Sunday, 11 September 2016

Space Delusion

Thinking of road transport (in the broadest sense), how much space does each mode take up. The answer is a little bit more complicated that you might at first imagine.

Take a human standing in the street. How much space does this person take up? Perhaps a square with 600mm sides (0.4 square metres)? What about a parked bicycle, or a tandem or a cargobike? Perhaps a space up to 2.3m long by 0.9m wide (about 2.1 square metres). A car will generally fit into a space about 4m long and 1.8m wide (7.2 square metres). A bus on the other hand is about 12m long and 2.4m wide (28.8 square metres).

OK, let's look at "static person density" for these modes. Pedestrians have a static person density of 2.5 people per square metre. My "standard" cycle gives about 0.5 people per square metre (tandems give 1 per square metre). A car can carry 0.14 people per square metre for single occupancy and just under 1 person per square metre if 7 people are being carried (although 5 people tends to be be maximum - 0.69 people per square metre). Buses are interesting. If we have 80 people on my example, we are up to 2.8 people per square metre - with 14 people on this bus, we are about the same as the cycle. 

What about when those transport modes start to move? It becomes far more complicated as we are dealing with the "dynamic envelope" of each mode; that is to say we need a bit more width as  people don't (and indeed cannot) walk, ride or drive in a perfect straight line. We also need to think about stopping distances which increase the faster one goes - a person ambling along on foot can stop pretty much immediately and a car being driven at 70 miles per hour needs 96 metres within which to stop (a person driving 20mph still needs 12 metres to stop).

The movement of traffic (and in this I mean walking, cycling and driving) means we start looking at capacities. For pedestrians, we are looking at (approximately) 13 people per minute per metre width of footway as the upper limit of being comfortable (Gehl, p34). For a 2 metre wide footway, this is about 1,500 people per hour; it will actually be less because people don't like walking right next to the kerb (more so where traffic is fast or heavy) and clutter can reduce the effective width.

For those cycling, it gets a little more complex because (and I'm not going to detail every option). A 2m wide cycle track will permit around 150 cycles per hour and a 3m track, up to 750 cycles per hour (CEoGB). For two-way tracks of a decent width there is the ability to squeeze a little more flow (especially where flows are tidal), but the numbers may not feel particularly comfortable to cycle within - an unscientific count in the week on Blackfriar's Bridge in London saw over 5,000 cycles per hour (well OK, if you scale it up!). 

For motor traffic (including an element of HGVs and buses), the picture is just as complex and depends on the situation. Motorways and trunk roads generally have wide lanes and little interaction with external factors (crossings, bus stops, oncoming traffic!) which means capacity is highest at around 2,000 vehicles per hour per lane - it's actually less per lane with 3 or 4 lanes as the outside lanes tend to be faster and so there are longer gaps between vehicles. On a typical high street you might be around the 1,000 vehicles per hour mark (both directions added), rising to perhaps around 1,500 vehicles per hour (both directions added) on a wide suburban link road with some side roads and the odd pedestrian crossing. 

People moved per hour by car is (unsurprisingly) complicated. 62% of driving trips are single occupancy (UK Government, p20), although this is far higher for commuting and business trips (the current data is buried, but it used to be reported and was over 80% in 2012 - UK Government, p22). So in terms of people moved per hour, it's going to be a bit higher that on the commute and a reasonable bit higher at other times. Of course, off, peak, the roads are less busy and so we must remember that I am talking about maximum theoretical capacities here! 

I should mention buses of course. If we have a 10 minute service, we will have 12 buses per hour (both directions added) and with our 80 seater example filled up, this is 960 people per hour. Of course, this will be on the busiest section of route! With a 7 minute service (this is in a very dense city place) we could be up to 1,120 people per hour.

The thing which limits capacity for everyone is at junctions. On a main road carrying high flows, it is hard for people to get out of side streets or to cross roads and so traffic signals can be provided. Because of the competing movements (phases), we have to deal with conflicts by gathering phases which don't conflict into stages which all get green time and when the stages have had their turns, we have a cycle (my thanks to Oli of Urban Movement getting the concepts right in my head this week!). For any length of green, we can only get a certain amount of people through a junction. For motor traffic, we tend to add lanes to increase the capacity which leads to the horrific amount of space devoted to them at many junctions which squeezes pedestrians and mainly forgets people cycling. Remember, we use traffic signals almost exclusively to regulate the flow of motor traffic. It's a subject I will return to in the future.

What I have gone through can be hugely complicated as we are trying to look at both static and moving capacity at the same time. Static because we need to think about parking for vehicles and cycles (not pedestrians, unless we park them on a bench). Car parking is more than the size of the vehicle. If it's parallel to the kerb in the street, we need to add a bit at the front and back to get a car into a space - 6 metres does most people comfortably. If we are going to regulate a parking space, we need parking signs every so often and those generally mean a post placed in the pedestrian's space, or rather this is pedestrian space taken away. There is a push for conversion to electric vehicles (EVs) and for EV-friendly spaces, we are talking about a charger at each space (or serving pairs of spaces). That's half a metre or more gone from comfortable pedestrian space. For the same space, we could provide parking for 10 bicycles and not take any pedestrian space - non-standard, cargo and adapted cycles are a little more space hungry.

In terms of the movement of people it's also complicated, but in general terms, walking is the most efficient use of space over a short distance for moving people. Cycling is efficient for those medium distance journeys of up to 5 miles (more if the infrastructure is really good and even more with e-bikes). Buses are less efficient than cars in terms of people per hour, but given that they don't run nose to tail like cars (we don't have 20 second bus services!), they can move plenty of people over a range of distances, including people who cannot or don't want to cycle or walk. So at the bottom of the pile on city movement, I'm putting private cars. In theory, they are an easy door to door option, but they take up so much space and have led to the type of space-hungry junctions which marginalise walking and cycling.

From a parking and movement point of view, we don't talk about putting walking, cycling and buses in that order for a user hierarchy for no reason; they are efficient uses of city space and trying to put private cars at the top is completely illogical. From a parking point of view, motor vehicles take up significant amounts of street space and this is space which could be used for moving people by foot or cycle. It's a complex picture to be sure and the dynamic changes as one leaves the urban cores for the suburbs and beyond, although cars tend to be a popular choice simply because buses are infrequent and expensive, and cycling is scary as hell. But it reinforces the fact that we need to get private vehicles out of city centres because we don't have the space to move or park them. As a consequence, we can free city centre space for walking and for the longer distances we can use cycles and buses. If this becomes attractive, then we can extend the infrastructure out to the suburbs.

As mentioned above, there are people touting EVs as the future (some may even drive themselves), but as they take up exactly the same amount of space and have the same limits on capacity and many will continue to have one person sat in them, it is a physical impossibility as a city centre transport option. This is the delusion of space.


  1. The other main reason that buses are not used nearly so often as they should is because of a lack of reserved lanes with good quality enforcement that continues through junctions with independent signals, which would offer time advantages over the car. They also tend to be less direct than they should. Cars should be made to go on expressway style 70-100 km/h urban through roads that bypasses most of the residential and business areas while buses can go straight through. Adding bicycle parking and bikeshare scheme locations to the stops and they become even better.

    I also like the electric vans and buses or at least hybrids that depend on electricity and biofuel (canola, vegetable, hemp, whatever other plants, as the fuel). They do really need to be in the urban area, unlike most cars.

    And there are several ways to get rid of traffic lights. You can turn the junction into a roundabout. If you have bus lanes, consult BicycleDutch on the tram+roundabout in Amsterdam, or you can end the bus lane just before the roundabout to mix into general traffic. You can also turn signal controlled pedestrian crossings into zebra crossings and occasionally informal crossings by narrowing the road to one lane per direction (counting parking), lowering the speed to 30 km/h, and add measures like a raised table or a central reservation if possible. As for normal junctions, limiting the road to one lane per direction plus turn lanes for the turn accross traffic (left turn in the NL, right turn in the UK) and or a central reservation with marked zebra crossings and no signal works well, if you can make it a T junction, even better. If you have to have signals, they don't have to be on all day, they can go to flashing amber mode at night and sometimes during the off peak as well along with give way and priority road signs in their places. Even when signal controlled, you can use more independent signals and fully actuated signals with no green waves for anyone with different priority networks for pedestrians, cyclists, motor traffic and public transport like in Den Bosch to help out.

  2. "Cars should be made to go on expressway style 70-100 km/h urban through roads".

    We did that. We destroyed the very thing we were trying to save.

    1. OK, bad context. I meant things like ring roads around urban areas, less so through them. A couple urban roads can be 70 I suppose, Assen NL has one, but it's no motorway, just a slightly bigger distributor road, sometimes with 4 lanes and a median. But I don't want motorways in the city centre. Sorry for the bad wording.

  3. It looks like you're using 150 & 750 b/hr for 2m and 3m wide cyclepaths as the upper limit of capacity, when these are actually lower limits for determining when the path needs to be wider. IIRC Botma and Papendrecht measured flow rate of 2.5m path to be about 9,000 b/hr.

  4. While I agree with the conclusions I would qualify the statement "Buses are less efficient than cars in terms of people per hour" as this is only true because for economic/political reasons we don't run them nose-to-tail like cars. From an engineering perspective the maximum theoretical throughput at 30mph would be with each bus separated by minimum safe headway ( about 3 second gap at 30mph ). A 10m long bus would take about 1 second to pass at 30 mph. So theoretically we could have a bus every 4 seconds ( call it 5 for ease of calculation ). That's 12 buses a minute, 720 per hour which equates to 57,600 people per hour. Obviously at this interval it is the dwell time at stops that predominates. I'm no bussist ( I think they don't mix well with pedestrians or cyclists) but I don't want to give the car lobby an opening to say that cars are more efficient than buses; from a road use perspective they aren't.