Slip lanes: slipknot?

A spotter’s guide to some common feature of our towns. Here: the “free left turn” or slip lane.

When we’re driving we love ’em: the “free left turn” feels like a free pass. Don’t have to queue with the straight-ahead traffic at the intersection, or suffer with those poor turning-right people. No, you have a slip lane: just whip your head to the right and be ready to accelerate – you can probably go, wheeee round the corner!

Here’s some local examples.

Cobham Drive / Evans Bay Parade, Poneke
Brora Crescent / Papakowhai Road, Porirua

But slip-lanes are pretty rough if you’re there being anything other than in a car.

“Making it simpler for motorists”

Not being an engineer, I googled slip lanes. The following from NZTA’s guidance on slip-lanes seems… a little bemusing. They say approvingly that one of the reasons you would want a slip-lane is:

Decision-making processes for motorists are generally simplified, resulting in a safer intersection layout.

This is some … ah… interesting causal logic implied here. It doesn’t seem to stack up.

The following are some “anecdata” from my personal experience as a driver. (Note: I’m pretty average, so if someone can show I’m uniquely stupid that would be good news. I figure I’m pretty representative.)

When my driving environment is simplified I tend to pay less attention to the task of driving. I believe I can make decisions quicker, because everything’s more straightforward.

The result: when turning left with a nice slip lane, I feel I needn’t apply my whole brain to the situation. While turning I can keep holding my drink with one hand. While turning I can keep singing along to the stereo (and trying to remember the words). And while turning I’m also looking away from where I’m going: I’m checking right to see if I’m going to get a gap in the traffic oncoming from my right, because you bet I’m poised to gas it to get in ahead of them. Wheee!

What I’m not doing is paying so much attention to who or what might be just ahead of me in my lovely slip lane.


[Update: this kind of distraction is, depressingly, by far the norm. Check out the fascinating research on our driving minds from Burdett / Charlton & co at the University of Waikato]

“WHEEEE” for driving, “EWW, AARGH” for walking

Auckland traffic engineer Luke Christensen makes the point well :

Cars often signal late, or even not at all. It is very difficult for a pedestrian to judge if this car is going to turn into the slip lane, or keep going straight. “Couldn’t the person walking just wait to cross” you might ask? Well they could, but on very busy road they could be waiting a long time, and it is very easy to misjudge when a car might be turning and when they might be going straight. So, it is very easy for a situation to occur where a small misjudgment by both the person walking and person driving could lead to serious injury or even death.

This type of situation makes what should be common everyday tasks like walking to school, the bus or the local shops very scary. For those with lower mobility, or with young children, these might amount to an impassable barrier. The result of this is that people are encouraged to use their cars for short local trips, or are not able to get to local destinations at all. This is unacceptable if we want to make our city streets safe to walk around, with the all the resultant health, community, environmental and traffic reduction benefits this brings.

(The Auzzies’ rules, interestingly, say drivers have to give way to people walking when using a slip lane. I wonder, is that going well for them? A Victoria Walks legal beagle unwittingly answers that question: Yeah nah.)

And it’s not just impeded, more stressful walking.

This below is what you say you’re happy to have more of, when you make it easier for people driving to pay less attention turning:

The moment where someone driving the slip lane smashes their car straight into a person. NZ Herald Link / video from the photo (beware, the video is a bit distressing)

So why do we build such dangerous things in towns?

Strong Towns’ Stephen Lee Davis sums it up:

It’s important to remember that slip lanes were created to solve one specific set of problems: vehicle speed and delay. 

They were borne of the simple realization by traffic engineers that cars turning [left]—even on a green light—can produce dreaded congestion because slowing down to a safe turning speed can delay traffic travelling straight. So to solve this one problem, they started adding lanes that allow traffic to make [left] turns without being required to slow or come to a stop, often accompanied with an additional lane on the approach or the exit.

(left/rights swapped for NZ context):

Are we just blithely ignorant?

Well, no. Slip-lanes are fine in motorway environments where everyone’s in a car. (And as long as those existing the motorway on a lovely sweeping slip-lane slow down to a survivable speed for the local streets and local activities they pop out into.)

But slip lanes are definitely dangerous in busy urban environments where we really should be prioritising people-movement over vehicle speeds.

Oddly, we seem happy to downplay the downsides or just keep doing ’em anyway.

The official guidance says “While some pedestrians voice reservations about slip lanes, they do remove the conflict that occurs when left turners and parallel pedestrians proceed together” and then a mere 50 words later, “Poorly designed slip lanes can lead to: [first of several bullet points] poor intervisibility between pedestrians and left turners, if the [pedestrian] crossing position is too far around the corner”.

Translated: “In a perfect world, slip lanes would be well designed” and would cause less harm….

Sorry but so many of them are poorly designed, especially on intersections with “arterial” roads like the below. (And, let’s note, any major “arterial” people propose to put through your town to “fix the traffic”, because the designer will prioritise traffic flow over your safety walking and not want people driving to slow down around that corner. Sound familiar from any local politicians lately?)

The former state highway in Porirua, Papakowhai Road, a 70km zone. Here, one of several left turn slip lanes.
Good luck, anyone on foot, crossing the side road here (also cycling: apparently the wafer thin sliver on the left is not a footpath, it’s a shared path!)

The official guidance here and the US provides lots of ways to reduce the inherent danger to humans that sliplanes create.

But why not just get rid of them in busy urban areas?

Auckland is steadily getting rid of them in places people should be comfortable getting around as “fresh (not canned) humans”, and so are lots of other towns. There are some nifty retrofits being used, quite cheaply – see Strong Towns. And here’s an intersection on a big arterial – Beach Road in Auckland.

After. Much, much safer and less stressful for fresh humans

If you need any more convincing, this excellent thread from Transportation for America sums it up in one of their themed social media campaigns.

So… in places where there’s more value to everyone from freer and more comfortable walking, scooting, cycling…

Death to the sliplanes so there’s less deathliness from them!

Banner gif credit: Strong Towns

Auckland slip lane and Beach Road photos: Luke Christensen

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