Removing centrelines can be done easily when carriageways are resurfaced, with an immediate saving in capital and ongoing maintenance costs. – Manual for Streets 2
Interesting research was recently published on how removing (or not reinstating) centre lines reduces vehicle speeds. This is a similar conclusion to research conducted in the States over 10 years ago, but perhaps with a different imprimatur and accent this study may be more useful in the New Zealand setting.
The study by Transport for London measured three residential streets: Seven Sisters Road, Harringey (Streetview), Wickham Road, Croydon (Streetview), and Brighton Road, Coulsdon (Streetview). The streets all have a medium density residential context and appear to have a modest role in the transport network including some bus and bike facilities. I’m guessing that there are over 7,000 cars a day on these streets.
As an aside, it’s interesting to see how different the streets and network structure is here compared to Auckland. It’s hard to think of a direct local comparison for these streets. It is much easier to compare our streets to North American cities like Calgary, Seattle, and San Diego.
So back to the study. After removing the centre lines (in cases striped medians, yuck) the following results were measured.
Wow. So roughly speaking by removing the centrelines the vehicle speeds have dropped about 5 kph. Critically, this vehicle speed range is where stopping distances are significantly longer and accident severity becomes increasingly life threatening so even modest speed reduction is significant.
The psychology of the results is discussed briefly, but somewhat inconclusively- “getting into the minds of drivers is not easy”.
A theory is that centre lines and hatching can provide a psychological sense of confidence to drivers that no vehicles will encroach on ‘their’ side of the road. There can also be a tendency for some drivers to position their vehicles close to a white line regardless of the traffic conditions, believing it is their ‘right’ to be in this position. Centre line removal introduces an element of uncertainty which is reflected in lower speeds.
The study doesn’t stop there. Using a control site on a different section of Wickham Road they compared the results to a status quo resurfacing project. The resurfacing project alone increased vehicle speeds by 7.2 kph. This is due to the fact that drivers have more confidence since road surface irregularities are removed and bright new lanes are added. The researchers concluded that for resurfacing projects the speed reduction value should be adjusted for that difference, resulting in an absolute change listed below.
Removing centrelines along residential streets in particular during resurfacing projects slows vehicles more than 10kph. This is something that should be investigated further in New Zealand.
Meanwhile, below is a residential street in Mt Eden/Balmoral. Like the first photo above this one runs in front of a school. I don’t know what the double stripes mean (I got my drivers license from a Wheetbix box), but it seems to be suggesting that cars can overtake?
This situation, like slip lanes, flush medians, splitter islands, and pedestrian refuges is a good example of how road design has creeped into the urban context of street design. How long would it take to identify the residential streets across the city that don’t need stripes? How much money would be saved from stopping the endless re-striping of these streets?
As I’ve written before, the default speed limit of 50kph on New Zealand’s streets is politely, inconsistent with international best practice, and more accurately crazy. One of the standard traffic engineering excuses for not lowering speed limits is that the design of the street influences the vehicle speed, not speed limits, so it’s not worth bothering. I get that to a point. The presence of car parking, lane widths, mature street trees, intersection density and other factors combined have a much bigger influence on travel speed than sign posts. But is there any other profession that would sit back and not actively address the serious safety issues of their own domain?
In the absence of a road rule change, narrowing lanes, adding mature trees, and tightening intersections, I am suggesting that this “do nothing” solution of removing centrelines may be something that can be done to make our neighbourhoods safer and allow more people to get around on foot and bicycle. In a subsequent post I will describe another simple proven technique to lower speeds.
Bringing the default speed of neighbourhood and urban streets to 30kph, both by design and by law, should be a high priority for the world’s most livable city.