The Wrong Side of my Car
The blog that wants to go obsolete
How much do we get out of that 8th lane?
The previous post was about whether or not it is the right time to liberate the lane. One thing that was mentioned was that the average lane on the harbour bridge is used by about 10,000 people.
However not all lanes are created equal. How much capacity do we lose when switching to 7 lanes?
When looking up some numbers for the previous post I came across this graph on Greater Auckland.
The interesting lines on this graph are the thick grey one, the estimated capacity, and the blue line, the measured traffic.
The southbound peak capacity is a bit lower than the northbound one. I think bottleneck southbound is at Onewa Road: there is only three motorway lanes plus one lane from the Onewa Road on-ramp, and the busway. Not quite five lanes.
Northbound the bottleneck is the bridge, you can see traffic queued up on the foreshore to cross the bridge.
You can see the blue line just below the grey line at times. Having commuted counter-peak before, I can 100% confirm that between 3pm and 5pm those 3 lanes on the bridge are chocka during that time.
Minus one lane
So, what would happen if one lane is changed to carrying bicycles and pedestrians? The peak setup would presumably be 3+4 lanes instead of 3+5. But also we can no longer have 4+4 interpeak. Let’s assume the barrier gets moved once around noon. This will probably reduce the traffic to just below our new grey line.
So the chopped-off areas are painted red. And it doesn’t look too bad. It seems that 8th lane doesn’t buy us that much capacity after all. By my (ahem) rough eyeballing it would be about 1,600 movements southbound and 2,900 northbound. That asymmetry is because the southbound capacity is already constrained by the approach.
Wait — how do we not get 1,300 cars stranded on the North Shore every day? (well it would maybe be a good thing for the city but we digress)
Probably the northbound peak will flatten out a bit, similar to the southbound peak right now. In other words we get a somewhat longer evening peak. Let’s call it 2,000 less movements. That would be both southbound and northbound. In reality some of the removed capacity will also be absorbed by the bus network.
Plus one lane
Now that we have one lane, we can use it for walking and, probably mostly, cycling.
If we get 4,000 cycling movents per day across the bridge, the operation did not cost us much capacity, it would only have shifted some people to another mode of transport. (car occupancy is in reality a bit larger than 1.0, but the difference is small enough that today we will ignore it).
Would that amount be possible? Not right now — it is about three times that of the currently much better connected northwestern cycleway. But it is not far-fetched either. Bike lanes are easy and cheap to build so this can change practically overnight.
And a serious investment in cycling could easily get us a much bigger increase. The inner west has a cycling mode share of about 3 to 4 percent. There are cities with literally 10 times this bicycle mode share. 14,000 movements per day? You will never get that out of that extra car lane.
Congestion
But what about congestion? Won’t it make congestion much worse?
And as things stand now, the answer is yes, kind of.
I mentioned my experience driving southbound in the afternoon. Traffic will be backed up, often well past Northcote, basically waiting in line to cross the bridge. Traffic bean counters may call it ‘at capacity’, but lay men call it ‘congested’. Look again at that second graph. Things will be ‘congested’ from 12pm, right from the moment the barrier gets moved. Oops. And on the other side, it will be the same right until the barrier is moved.
For politicians, the common response to this situation is “oh, shit”.
Even though it is maybe not that bad in reality, perception will be terrible. The harbour bridge is one of those things that always attracts outsized attention.
But the long term plan officially is to have less car trips than now anyway so maybe we should just do it regardless.
You may remember that a few years ago a lane on Nelson Street was converted to a bike lane and that contrary to some expectations the sky didn’t fall. There was pretty bad congestion for a couple of weeks, presumably until some people adjusted and the most dumb mistakes in traffic light phasing were ironed out. However this is a poor example, since Nelson Street and Hobson street both had a lot of spare capacity to begin with.
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