On a recent trip to the supermarket I happened to notice a driver turning into the car park at roughly the same time as me. Obviously this isn’t something you would normally dwell on, but in this particular instance I happened to notice the same driver entering the supermarket building itself some time later – when I already done a good deal of my own shopping.
What had happened? Well, I can park my ‘vehicle’ right by the entrance of the supermarket, something the driver wasn’t able to do.
They had obviously had to circulate around the car park, looking for a space, parking their car, and then walking all the way back to the supermarket entrance.
This set me to pondering on a bit of maths in an attempt to establish just how quick a car is at short trips, compared to cycling. You might think a car will ‘obviously’ be quicker at getting from A to B – after all, it just goes faster. But as my anecdote hints at, the basic problem this straightforward analysis overlooks is… parking.
Cars are big, and difficult to store. That means when you get to where you actually want to go to, you won’t actually be able to get there. By that I mean, it is very, very unlikely that you will be able to park your car right where you want to go to, either because someone else has got there first, or because there’s so much (induced) demand for parking where you are going to it has to be spread out over a large area (or on multiple levels), or because the area you are going to is somewhere that restricts parking altogether, because it’s not very nice when streets you want to visit are clogged up with cars that are either parked, or being driven around in search of parking spaces.
This isn’t the case with cycling; you will almost certainly be able to park exactly where you want to, especially if you have the kind of bike that has a built-in lock (the convenience of which I’ve written about before). So we have to factor in something ‘extra’ into the time taken to get from A to B by car – the time you are walking to or from your car, once you have parked it, to actually get to or from ‘B’.
So I came up with this rough little equation to establish the distances at which cycling time is approximately equal to driving time, adding in the extra walking time involved with driving. It equates cycling time (on the left) with driving + walking time (on the right).
- D is the actual distance from A to B;
- SB is cycling speed;
- SC is car (driving) speed;
- DW is the walking distance, from parking stop to final destination; and
- SW is walking speed.
Now we can plug in some values. If we take cycling speed to be 10mph, walking speed to be 3mph, and driving speed to be 20mph, we get the following –
And with a bit of rearranging, we arrive at –
What does this mean?
Well, it tells us that for our starting assumptions of speed (20 for driving, 10 for cycling, 3 for walking), cycling time is equal to driving (+ walking) time when the walking distances is 0.15 of the distance from A to B.
So – to take an example – let’s say I had to choose between cycling or driving for a short trip from A to B of 1 mile. In this case, if the walking distance from the parking to the destination is 240 metres (0.15 of 1 mile), then I can expect to arrive at the destination at exactly same time if I cycled or drove. If the walking distance is greater than 240m metres, then obviously the bike will be quicker.
For shorter trips the equation obviously tilts further in favour of cycling – for a trip of half a mile from A to B, you’d have to be able to park within about 100m of the actual destination for driving to match cycling.
How realistic is this? I think it’s fairly accurate, and if anything a little generous towards driving, for a couple of reasons –
- 20mph is probably quite an optimistic average speed for driving in urban areas – it assumes no queues or congestion, and no traffic lights.
- the equation doesn’t account for the extra driving time spent driving around looking for a parking space near the destination.
- nor does it account for the actual ‘parking’ time; time spent shuffling your car in and out of a space.
To return to my supermarket example, I think the driver who entered car park at the same time as me probably had a walking trip of around 100m – a reasonable assumption based on the size of the car park. I’ve shown a typical ‘walk’ below, from the mid-point of the car park.
Of course the driver would have to have driven to this spot, and maybe a bit further, circulating around to look for it. That means if we both had to travel half a mile to get here, he would have gained nothing (in time, at least) by driving.
The equation tips further in favour of cycling when we examine ‘as the crow flies’ distance, rather than the actual travel distance, because driving – even somewhere as car-friendly and cycling-hostile as this town, Horsham – tends to involves longer routes than cycling. To take just one example –
Here a short car trip to Sainsbury’s of nearly one mile is significantly longer than one by bike, principally because someone on a bike can use the short cut indicated by the red arrow, but someone driving can’t. The ‘crow flies’ distance here is around 600m; the cycling distance approximates to 900m, while the driving distance is a far less favourable 1400m.
The red arrow is actually an example of filtered permeability – a residential area which drivers can access with their motor vehicles, but can’t drive through. This makes it a pleasant area to live in, and has the side benefit of making cycling and walking trips more closely aligned with ‘crow flies’ distances, compared to driving.
This whole mathematical exercise got me thinking about filtered permeability in a different way. Essentially –
Filtered permeability only ‘punishes’ the kind of car trips that weren’t worth making in the first place.
Yes, filtered permeability will make your 0.5 mile car trip significantly longer, perhaps even twice as long. But that’s the kind of car trip it really doesn’t make sense to drive, because cycling will almost certainly be quicker over that distance, once we factor in the kind of details considered in the maths here. This is, in fact, precisely the case with the example I’ve used above. A car trip from A to B (Middleton Road to Sainsbury’s) would actually be costly in time terms, compared to cycling, even without any filtered permeability in place.
For longer car trips, however, of say 2-3 miles, the effect of filtered permeability is more negligible, perhaps adding only 5-10% to the overall journey time. So filtered permeability is only really a ‘problem’ for driving for those trips that are actually more time-consuming to make than going by bike, or even walking.
Now of course I fully acknowledge that cycling isn’t an option for most people in urban areas because of the hostility of road conditions – indeed, that’s pretty much what this blog is all about. So the kinds of comparisons here won’t work for most people, simply because they have to choose between walking and driving, and the equation here isn’t anything like as favourable as a cycling/driving comparison, because of the lower speed of walking.
This might explain why new ‘filtering’ schemes attract such a great deal of opposition in Britain; it’s because people are driving short trips of under a mile, and because the only realistic alternative is walking. Cycling is the missing piece of the puzzle, one that will unlock the benefits of ‘filtering’ and demonstrate just how inefficient short car trips in urban areas actually are, compared to the alternatives.
Of course to unlock that potential cycling has to enabled, and that means constructing environments that work for all users – protected routes on main roads, and genuinely quiet routes on residential streets, which will involve (ironically enough) filtered permeability. So this is something of a chicken and egg situation – the arguments in favour of filtered permeability rely partly on the benefits of a mode of transport that people aren’t currently prepared to use, and won’t be using until these kinds of schemes are in place.
But I think it is certainly helpful to consider just how painful driving is, in time terms, for short trips, when arguments and discussions about ‘filtered permeability’ are happening.