loops everywhere with no preset sequence).

Here, it looks like:
a) somebody forgot to activate “presence” on that loop; or
b) the jurisdiction intentionally doesn’t want to service this phase for only a vehicle or two, for reasons that may be valid; or
c) the stop-line detection was broken and a poor repair job was done.

A broken detection loop should send the advanced green into automatic recall, no queuing required. In our blogger’s case, it could be that a broken stop-line loop was bypassed and the set-back loop was left as-is. Oops.

The Take-away

In the 21st century, traffic engineers use cool stuff: radar, cameras, infrared, microwave or lidar. Look for thingamabobs hung on traffic signal posts and masts – size, colour and shape will vary. So, don’t look for cut lines to stop on – loops are often broken or abandoned, and we use the no-digging high-tech stuff to replace them. Repaving is (often) optional.

Instead of blogging your solution, send an email to your local public works – they have a loop to fix. Your mileage may vary, but in this case: traffic myth – busted.

The straight-up on turning left 9/29/2016 10:00:34 AM