As Easy As Riding A Bike

The 85th percentile as a tool for improving roads and streets

The “85th percentile” speed is a speed at which 85% of traffic will be travelling at, or below, along a street or road (under free flow conditions). It’s typically associated with the setting of speed limits, and (more controversially) often used as an argument against lowering them, or enforcing limits.

In particular, some police forces have been reluctant to enforce 20mph limits that have been introduced on roads that previously had a higher speed limit, without any changes to the design of the road, on the basis that enforcing this lower speed limit will prove to be too much of a drain on their resources – too high a proportion of drivers will be exceeding the new (lower) limit.

I have to admit I have changed my position on this issue over the last few years. Previously, I had been of the opinion that a speed limit is a speed limit, and that it should be enforced, regardless of how many people are breaking it. That any refusal to do so was effectively a ‘cop out’ (excuse the pun) on the part of the police.

But I think the police (or ACPO) are exactly right when they say

Successful 20 mph zones and 20 mph speed limits are generally self‐enforcing, i.e. the existing conditions of the road together with measures such as traffic calming or signing, publicity and information as part of the scheme, lead to a mean traffic speed compliant with the speed limit.

To achieve compliance there should be no expectation on the police to provide additional enforcement beyond their routine activity, unless this has been explicitly agreed.

In other words, the 85th percentile speed (the speed at which 85% of drivers are travelling at, along a road) should correspond much more closely with the posted speed limit through the kinds of measures the police list here – in particular, the design of the road. Research carried out for Manual for Streets shows that the speed at which drivers travel along a road is influenced by its design – principally its width, and forward visibility. If plenty of people are breaking a limit, that probably tells you either the limit is wrong, or the design of the street is wrong. Something has to give.

And this is the reason I am suggesting that the ’85th percentile’ could actually be a force for good – it cuts both ways. While it can be used to reinforce the status quo, it can also tell us that the design of a road is inappropriate for the posted speed limit.

Take, for instance, a situation in which a residential street with a 30mph limit has that limit lowered to a 20mph limit, without any changes to the design of the street, or to the motor traffic network. Let’s then say that the 85th percentile speed of motor traffic on this street, after the introduction of the lower limit, is much more than 20mph – close to 30mph, for instance.

What does this tell us? It tells us that the design of the street isn’t doing its job. While it might be a good idea in the short term to get the police out with speed cameras, a long-term solution should be to change the nature, character (and usage) of the road so that the 85th percentile speed on it is much closer to 20mph.

So the 85th percentile is an effective way of demonstrating when speed limits and road design are out of kilter. Take, for instance, this 20mph limit on Midland Road in London, running between St Pancras and the British Library – just one of many main roads in London that have, in recent years, had their limits lowered from 30 to 20mph without any change to the design of the road.

I don’t know what the (free flow) 85th percentile speed of motor traffic is here, but I’d be willing to bet good money it is way, way over 20 mph – this is a wide road, with three lanes of motor traffic bearing down on Euston Road, in one direction.

Again, we could get the police out here with speed cameras, but really, the discrepancy between the posted limit and the way people are actually behaving on the road tells us that something more serious is wrong here – the messages the road is sending out to drivers don’t correspond to the limit that has been painted on it. Something has to give.

By contrast, in this early-1990s 20mph zone in Horsham – designed to be self-enforcing – it’s pretty much impossible to drive at 20mph (despite it being one-way!).

A combination of speed humps, tight corners, limited forward visibility and surfacing means that the 85th percentile speed is likely to be at (or even below) 20mph, which tells us that the speed limit and the design of the road are in agreement, and there’s little or no need for enforcement.

The same logic can be applied to 30mph roads too. This road in Wageningen, NL, has a 50km/h speed limit – and it’s reasonable to assume that the 85th percentile speed will be at or below that speed, due to the design of the road.

The carriageway is very narrow, with motor vehicles barely passing each other, and has no centre line.

And there are other ways of bringing the 85th percentile speed into line with a 50km/h (or 30mph) limit on these kinds of distributor roads – for instance, pinch points for motor traffic (that don’t affect cycling).

So if the 85th percentile speed on a 30mph road near you is (under free flow conditions) closer to 40mph, that should tell us that action is needed to bring driver behaviour more closely into line with the posted limit, through these kinds of measures. Principally, perhaps, by reclaiming a good deal of the carriageway for cycling, consequently narrowing it down for motor traffic.

I hope this explains why I’ve changed my mind, and why the 85th percentile can be a constructive tool for improving streets for walking and cycling!