By Brendan Moyle 11/02/2020

Once again the Aotearoa Bike Challenge is on us, and once again I’m participating. In principle this is not hard because it does not represent a significant switch in behavior for me. I already bike nearly every day.

The Bicycle

The fascinating thing about cycling is the realisation that this is not an archaic form of transport. The bicycle is an incredibly advanced machine that can easily transport many times its weight. The basics of the road bicycle was settled in the 1880s and 90s. This is a diamond-shaped frame, a crank (the bit the pedals attach to) that turns the rear wheel via a chain, spoked wheels, and pneumatic tyres. And that design is basically brilliant. That’s largely why the improvements to it have come largely from better materials and more efficient gearing. Fundamentally, we still ride an A-shaped frame, on pneumatic tyres, with the wheel turning via a crank mechanism powered by pedals. The other great thing about a bicycle is it is cheap to run. A lot of repair and maintenance can be done by the cyclist.

The Urban Cyclist

Anyway, what sort of things should we be aware of on the road? Well, one is the dreaded ‘close pass’.

In principle, a motorist should give a cyclist a 1.5 metre space when they overtake. If it is not safe to overtake the cyclist, then the motorist hold back, wait and pass when it is safe to do so. Of course, in Auckland the odds of someone giving you 1.5 metres space or waiting is pretty low. I mean it does happen and I’m grateful to all motorists that do so. But it is not really a common thing. See the example below.

Close Pass- the wide angled lens on my bicycle does stretch horizontal distances in the frame. I also stick out a bit further to the right. We are close (<30cm). The cables crossing center of my bicycle’s stem shows how far to the left I am. 

The Risks

The moment a motorist drives closer than 1.5 metres they put the cyclist at risk. Which given they have a 1.5-2 ton car a lot of the time, isn’t optimal. The level of risk however from a close pass depends on several factors. One is the ability of the cyclist to maintain their line. That’s not always easy as the side of the road accumulates debris or uneven surfaces that can throw you off your line.

The three big risks however come from the mass of the vehicle, the speed it is moving, and how close it is. As a rule of thumb, the faster a vehicle is going and the bigger the vehicle is, the more clearance space is needed. A small vehicle moving cautiously around you at a slow speed is not a big risk. The problem is turbulence. Air weighs about 1.3kg per cubic metre. This means air resistance increases exponentially as your speed increases. This after all why cyclists adopt low aerodynamic positions or use ‘aero’ designs for their bikes. That mass of air you are moving into provides more and more resistance. It’s weight is dragging at you. (This is also why fuel economy for cars is much better in the 80-100km range than the 100-120km range.)

So if a large vehicle is barreling down a road at a high speed it’s going to be hitting a lot of wind resistance. And as the vehicle makes its way through the air, all that air has to go somewhere. and if a cyclist is beside the vehicle, a lot of that air (at 1.3kg per cubic metre) is going to pushed into the path of the cyclist. And that makes the close pass very dangerous. It is now much more challenging to maintain a straight line. And if you deviate, you can crash. There is a good chance that deviation will be in the path of the vehicles around you. Which is not optimal in the sense it can be suddenly fatal.


Here’s a short video clip to try to illustrate. Every risk factor is multiplied here. The truck beside me is on a dual carriageway and there is space to give me a bit of clearance- the adjacent lane is clear. This does not happen. For context, note that my shoulder/elbows are pretty much on the edge of the white line. We are very much within the 1.5 metre clearance space.

The slow motion clip gives a clear idea of my response. I do a controlled brake (if I’d panicked, braked too hard that could have thrown me off my line) and focus on staying on that line while the air is hitting me from the front. One of the risk factors for the cyclist is the speed you’re moving at. At 40-50kph you don’t have a lot of reaction time.

You can see I have a slight wobble even with the brake. Note that the air the truck is pushing through has another low-resistance space to flow through. That’s under the truck. We are so close, the wind is trying to suck me under the truck’s wheels. So I have wind to my front trying to knock me over whilst the wind to my side is trying to suck me under the wheels. It is a very disconcerting experience.


In short, passing closer than 1.5m puts a cyclist at risk. Motorists can reduce that risk by giving as much space as possible and at a prudent speed. Also it’s pretty terrifying having large vehicles pass you well within 50cm.

0 Responses to “Auckland Bike Life”

  • I’m a big fan of direct action targetting n these situations. A spray can of dazzle colour paint (pink works well) can quickly change a driver’s understanding at the next set of lights. They fit nicely in a drink bottle holder too…

  • I am getting more and more pissed off by cyclists who come up on the inside and overtake – or undertake? When challenged, they say the road is too busy to go on the outside. That is why we need bells.

    Nothing affects the “ability of (this) cyclist to maintain their line” like a bike appearing 30cm to your left.

  • Hi Don- I note your response does not seem particularly germane to motorists engaged in close passes and trucks sucking cyclists under their wheels.