** DRAFT FOR REVIEW BY CPTT, NOT FOR WIDER CIRCULATION ***
Will 20mph limits on main roads reduce traffic capacity?
Categorically, they will not. The easiest way to understand this as follows: a vehicle occupies the amount of space on the road equal to the stopping distance (thinking and braking) needed at the given speed. Given the speed in feet per second, you can convert that to a time window (in seconds) for which the vehicle occupies any given point on the road, and from that you can calculate the number of cars per hour the carriageway can accommodate.
What is that stopping distance? Sources vary: they assume different values for peoples’ reaction time (from half a second to two seconds) and for the braking rate of a “typical” car (can you take it for granted that the car behind yours has ABS and well-maintained tyres?)
According to the highway code book, which uses a reaction time of 2/3rds of a second (a good figure for a healthy but unexceptional adult paying full attention to the road in dry, daytime conditions), the overall stopping distances are:
- 20mph: 40 feet
- 30mph: 75 feet
This number can be reduced by 1/3rd for an attentive, expert driver with an ABS equipped car, or doubled for someone distracted, tired, medicated etc. Wet weather has an impact, and larger vehicles, especially those equipped with air brakes, have longer stopping distances.
- 20mph is equivalent to 30 feet per second
- 30mph is equivalent to 45 feet per second
So, given the above numbers, a car occupies a given bit of carriageway for (40 feet / 30 feet/sec) = 1.33s at 20mph, and (75 / 45) = 1.66s at 30mph. This gives you a theoretical maximum capacity, for the type of single lane main road typical in this area, of:
- 45 vehicles/minute at 20mph (60/1.33)
- 36 vehicles/minute at 30mph (60/1.66)
In practice, however, the ability of the road network to support free-flowing traffic is limited by junctions and traffic lights long before you reach those figures (which is the rationale behind highly cycle-unfriendly “stack” junctions e.g. Church Road / Anerley Hill, where traffic flares for 2 lanes ahead of the lights and then merges back to 1 – this design doubles junction throughput per light phase, never mind if a few cyclists get squashed). 20mph limits have essentially no effect (good or bad) to junction capacity – when traffic is free flowing, one or two more vehicles per phase may be able to get through safely (because they’re flowing much closer together), but the effect either way is likely to be minimal.
Which roads should be 20mph?
We began from the premise that all residential roads should be 20mph, and all main roads 30mph except in very specific circumstances:
- In district / town centres where there are very high volumes of pedestrians, some of whom may want to cross informally (enabling them to do so is generally considered good for business).
- On designated London Cycle Network routes where there is no effective cycle lane, nor any near-term prospect of one, and no realistic prospect of the route being sent down a quieter alternative route – so the road will be expected to have significant volumes of cyclists, and they are required to cycle in the traffic.
- Within 0.5km of a school, where significant volumes of children walking/scooting to or from school will be present. Children under 13 are known to be unable to judge vehicle speed/distance well – this makes it difficult for them to use even a Zebra crossing safely unsupervised, where traffic speeds are much above 20mph.
However, once all these things are mapped, there’s realistically very little 30mph left in Thornton Heath, Upper Norwood and South Norwood. Even using a smaller 250m radius around schools doesn’t change that very much – instead, it creates the kind of broken-up 30/20/30/20 short segments that are guaranteed to frustrate and annoy.
Upper Norwood, South Norwood and part of Thornton Heath wards. LCN route without viable alternative or feasible segregated route in orange; district and local centres in green, school 0.5km radii in blue.
Will 20mph on main roads affect journey times?
In general, very little or not at all. The difference between 20mph and 30mph is just one minute per mile, and the distance between the northern end of Wellesley Road (a road which is clearly not suitable for a 20mph limit, along with central and south Croydon’s other large multi-lane roads) and the northern tip of the borough (the border with Lambeth at the top of Gipsy Hill) is just three miles – giving a worst case additional journey time of three minutes. In practical terms, though, much of a journey is not made at full speed – any distance which is already covered at less than 20mph will be completely unaffected, and any covered between 20 and 30 will be affected proportionally less. This means that most daytime journeys of three miles will be affected by less than 90 seconds – and those which are affected will tend to be the quicker-than-average journeys where the time you’d save is time you’ve allowed for the journey anyway.
How should 20mph limits be enforced?
We suggest a gradualist, education-first approach towards enforcement. A camera-based enforcement mechanism is preferred, but during the early operation of the scheme should trigger only a letter to the vehicle’s registered keeper, educating them about the limit and cautioning them as to the consequences of not keeping to it. After a few months of operation, repeat offenders should be sent on mandatory speed awareness courses, with fines and points reserved for those who fail to attend.
20mph – good for cyclists
A speed limit of 20mph is a vitally important safety measure in situations where cycles and cars are expected to share the road – that is, on minor roads which do not typically have cycle lanes, and tend to have limited overtaking space, and on main roads which have been designated as part of the Cycle Network but do not provide a functioning cycle lane. (A cycle lane in which car parking is permitted is not a functioning cycle lane). Adult cyclists typically have sustained average speed of 12-20mph. This gives a difference in speed of 8mph to 0mph. To put it another way, cars approaching from behind close at, at most, a fast jogging pace; this gives both parties far more time to react. At 30mph, the closing speed with a slowish adult cyclist is 18mph, allowing half the reaction time for either person.
A further benefit, for faster cyclists capable of sustaining 17+mph, is that drivers should no longer have any real need to overtake, which in turn allows such cyclists to ride in primary position (i.e. centrally in the carriageway) at all times without causing undue inconvenience. Primary position is often the safest place for a cyclist to be – they are easier to see, and it allows more room to avoid car doors, potholes, drain covers and other obstacles; and being further from the kerb, gives them more time to react if a pedestrian steps off the pavement or out between parked cars. One of the most stressful things cyclists experience regularly is a close pass overtake, and widespread 20mph limits help make this a thing of the past.
And if somebody does make a mistake, or suffers a mechanical failure, the consequences are likely to be less severe – see “Fairer for everyone” below.
20mph – good for pedestrians
20mph – good for drivers (no, really)
Yes, really. Although some journeys will indeed take a bit longer – a whole two minutes, in some cases – what typically matters most is not “best” or even “average” journey time, but those bad days where everything takes three times as long as normal. Journey times on the roads are pretty unpredictable, and that’s why most sensible people allow twenty minutes for a journey that usually takes fifteen. 20mph limits reduce serious collisions – and serious collisions result in the road being closed for an hour or more (3-4 hours in the case of fatalities or life threatening injuries) while the necessary investigation work happens. Reducing these serious crashes means the major delays and disruption they cause will happen less frequently. And while the all-too-common minor prangs and scrapes may not cause you much delay on the day (none at all, if you’re parked up at the time), that busted wing mirror or indicator lamp is going to eat a good couple of hours of your life getting the wretched thing sorted out.
20mph – fairer for everyone
People make mistakes. Whether behind the wheel, on foot, or on a bike, people are fallible, and this should come as news to nobody. However, when somebody makes a mistake on the road – whoever they are – it’s almost always the more vulnerable party that comes off worse. And yet, the benefit of being able to travel at high speed is almost always enjoyed by the less vulnerable party. There are exceptions to this, especially relating to motorcycles, but in general: the person who gets to enjoy the benefit of traveling above 20mph is the one who’ll walk away unhurt from a collision – regardless of which party was at fault. Reducing speed lightens the burden of injury which is mostly borne by those who don’t get to benefit from moving fast in the first place.
Are there any other reasons not to adopt 20mph on main roads?
An argument put forward by some people is that making residential roads 20mph, while keeping main roads at 30mph, will help reduce “rat running” on the minor residential roads. While there may be a grain of truth in this, most people rat-run in order to avoid congestion. Even at 20mph, rat-running is still quicker than using the official route, if the official route is congested. It’s preferable to establish an area-wide 20mph limit and have the rat-runners stick to it too, than have people turning off 30mph main roads on to 20mph side streets and probably not dropping their speed. If the council is serious about stopping rat running – which we believe it should be – more robust measures than 20mph are needed. Some such measures that should be considered are banned turns, modal filters (closing roads to through traffic, but allowing cycles and in some cases buses through), electric bollards activated by a residents’ keyfob, 10mph limits together with shared-space style resurfacing (Dutch “woonerf” model), but we do not believe that 20mph limits will significantly reduce the amount of rat running in the area.