Highway widening and CO2 emissions

One of the best things about this blog becoming more and more widely read is that I am increasingly having people email me interesting links to transport research or transport-related events happening around Auckland. I also seem to have ended up on ARTA’s emailing list for all media releases these days (perhaps I am now an important stakeholder?) This is really great, as I am provided with so much more information than I could possibly ever discover myself, and by reading the various links that I am sent I find myself becoming more and more educated. So keep it coming!

Today, I got sent a link to a study on the relationship between highway widening and CO2 emissions.  It is based on American data, but generally the conclusions found could be easily applied to Auckland. Overall, the study finds that widening highways and building more roads will lead to more CO2 emissions, rather than less – which is often the reason given by road-builders to promote their projects. Even in Auckland, if you look at the (dodgy) cost-benefit analysis of the Waterview Connection, $4.3 million of benefits have been allocated to “carbon dioxide”, which one must assume means that it is anticipated that building the motorway will reduce CO2 emissions. The study suggests that might be a misguided conclusion:

Road-building proponents often suggest that adding lanes to a highway will reduce greenhouse gas emissions. By easing congestion, they argue, new lanes will reduce the amount of fuel that vehicles waste in stop-and-go traffic, leading to lower releases of climate-warming gases from cars and trucks.

Over the short term—perhaps 5 to 10 years after new lanes are opened to traffic—this argument may hold some slim merit. But considering the increased emissions from highway construction and additional vehicle travel, adding one mile of new highway lane will increase CO2 emissions by more than 100,000 tons over 50 years.

This is just another example of ‘induced demand’ being ignored by transport planners it seems. Yes, a less congested road will mean that vehicles driving along it emit less CO2 per vehicle per kilometre. The problem is that by building a ‘better road’ you are encouraging more vehicles to use it, and for each vehicle that uses it to drive further. Quite soon that ‘per vehicle per km’ saving is likely to get swamped by the induced demand, and in the end we find out that the roads we built at least partly on the assumption that they’d lead to lower CO2 emissions, have actually done the opposite. The image below shows how this happens over time:

One thing that seems to often be ignored in the calculation of CO2 emissions of a proposal are the resource that are used to build the thing in the first place. This study calculated that building one mile of roadway (at the width of a lane) would release between 1,400 and 2,300 tonnes of CO2, while its maintenance and renewal over time would lead to a further 3,100 – 5,200 tonnes of CO2 over the course of 50 years. Sightline (who did the study) conservatively estimated that constructing and maintaining a mile of roadway would emit around 3,500 tonnes of CO2 over the course of 50 years.

Now obviously there are some benefits to CO2 emissions from reducing congestion – the benefits that we see picked up in the Waterview Connection study and the benefits picked up in many other international cost-benefit analyses I imagine. Putting aside induced demand for a moment, traffic which flows more freely will emit less CO2 per kilometre as the vehicles will not have to slow down, then speed up, then slow down, then speed up again. The more consistent running of the engine leads to better fuel efficiency per kilometre travelled for a vehicle, so therefore emissions are reduced. However, the study also indicated that once you start to consider induced demand, these benefits disappear fairly quickly. The study indicated that a mile of roadway would probably reduce CO2 emissions by around 7,000 tonnes over the course of 50 years (although once again that ignores induced demand for now).

Things become more interesting when we stop ignoring induced demand actually. Interestingly, the study examines two different types of induced demand – ‘on-highway’ and ‘off-highway’. Looking first at ‘on-highway’ induced demand, the study states the following:

It is well documented that highway expansion can result in an increase in the number of vehicle trips on a roadway, particularly in congested urban areas. Indeed, accommodating additional trips is typically the point of adding new lanes to a highway. Still, the speed at which additional traffic floods new lanes often comes as a surprise. One recent California study estimated that more than roughly 90 percent of new lane capacity in congested urban areas is filled within five years after a project is completed. Other studies have found similar “induced traffic” effects from adding lanes to congested roads.

Even assuming that vehicle fuel efficiency improves by 2.5% a year (which the study says is optimistic, considering that fuel efficiency in the USA has stagnated for decades due to their SUV craze), the new vehicle travel encouraged by the wider (or new) highway will result in 83,000 tonnes of CO2 emissions over the course of 50 years per mile of additional highway lane.

The final measure of induced demand is seen as ‘off-highway’ or ‘indirect new traffic’, and is very difficult to measure as it relates to the additional travel people are likely to make on parts of the road network other than the highway in question, because the construction of the highway has led to more dispersed land-use patterns. While Auckland’s MUL mitigates this situation to some extent for now, in the future who knows what’s going to happen – and it certainly seems like projects such as the Puhoi-Wellsford motorway are being specifically planned to allow Auckland to sprawl further to the north in the future. The potential effects of these changes to land-use patterns are detailed below:

Adding lanes—particularly on roads leading to low-density suburbs and undeveloped land on the urban fringe—tends to accelerate low-density sprawling development. Many studies have linked lower-density land use patterns with increased driving. In a sprawling suburb, virtually every trip must be taken by car, and everyday trips can require many miles of travel. In contrast, residents of more compact suburbs and urban neighborhoods typically drive less, and can walk or use transit for many trips, which reduces the carbon emissions from their daily transportation. Accordingly, low-density development is associated with increased vehicle fuel consumption.

Sightline estimates that if as little as one-tenth of new highway trips represent a net shift to lower-density land use patterns (i.e., new sprawling suburban development with modestly higher per-household driving than in compact suburbs), then greenhouse gas emissions from additional off-facility driving could rival or exceed the increases from driving on the facility itself.

All this induced demand means that the final figures are actually kind of scary, in terms of the significant effect they could have on the cost-benefit analysis of a project. Here’s a table showing the ‘final results’ of Sightline’s study:

To put this in a bit of context, let’s look at what effect this could have on the cost-benefit analysis of the Waterview Connection. I’ll take a total of 150,000 tonnes as a midpoint per mile,  take the Waterview Connection as about 2.8 miles (4.5 km) long, and have it as a six lane highway. This would mean a “total lane miles” of 16.8. Multiply that out by 150,000 tonnes/mile and we can see that the Waterview Connection would result in an additional 2.52 million tonnes of CO2 being emitted over the next 50 years. At a cost of $100 a tonne (a reasonably likely medium-term level), that’s $252 million in costs that are just being ignored by the current analysis over 50 years. Hardly a $4.3 million benefit is it?

Share this