Do more roads really mean less congestion for commuters?

Do more roads really mean less congestion for commuters?
Do more roads really mean less congestion for commuters?

By Matthew Beck and Michiel Bliemer (University of Sydney).

Congestion is a major source of frustration for road users and has worsened over time in most cities. Different solutions have been proposed, such as introducing congestion charging (a favourite of transport economists) or investing in public transport. One solution that is most often put forward is to build more roads, but does this approach work?

A recent study in the United States identified Los Angeles, Honolulu and San Francisco as the top three most gridlocked cities in the United States. All of these cities use almost exclusively road-based solutions to transport citizens.

While China has increased its expressway network from 16,300 km in the year 2000 to around 70,000 km in 2010, the average commute time in Beijing for 2013 was 1 hour and 55 minutes, up 25 minutes from just the year before.

Why, then, do residents of these cities with large amounts of road capacity, not live in a driving utopia?

Induced demand

The first concept you need to get your head around is called induced demand.

Think about the street on which you live. If a new road makes driving to work quicker, you may benefit from that, but this reduced travel time might be enough to encourage two other people in your street to start driving; and two more people in the next street; and two more people in the street after that; and so on. Very quickly the drive to work takes just as long as it ever did.

In transportation, this well-established response is known in various contexts as the Downs-Thomson Paradox, The Pigou-Knight-Downs Paradox or the Lewis-Mogridge Position: a new road may provide motorists with some level of respite from congestion in the short term but almost all of the benefit from the road will be lost in the longer term.

Further, while more roads may solve congestion locally, more traffic on the road network may result in more congestion elsewhere. In Sydney, for example, the WestConnex may improve traffic conditions on Parramatta Road, but may worsen congestion in the city.

Weakest links

Congestion is determined by the weakest links in the road network. If road capacity expansion does not involve widening of these bottleneck links, congestion may simply move to another part of the network without solving the congestion problem. Moreover, it could potentially make congestion even worse.

The Braess Paradox is a famous example in which building new roads in the wrong location can lead to longer travel times for everyone, even without induced demand, because such new roads may lead more car drivers to the weakest links in the network. The reverse may also be true: removing roads may even improve traffic conditions.

This paradox occurs because each driver chooses the route that is quickest without considering the implications his or her choice has on other drivers. Car drivers only care about the number of vehicles in the queue in front of them and do not care about vehicles queueing behind them. This is a classic problem in game theory, very similar to the type for which John Nash was awarded a Nobel Prize.

What does the data say?

One US study has shown a strong relationship between the amount of new road length and the total amount of kilometres travelled in US cities, a finding the authors of that study termed “the fundamental law of road congestion”.

Similar findings are reported in Spain and in the United States, where even major road capacity increases can actually lead to little or no reduction in network traffic densities. It has also been found to exist in Europe, where neglecting induced demand has led to biases in appraising of environmental impacts as well as the economic viability of proposed road projects.

In Sydney, there is similar evidence from traffic volumes crossing the harbour. The Sydney Harbour Bridge was carrying a stable traffic volume of around 180,000 vehicles per day from 1986 to 1991. The Sydney Harbour Tunnel opened in 1992, and the total volume of traffic crossing the harbour increased in 1995 to almost 250,000 vehicles per day. This 38% increase in traffic can be attributed to induced demand and not to population growth (which was around 4% during this period).

Empirical observations have also confirmed the existence of the Braess Paradox. For example, in 1969 a new road was built in Stuttgart, Germany, which did not improve the traffic conditions. After closing the road again, congestion decreased.

Similar observations in which road closure led to improved traffic conditions have been observed in New York City, where upon closing 42nd street (a major crosstown street in Manhattan) it was observed that traffic was significantly less congested than average.

A recent experimental study confirmed that this paradox still exists by showing that expanding road capacity can result in worse traffic conditions for everybody.

The theory of induced demand is accepted by a large majority, but not by everyone.

For example, authors of a 2001 paper have argued that induced demand does not exist. However, UK researchers Goodwin and Noland have criticised this study.

In isolation, building more roads can certainly improve traffic conditions but these effects may only be local and only in the short run. Congestion may become worse in other parts of the network and experience shows that spare road capacity is quickly filled up with new cars.

Even without the extra road users that new roads create, if the new roads are built in the wrong locations congestion may actually become worse simply because of the way people behave. Roads alone do not solve congestion in the long term; they are only one (problematic) tool in a transport management toolkit.The Conversation

This article was originally published on The Conversation. Read the original article.

Lead image credit: Wikipedia.

The Conversation

The Conversation

Tags: 
Roads The Conversation Urban Affairs

Comments (19)

Help contribute to the Urban community by leaving your comments about this article
What would you like to say about this project?
Adrian's picture
The 1970's Melways had the rail line marked out its 30 years overdue. The Patk & Rides would be built on the broad space in the median strip of the Freeway where the rail would go thats exaclty what it was planned for - just like you see similar ones in the feeway medians in Brisbane & Perth. No parks would be touched. Nobody would use an extended tram line that would take an eternity to get to the city and more buses mean more pollution and road congestion in the city. Rationalise it all you like its way overdue and now by another 10 years.
Helpful
(0)
Not helpful
(0)
Reply

Reply to this comment

What would you like to respond to this comment?
michael.bell
Adrian: I don't share the views that "the residents of the East have suffered enough for 30+ years being disconnected from the rest of Melbourne", or that "the people of the Eastern corridor are the one's being left to dry being the only part of metropolitan Melbourne without Train or Tram services". It is true that some parts of the "East" do not have fixed rail services. However, some parts clearly do: inner "East" areas like the Alphington to Greensborough corridor, and the Richmond to Box Hill corridor; and the outer east corridors from Box Hill to Lilydale and Ferntree Gully. Yes, there is a fixed rail gap along the Kew to Donvale/Templestowe corridor. But these areas have rapid bus services to provide a transit connection to central Melbourne, so I'd hardly call that being "left to dry". Far from being "disconnected from the rest of Melbourne", the "East" is particularly well served by motorway connections, being fortunate enough to have both the east-west aligned Doncaster Fwy, and the North-South aligned Eastlink Tollway. I also disagree with the subjective use of "suffered". If there are services that the "East" is missing, when compared to other parts of Melbourne, there are bound to be some compensating factors to offset them. For example, house prices and rent are probably lower, thanks to the demand to live there being lower. On the positive side of the ledger, property size and tree and garden cover are probably greater; there may be better access to parks and recreation and other forms of local amenity. Everybody makes choices when they decide where to live, and everybody makes tradeoffs when they make those choices. If the lack of a fixed rail connection along the Kew to Donvale/Templestowe corridor is seen as a negative factor, then that negative factor is baked-in (consciously or unconsciously) to the decision to live in the "East". There are many things that make it worthwhile living the in the "East", because in spite of the lack of a fixed rail connection along the Kew to Donvale/Templestowe corridor, hundreds of thousands of people still choose to live there
Helpful
(0)
Not helpful
(0)
Reply

Reply to this comment

What would you like to respond to this comment?
Llib
Surrounded by parklands and quite low density along the corridor (apart from Doncaster Hill). Again the park and ride would be expensive and only utilised by peak commuters which is a costly service to provide. In regards to the greens they are against all types of cars and roads and even all sort of consumerism and the market economy. I have tried to work with the greens as an alliance of convenience to promote worthwhile PT investments such as extending trams to Doncaster and getting more bus lanes on that corridor but they refuse to work with me even though I have extensive marketing experience. I believe the greens are all about being in the limelight and are resentful of suburban and middle class people. They are unrealistic about the fact that suburban people will continue to need their cars for day to day transport and the solution to Melbourne's transport problems is a market based approach to supply and demand which is why Citylink has been successful as it would not have been constructed without private sector investment funded by tolls. This does not mean that rational people should still not actively support feasible projects whether they are road, rail or other alternatives.
Helpful
(0)
Not helpful
(0)
Reply

Reply to this comment

What would you like to respond to this comment?
Martin Mankowski's picture

The problem with that is that the majority of the eastern freeway runs through parkland. Surely you are not suggesting we rip up parkland for multi level carparks? And the car parks would have to be huge - there would be very little walk up commuters. Then you start to run into the problems we have in some of the outer suburbs - the car parks filling up early, and people just saying 'Screw it, I may as well drive in'.

Massive park rides are not only ugly, they are the most inefficient and economically unsound use of space you can get. And as just mentioned, are also self limiting.

Also as LLib pointed out, there is no capacity to add an extra line to the South Morang/Hurstibridge corridor until MM2 is built.

The long term fix should be to upgrade the 48 Tram to Light Rail and extend to Doncaster. The short to medium term fix would be to have dedicated bus lanes on the eastern Freeway AND Hoddle Street. But of course all the freeway supporters demanding extra lanes want them for more cars, not buses, which just exacerbates the problem.

I agree with you about the Greens. As I have mentioned here before their obsession with Doncaster rail borders on the insane - they are just interested in feathering their own nest rather than coming up with a broad ranging solution.

Doncaster is not far from the city. The whole corridor does not have a hugely dense population, even taking into account the recent apartment boom. RBT and Light rail would more than suffice if adequate road space was dedicated to them.

Helpful
(0)
Not helpful
(0)
Reply

Reply to this comment

What would you like to respond to this comment?
Llib
The problem with Doncaster Rail is that it will be difficult for commuters to get to the stations if the railway runs in the middle of the freeway. It will also need the MM2 to allow capacity into the city.
Helpful
(0)
Not helpful
(0)
Reply

Reply to this comment

What would you like to respond to this comment?

Pages