Do cycle detector loops trigger with aluminium bikes?

This is often an issue when a city first gets going with wanting to detect bicycles at traffic signals. There are a lot of cogs that need to be aligned for the system to work reliably:

• The designers need to know what kind of loops to specify.
  • Often, they need a larger offset to adjacent lanes so that they can be tuned with high enough sensitivity to reliably pick up bicycles without detecting cars in adjacent lanes.
  • You’d use different loops for shared traffic lanes than those you’d specify for exclusive cycling infrastructure.
• Next, the contractors need to know a thing or two as well. Things that commonly go wrong:
  • Loop gets cut too close to adjacent lane
  • Loop gets installed too deep (I recall a case where they installed the loop in a conduit before sealing everything over; the idea was that there were no cuts in the asphalt surface and you thus get a much longer life out of the surface. But it was all too deep and didn’t detect, so they cut new loops through the brand-new surface.)
  • When tuning the loop, they may be using their metal toolbox, which contains a shitload more metal than you’d ever find in a bicycle with an aluminium frame (pro-tip: use a bicycle when tuning loops that need to detect bicycles)
• And then there’s those who operate the signals. They need to know the above, too. If they don’t, they look at their operating system, see that bicycles get detected, and tell people who ring in that everything is working just fine (when in reality, people are ringing in because it’s not working for them).
• It helps if the road markers get told to mark the loops with diamonds where they are most sensitive (with a vehicle loop, there is an area where they are most sensitive, an area where the sensitivity is about half that – which should still be fine – and in between the sensitivity is poor). But this has to somewhat align with how people would use a traffic lane if it’s shared with general traffic. Rule of thumb is that an engineer who understands how loops work and how one would ride a bike in traffic is in charge of this. If you leave this part to a maintenance contractor, it generally won’t work.
• And if a city marks its loops like this, they ought to tell people on bikes what these diamonds are for.

It’s generally a painful process to get it all working. It generally helps when somebody is on the operations team who really cares. If so, it’ll sort itself out over time.

Axel Downard-Wilke