Car bloat (or Carspreading) describes how new cars are being designed increasingly bigger, and how oversized ‘SUVs’ have come to dominate our roads. 

The charity ‘Clean Cities’ have recently run an extensive campaign on the range of harms this causes, and have a fantastic website here, which we fully endorse.  We can’t cover this nearly as well as they have, so we won’t repeat the same information.  But we urge our readers to spend some time having a look at their site.

This blogpost is not about the obvious and avoidable danger these excessively large vehicles pose, or about the extra resources they use, or the pollution and harm they cause to the environment.  It is about how carspreading is causing congestion, snarling up Hackney’s roads and delaying buses. 

We at Low Traffic Hackney focus strongly on evidence, so it has sometimes been puzzling for us to hear some people’s lived experience of congestion does not always reflect the hard data of trends in number of vehicles. 

We previously wrote a blog post on evidence and why we have to be careful how we interpret ‘lived experience’.  But what is going on here?

We must start by saying the causes of main road congestion are complex and we don’t pretend there is one simple reason to explain it.  But we do think changes in vehicle size may be having a very significant role on congestion in London.

How car size has changed over time

Have a look at the image below to see how car models have grown over time:

How SUVs cause more congestion

Length:

The first and most obvious point is that longer cars mean quite simply any length of road can hold fewer of them.  Fewer can move across the space at one time and so the flow of vehicles per minute is lower. 

At junctions fewer vehicles can get through on each cycle of the lights meaning tailbacks are more likely to build up.

A recent study examined this effect in Minneapolis and found between 1995 and 2019 the number of larger vehicles increased by over 150% and this resulted in a 13.5% decrease in the average ‘throughput’ of vehicles (the number of vehicles per lane per hour) in the city network. 

Another study in Ireland found longer vehicles reduce road capacity in general and junction capacity in particular.

Longer cars reduce capacity and mean more congestion and delays.

Height and 'Dynamic Envelope':

Then there’s the effect of height.  Larger vehicles increasingly have higher bonnets.  These obscure the view to the road directly in front, which is a huge safety risk, but also means the tendency is for drivers to leave greater empty space between each vehicle so they can see the road.  The technical term for this is needing a larger ‘dynamic envelope’. The result: A further reduction in the capacity of the road.

Width:

The fact that longer cars use more roadspace is fairly obvious, but an equally significant issue is width.  Almost all modern cars, and particularly SUVs, are much wider than older cars.

London’s roads are old, often narrow and the usable width is further reduced by car parking.  Some SUVs don’t even fit in standard parking bays and project out further into the carriageway (such as the one in our first picture above).  The effect of this on many roads is to reduce the available space and ability of vehicles to pass, especially when two larger vehicles are going in opposite directions at the same time.  One vehicle must wait causing delays and build-up of traffic behind. 

SUVs Delay Buses

Buses particularly are becoming stuck waiting by parked cars because there is now insufficient space on many streets to pass with oncoming traffic.  TfL usually seek a minimum lane width of 3.5m to run bus routes, and whilst these exist in many places in theory, the useable width on existing routes is becoming increasingly lower over time as wider and wider cars park on these streets, narrowing the usable space.  The effect is insufficient space to efficiently operate the bus, more waiting for the opposite lane to clear in order to fit the bus through, and tailbacks of cars developing behind the stuck bus. When it clears they all hit the next junction at once, making things worse.

At narrower side roads, where cars could previously turn in and out of the side road simultaneously, now the lack of space means the car on the main road often has to wait, blocking the road behind until the side road is clear to turn in to.  Two large vehicles often now means insufficient space for both to turn at once.

A recent article in The Guardian showed London's slowing bus speeds, with the worst affected areas in outer and west London, where SUVs and huge cars have become increasingly popular.

How bigger cars cause more road damage

What about weight?  SUVs are much heavier than smaller cars. 

Well, road damage is proportionate to the fourth power of axel weight.  This means larger heavier vehicles are disproportionately harmful to road surfaces.  In simple terms doubling a vehicle axle weight results in 16 times more damage to the road.

The resulting epidemic of potholes may sometimes directly cause congestion as cars brake and drive unevenly to navigate them, but certainly we can all see the increasing need for roadworks and resurfacing to fix potholes causes chaos and congestion.

The psychology of sunk costs

Then there’s the psychological factor.  SUVs are hugely expensive. Often twice the cost of a smaller car that would be equally as suitable for the vast majority of people.  This leads to a ‘sunk costs’ motivation.  If you’ve spent £50k+ on an SUV, or you’re shelling out £500/month to lease it, then you really want to drive it everywhere, otherwise why bother? Higher fixed costs with minimal additional cost per mile generally means a greater incentive to drive more miles.

Who Gains?

Cars are rarely used to their maximum space capacity.  TfL research shows the vast majority of car journeys (59%) are single-occupancy with 85% of journeys having one or no passengers.   However, car makers make more money from SUVs, which is why they have been pushing them so hard for years.  Society in general pays the external costs of this.  These costs are not just to bus users, pedestrians or cyclists: A key point to remember is carspreading makes roads worse for drivers too.  

What can we do locally in Hackney?

It may feel like the trend for larger vehicles is a force that can’t be controlled locally, but there is a great deal Hackney Council could do to tackle this problem. 56% of traffic in Hackney is local meaning their journeys start or end in the Borough. 

Hackney has control over local roads and parking and these are very powerful tools to influence choice.

We would like to see the following applied:

• Parking charges graded by vehicle size, with punitive charges for the very largest domestic vehicles
• The Council to simply refuse to issue on-street parking permits, or to allow parking of vehicles on public streets, that do not fit into a standard car parking bay
• Hackney Greens proposed a ‘road safety premium’ (road user charge) for larger vehicles and we support this in principle (although with the big ‘if’ that this requires TfL approval)
• Complete removal of parking at main road pinch points. Removing all parking from bus lanes and properly enforcing compliance.
• Bus prioritisation measures such as bus lanes and bus gates
• Low Traffic Neighbourhoods to reduce turns to and from main roads by rat running vehicles, which cause delays on through-routes for others

If nothing is done about car bloat then the trend will continue.  However, like every aspect of urban transport, harms come from policy choices; they are not inevitable.

Car bloat exists because bigger cars have proven more profitable for carmakers, and because authorities have failed to act to stop harms.  This can and must change!

Join our campaign to help us fight for safer, more inclusive and more efficient roads.