News & Discussion: Roads & Traffic

[skyliner]

That theory really is not appropriate.

There are plenty of cities Adelaide's size that function much better with Freeways, Perth is a the best example.

You can drive from Joondalup to Mandurah (the equivelant of going from Gawler to Victor Harbour) and NOT STOP AT A SINGLE TRAFFIC LIGHT.
People in Adelaide, or atleast some of them, just dont get it. I fantasise of driving anywhere in Adelaide, without it taking twice or three times longer than it should.

Not everybody can use public transport, public transport will never get people EVERYWHERE they need to go and public transport can not freight goods. An efficient public transport system, in conjunction with an effective road system is the solution, not one without the other.

With respect to the comment on induced demand, that's just stupid. The same amount of cars will be on the road, unless there is an explosion in the population. How does widening a road increase the amount of traffic that would otherwise be using the road anyway?

The THEORY of induced demand and is based on observation of some kind of phenomenon. It here means a generalisation waiting for final unquestionable proof.It presumes population growth as well. It presumes more and more people will use the way of least resistance until it becomes a resistance in itself. I personally believe there is at least some credibility to the theory, hence appropriate planning is needed.

Sorry about the double post guys - not intended.
SA - STATE WITH GREAT POSITIVE POTENTIAL

[Prince George]

The idea of Induced Traffic (aka Latent Demand) has come up in a couple of places and it's an idea that's confusing at first glance. I think that it's actually not too difficult to understand what exactly it is -- what is hard is predicting just how big an impact it has -- so I thought it would be useful to post some notes about it in one place. Remember, this doesn't come from hemp-wearing tree-huggers, this is the work of economists and traffic researchers. If you're interested, you can look at this paper (PDF) for a review of the literature.

Induced Traffic says that when you try to solve congestion by increasing the size or speed of roads, the net effect is to increase the total amount of road use to the point that it becomes congested again even if all other factors, including population, remain the same. And this is because there are trips that people aren't taking on the roads precisely because it is congested; when that improves, now people start taking these extra trips, increasing the total road use until the roads start to get congested again and things level out -- not as bad as it was before, but worse than it had been a short time earlier. And all that without increasing population.

This is really a special case of what they call Latent Demand: when something gets cheaper, people will buy more of it. Since that's a bit easier to understand, let's start there.

Think about foods that you like - there are some that are "special occasion" foods, because they're too expensive to have very often, you need an excuse. I'm happy to buy a 6-pack pretty regularly, but I'm only going to get a bottle of Moet for an anniversary or an engagement present. It's not that I don't like champers, I just can't afford it. So what would happen if the price of Champagne dropped sharply - say by half - now I might get it much more often. So the amount of champagne getting bought will increase without there being an increase in the number of people buying champagne.

Or think back to when DVD was first introduced - those first players cost upwards of $1500, so hardly anyone had one. People could see it's advantages, especially the high-quality picture and sound (compare it to VHS!), but almost no-one could justify that price. By the time they had dropped to $500, many people had them; once they arrived at sub-$100 and you could get them in BigW, you could safely say that almost everyone had one.

Now, travelling somewhere has it's own kind of costs: apart from things like the petrol, there is the cost of time for the trip and the frustration for being stuck in traffic. There are times that people will decide that the cost is too much sometimes and choose not to drive.

• "Driving into town in the morning is crazy, I'll take the bus"
• "It takes ages to drive to and from AAMI, I'll just stay home and watch it on TV"
• "I could probably get this cheaper at Bunnings, but parking's terrible down there; I'll get it down the street"
• "I'll look for a house close to work, I don't want a long commute"

When you increase the capacity of the roads, you will reduce the congestion at the start. That reduces the cost of driving, and now some of those trips that people had avoided start getting taken. Some people will drive more often, some will drive further, and these extra trips now increase the amount of congestion - not because the number of drivers, or even the number of cars, has increased, but because their amount of road-use has increased. Eventually, things reach an equilibrium again: the cost of driving is less than it was before improving the roads, but more than it was just after you made the improvement. You've lost much of your benefits before any new people have arrived. When the population does increase, there isn't as much spare capacity as you had thought, and you're back at square one.

[Prince George]

With respect to the comment on induced demand, that's just stupid. The same amount of cars will be on the road, unless there is an explosion in the population. How does widening a road increase the amount of traffic that would otherwise be using the road anyway?

The idea of "induced demand" is strange and certainly pretty unexpected to most people, and, no, it doesn't need a population explosion. Rather than clutter this thread with details, I've put a longer post about it here.

[drwaddles]

Excellent thread idea. Hopefully some posters can post links/reviews of documents they find, perhaps case studies and any thoughts/questions/discussions.

Here's something off the top of my head.

There's a couple of facets to induced traffic, assuming population remains constant:
1. Modal shift - i.e. travel formerly undertaken by another mode is now undertaken by road due to the improvements. Total travel does not necessarily have to have changed.
2. Latent demand - i.e. travel that was previously not being undertaken due to congestion etc. is now undertaken due to a release of that congestion
3. Induced travel demand - i.e travel that was not undertaken/considered is now encouraged due to reductions in travel time increasing the possible origin/destination combinations

The theory behind the phenomenom, especially for 3., is that everyone has a 'travel budget'. That is, people are prepared to travel for a certain amount of time in order to reach certain facilities and services. Instead of road improvements reducing the time spent travelling, what happens is that people use the time saved to travel further to access 'better' facilities and services. Thus you get your induced travel demand and very minimal improvements in congestion, if at all, over the medium to long term. The extra travel obviously increases Vehicle Kilometres Travelled (VKT) which is a key indicator of the environmental impacts that the travel is causing. Very few road projects ever reduce VKT, despite it being an objective/target in the policy of most Australian governments.

[AtD]

Great thread Prince.

Transport policy is an interesting application of economics. There's all sorts of phenomenons at play: induced demand, negative and positive externalities and so on. Transport economics have a huge influence on where people chose to live, work and play, so it's something I would like to see a greater understanding of.

[Aidan]

Induced Traffic says that when you try to solve congestion by increasing the size or speed of roads, the net effect is to increase the total amount of road use to the point that it becomes congested again even if all other factors, including population, remain the same.

Close, but induced traffic is a phenomenon not a theory!

Induced Traffic is the main reason why when you try to solve congestion by increasing the size or speed of roads, the net effect is to increase the total amount of road use to the point that it becomes congested again even if all other factors, including population, remain the same.

But induced traffic will not always have that effect, and other things can also have that effect - notably unaddressed bottlenecks. And particularly where population is increasing, the influence of the road improvements on development patterns can increase demand, and this can also be regarded as induced traffic even though it is not the result of latent demand.

[jk1237]

I forgot what day it was last week, but the day before the media made a big hoo-haa about the major price drop in petrol, and the next day when I went to work, North Tce was completely clogged during morning and arvo peak. I didnt notice any roadworks anywhere, and I got a seat on my bus. Cheaper petrol does encourage more driving, which is a bugger

[drsmith]

That theory really is not appropriate.

There are plenty of cities Adelaide's size that function much better with Freeways, Perth is a the best example.

You can drive from Joondalup to Mandurah (the equivelant of going from Gawler to Victor Harbour) and NOT STOP AT A SINGLE TRAFFIC LIGHT.

Not quiet but the last bit between Rockingham and Mandurah is currently under construction and due for completion next year.

My own experience as a road user suggests very strongly that urban traffic planners take into accound induced demand and as an example I offer the Causeway which connects the southeast end of the Perth CBD to Shepperton Road to the southeast across the Swan River. This can be viewed using google earth.

The Causeway contains six road lanes. In it's current configuration the two centre lanes are dedicated bus lanes. Prior to 2000 all six lanes were traffic lanes and the intersections at either end were controlled by simple two phase traffic signals. At that time traffic volume across the causeway was about 110000 vpd and apart from some congestion at peak periods that configuration coped with the traffic well. After the completion of a freeway bypass to the north of the CBD in 2000, traffic volumes across the causeway dropped dramatically to around 60000-65000 vpd. This was expected. For a couple of brief months it was a pleasure to drive across it during peak periods with no congestion whatsoever.

Then came the bus lanes. Once reduced to 4 traffic lanes with more complex signal phasing at both ends, peak congestion was worse than when it carried 110000 vpd. While buses now have priority under the new configuration I could see no advantage for them specifically as there was no congestion to hold them up in the general flow of traffic after the freeway bypass to the north of the CBD was completed. There was therefore another reason to construct the bus lanes and I suspect it was two fold.

One reason which I know to be the case as it was advertised at the time was to encourage drivers to use the new freeway to the north instead of the causeway. The second I suspect was to give bus transport an advantage over cars by causing some congestion during peak periods. Both reasons are in part about managing the potential for induced demand.

With regard to Perth's north/south freeway it is infinately better than South Road but it too is not without it's problems. It typically becomes very congested during the morning and afternoon peaks near the CBD. This is a consequence of rapid urban sprawl in particular to the north under which the transport network is struggling to cope.

[Prince George]

My own experience as a road user suggests very strongly that urban traffic planners take into accound induced demand and as an example I offer the Causeway which connects the southeast end of the Perth CBD to Shepperton Road to the southeast across the Swan River. This can be viewed using google earth.

The Causeway contains six road lanes. In it's current configuration the two centre lanes are dedicated bus lanes. Prior to 2000 all six lanes were traffic lanes ...<snip>... After the completion of a freeway bypass to the north of the CBD in 2000, traffic volumes across the causeway dropped dramatically ... [f]or a couple of brief months it was a pleasure to drive across it during peak periods with no congestion whatsoever.

Then came the bus lanes. Once reduced to 4 traffic lanes with more complex signal phasing at both ends, peak congestion was worse than [before the freeway] ... There was therefore another reason to construct the bus lanes and I suspect it was two fold.

One reason which I know to be the case as it was advertised at the time was to encourage drivers to use the new freeway to the north instead of the causeway. The second I suspect was to give bus transport an advantage over cars by causing some congestion during peak periods. Both reasons are in part about managing the potential for induced demand.

I don't know any more about the Causeway or Freeway than you have told us, but I am struck by a couple of aspects of this story that I hope you will agree are worth thinking on.

First, if your suspicion is correct, then they are trying to manage induced demand by making congestion as bad as it was before. Looked at another way, having built the freeway they are arranging for it to have not improved traffic conditions on the Causeway at all. I think that most people would not regard that as the outcome that they want from a major investment in their roads. And if you now say "Ah, but the Freeway itself was still uncongested", then it is there that you should be looking for the effect of induced demand.

Second, I really don't follow why they would need to "encourage drivers to use the new freeway"? Was the simple presence of a brand new freeway not enough? From what you said, there was a drop in the amount of traffic on the Causeway prior to the changes; presumably those people were now on the freeway, why would they encourage more? That goes rather beyond any idea of "induced" behaviour to something more like "compulsion" Since the Causeway is not going to be a solution for congestion on the freeway, just what are they going to do once the freeway is congested too?

It would be interesting to see just what the traffic is like in these places. Over here most states' Department of Transport provides live traffic maps and travelling time estimates, is anyone doing that for cities in Australia? Searching didn't seem to bring anything up ...

[adam73837]

Constructing Underpasses on South Road is not the long-term solution, it is simply a band-aid solution. This underpass is going to be 4 lanes wide, yes I know that it will be possible to upgrade it to six lanes, but really, its just more inconvenience. 'Build a freeway and be done with it'; I couldn't have put it better! Build a freeway and get the traffic off of these suburban roads, but wait, people are going to say that we'll need to expand the freeway to cope with the traffic'! What a great position to be in! (Adelaide ain't going to end up like LA,) Just add a lane or two on each side of the freeway and ta-da! Everyone's still going at high speed, meaning that the traffic's moving, (although in Adelaide people don't know that you need to stick to the left lane, so it may take a bit of time... ). It is a far better solution than building underpasses on a Suburban Corridor like South Road that has just gradually been extended to stretch from North-South. If we build a North-South Freeway, the people living North of about Cross Road/ Daws Road will continue to use South, Goodwood and Marion Roads, whilst everyone living south of Cross Road/ Daws Road will use the freeway! Meanwhile, people living south of Grand Junction/ Regency Roads will continue to use Prospect, Main North, South and Churchill Roads, whilst everyone living North of Grand Junction/ Regency Roads will use the freeway!
John Brumby is wrong to state that we're a backwater, we are not, yet. If we continue to have the stupid mentality that is driving these under/overpasses to be built, we will become a backwater, however if we get someone like Kennet or Playford to just come in and build the infrastructure that we need without being knocked back by vocal minorities that don't want to see change, we WILL NOT be a backwater!
That $2b that the State Government is asking for is going to be WASTED on building those stupid underpasses! It should be used to build that 'North-South Freeway' that we NEED!
BTW, I'm not asking for this freeway today, tomorrow, next week or next month; I'm asking for a vision similar to that of the MATS Plan that this city of ours needs.

[Adamo]

and where the hell could this freeway be built? are you seriously suggesting we have the capital to build a complete underground highway, or some obsene highway above ground, that totally will be blinding on the eyes. We don't need a freeway.....

[monotonehell]

Adam when will you realise that freeways are a 1950's experiment that has been found not to work in the long run? Most major cities are reducing and consolidating freeways rather than expanding them. Also you can't just "add another lane" at will. Very few trips in Adelaide go outside of two N-S quartiles, so we don't need great carved freeways, we need to concentrate on short hop inter-suburban travel for cars. PT can relieve the radial and cross town peak period rush, and for those who need to make obtuse journeys, the road system works fine.

You're focusing on relieving the symptoms, instead of the finding the cure.

(Also your posts could do with a little white space and less wall of text.)

[Norman]

Ooh, ooh, let's be like Perth and rid of all our heritage and build a plastic city

There are better traffic management options than building a eyesore freeway.

[Düsseldorfer]

There are better traffic management options than building a eyesore freeway.

Roundabouts, Underpasses/Overpasses

[Adamo]

This underpass will definitely be an excellent short-term fix, with less congestion at that particular anzac highway intersection like people have already stated but more needs to be done, but not in terms of upgrading south road into a freeway, or more lanes, but fixing public transport. And this will take time over the next 20 years. Once the north south rail corridor has been electrified and re-sleeperd patronage will increase, taking more vehicles off the roadway. Especially if the seaford extension goes according to plan, as well as the other proposed alignments further south.

I really want no more under/over passes to go ahead until our main rail corridors have been converted, because they are way past there time. Congratulations to the SA government for taking the first step, lets hope more exciting projects to come!