It's one of the more common questions asked by road users about cyclists.
The authorities have seen fit to provide cyclists with a lane – so why aren't they riding in it?
You may as well ask "how long is a piece of string?" – because there is no single or even simple answer to that question. A decision to enjoy or eschew a bike conduit can hinge on any number of factors, and is often not obvious unless you are the rider sitting in the saddle. Here are a few possible reasons:
It's in the door zone
I've written about bike lanes in the "door zone of death" before, but the point is worth repeating. If you are riding past a line of parked cars and someone opens a door on you, a stripe of white paint isn't going to save you. If I'm not sure that the cars I'm passing are unoccupied, I'll be keeping my distance.
Debris and other dangers
A bike lane can be a repository for all manner of items, especially when it's on the edge of a roadway. Other hazards include drain grates, potholes and slippery manhole covers.
Bike path fails
All bike ways aren't created equal – and some are a lot more unequal than others. I've been on bike conduits with poles, signage, pinch points and other challenges that significantly impede one's progress. Some cyclists still prefer these routes to sharing space with cars. Others – not so much.
Several routes I ride are flanked by off-road shared paths for cyclists and walkers. These conduits are great for many purposes – utility cyclists, cautious riders, parents escorting children – but with uneven surfaces, tree branches and regular intersections, I stick to the road if I want to go fast. And I'm sure pedestrians would agree.
Life in the slow lane
On separated cycle ways, bicycles are sometimes given the lowest priority by traffic signals – just a few seconds to scamper across an intersection. Some riders would rather avoid the bike-specific route and get more green light.
It's not a bike path
I've read online comments where people have complained that riders weren't using bike facilities in certain areas - but when I checked out the places mentioned, found that the infrastructure wasn't actually designed for cyclists.
Unthinking or inconsiderate cyclists
Some riders might use the roadway even when there is an acceptable alternative that would also get them out of the path of speedier vehicles. There are unhelpful people using every means of transport.
What about the law?
Australia's states and territories can have different rules for cyclists – you can see some useful links to relevant jurisdictions here. Rules about bike infrastructure are complicated and laced with legal definitions.
Chris Carpenter of Bicycle Network told me that riders are not obliged to use "off-road bicycle paths, separated footpaths or shared paths".
Meanwhile, a spokeswoman for VicRoads told me that "Road Rule 247 requires cyclists to use on-road bike lanes where they are provided, unless it is impracticable to do so", and I received a similar response from NSW authorities.
Of course, the issue of it being "impracticable" to use a bike lane is a key consideration in some of the scenarios I've described above.
But there's another pertinent issue - the markings of the lane.
Knowing what's what
As Carpenter explains, in Victoria "the guidelines require such lanes to be marked, of a certain width, to be signed, to have logos". Legally, there is a requirement for a 'Start Bike Lane' sign and an 'End Bike Lane' sign.
"Although a bike lane can have bike symbols in it, the presence of bike symbols does not mean there is a bike lane as legally defined. Bike symbols are sometimes placed on what is called the 'wide kerbside lane' - a traffic lane where there is sufficient space for bikes to pass to the left of cars. The use of the symbol in this way is to advise drivers that they may encounter cyclists on this road."
A Transport for NSW spokesman told me: "In NSW, we have shoulder lanes, bicycle lanes, bicycle paths, separated paths and shared paths and different road rules apply to each."
In other words, it can be difficult for road users to be sure of the status of bike infrastructure. How often do you look for the words "lane" and "end lane"?
Path or lane?
For example, the separated cycleways in the Sydney CBD are designated as "bicycle paths", not "bicycle lanes" - riders aren't obliged to use them.
I ride on several bike-signed routes when commuting in NSW, and it turns out none of them are actual "bicycle lanes".
This year, Queensland cleared up any possible confusion in their jurisdiction.
The mandatory requirement was removed, a Transport and Main Roads spokesperson told me, so that cyclists can now choose whether or not they want to ride in a bicycle lane.
The change was made "to improve the safety of cyclists as in some situations, riding in a bicycle lane may be dangerous for cyclists", the spokesperson said, citing issues such as parked cars and potholes. "In the same way that a bus is not required to stay in a bus lane, a cyclist should be allowed to ride outside of the bicycle lane."
Of course, laws are the framework of our society – but courtesy and commonsense should be the glue, and we certainly need more of it on our roads and footpaths. Road rules can be complicated, but focusing on safe behaviour should always be the priority.
So what is your experience? Are there bike paths you find impracticable, and for what reason? Let us know in the comments section below.
(Comments will be carefully moderated and only those addressing the topic will be considered for publication.)
Fairfax journalist Michael O'Reilly has written the On Your Bike blog since 2011. He has won a Cycling Promotion Fund media award and is a regular voice for cycling on radio and television.