Monday, 13 April 2020

Kerb Your Enthusiasm: Forgiveness

It's nice when we happen on something useful without searching for it and over the weekend, it turned out that the UK has deployed a 30° splayed kerb.

Why is this important you ask? Well, in the kerbnerd world, this type of profile is somewhat a holy grail type of artefact - often discussed, sometimes found, but always lost once more (unless you happen to be Dutch).


The photograph above shows a typical Dutch cycle track running next to the footway and crucially, the kerb between the two has a splay of around 30°; that is the sloped face is at 30° to the horizontal. The kerb type is known as rijwielpadbanden or "cycle track kerb". This creates two important conditions. 

First, it enables people cycling to make full use of the cycle track compared with "kerb shyness" created by a vertical kerb. In designing a cycle track, a vertical kerb sterilises width. The Dutch CROW Design Manual for Bicycle Traffic suggests that square edge of less than 50mm will create the need for a 200mm clearance and higher than 50mm will be 500mm (Figure 3.3 on p49 of the English 2017 for those following in detail). Losing up to half a metre is quite a bite out of a cycle track's width.

The reason for the clearance is simply in that where kerb heights are vertical, they create a risk of catching one's wheel and throwing one off - where the kerb height (upstand) is over 50mm, we have the same risk and the added potential to catch a pedal on the kerb.

Second, a sloping kerb means that people can more easily join or leave the cycle track mid-block, perhaps to access shops, premises or parking. This is good for most people, but vital for those who cannot dismount and who would otherwise have to try and find somewhere to leave/ join - a significant issue for some disabled cyclists. Here's a quick video of me cycling up and down a sloping kerb (a bit shallower than 30°) demonstrating why this profile is also known as a "forgiving" kerb.


One issue which isn't resolved in the UK is that of 50mm kerb heights and the use of 30° splay kerbs in terms of detectability for visually impaired people. The only study I'm aware of was under laboratory conditions which seemed to indicate 60mm was detectable by all participants with a 50mm not detected by one person in the test group. But the trial was also fairly limited and didn't consider splay kerbs. We really need to get some decent real-world research undertaken on this issue or some Dutch experience of how this all works.

So, where has the kerb been used in the UK and who manufactures it? My thanks go to Engineer Like a Girl and Toby Wells for the detective work that got us a scheme in Bristol which has used the kerbs manufactured by Aggregate Industries through their Charcon brand. I don't have the full details at the moment, but I'll speak to Charcon next week to get some more information, but the units are based on standard dimensions so they fit with conventional kerb types. Here's the scheme in Google Streetview on the A4044 in Bristol.

With the announcement of the access kerb (also by Charcon) a few months back, we now have two more elements for our design toolbox which takes us very close to being able to copy Dutch layouts. We still have traffic signal differences, but we can run them to protect people cycling and so the challenge is now there to transform a street with all of these tools used at once!

1 comment:

  1. This is good, I don't think the driving public understands the fear and danger that even a small kerb can bring to a cyclist. Approaching a dipped kerb has to be done with care, too acute an angle and the front wheel can go, putting you down hard. Forgiving is a nice concept.

    I don't do twitter, could you please possibly tweet Engineerlikeagirl and recommend the Kryptonite New York Fahgettaboudit chain for her new eCargobike, or the mildly upper end of the Pragmasis range. They look and indeed are rather tough.

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