How do you define “optimal conditions” in a transport sense? The graphic below paints a stark difference in opinion on what constitutes “optimal conditions” for different professions:

Image from Patrick Kennedy, D Magazine,

Now I’m both an engineer and an economist, so feel I have some insight into the motivation for whoever developed this graphic.

It highlights a very important issue: Engineers tend to measure performance using indicators that measure mobility, whereas economists tend to measure performance using indicators that measure value.

Now don’t get me wrong: I strongly believe that mobility has a value. I write this having just fanged up and down the northern motorway taking some visiting Australian guests to Tawharanui. But it’s not the only thing I value. Indeed, tonight I will take them to Fort Street for dinner at Ima’s.

I accept that what constitutes “optimal conditions” will change depending on the context, and that in some contexts mobility has a very high value. What I can’t fathom is why the transport/traffic profession have developed such simple and inflexible performance indicators to guide their work.

I’m going to say this right now: Level of service tells you *nothing* about what people value. Why? Because it’s measured independently of the costs associated with being able to move freely. We could, for example, enjoy great LOS if we bulldozed the entire city and replaced it all with twenty-lane motorways connecting to vast carparks, with the occasional office building or house dotted amongst the seas of asphalt. But I think it’s obvious that would be a really, really bad outcome for almost everything else that we value.

From an economic perspective, congestion in cities can be seen as a good thing. It’s an indicator that lots of people are using the city in lots of different ways – going to work, travelling to see friends or family, going shopping, visiting sports games or art museums etc. Congestion has costs, of course, but eliminating it entirely would be even more costly.

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    1. From what I understand optimal conditions, if you are measuring the maximum throughput and therefore best use of the corridor, comes at LOS C or D.

      To get LOS A you effectively squander most of your space on the stopping distance between vehicles. At LOS E you’re slowed down too much and back to moving far fewer people through than the maximum.

    2. I’d argue that we can’t reach a conclusion on what is optimal based on LoS alone, at least not from a long run perspective.

      Los E may maximise throughput and thereby be optimal in terms of short-run asset utilisation, but it does not tell you much about WTP for the long run asset, and in turn whether said asset should be expanded/reduced.

  1. No I agree with this. A full suburban train, for example, that might be slightly crowded and unpleasant feeling for the passengers but nonetheless doing its job, to me is optimising in both fields. The passengers themselves, and other stakeholders such as taxpayers, designers, urban communities benefiting from it, should be impressed that the train is full and running as planned. An empty one less so.

  2. The picture on the right looks like LOS A,B,C. LOS F would be bumper to bumper cars and everyone on the side walk would be deaf from the noise and choked by the fumes. If no one used cars you would have LOS A everywhere including Elliot St. So I’m not sure I entirely get your point. Engineers probably need LOS G,H,I to better classify congestion and LOS A isn’t the target. LOS C is the target and to minimise accidents as well.

    1. it’s fairly simple: Level of service measures vehicle density, or indirectly vehicle speeds; it does not measure value in an economic sense.

      It barely measures value even from the perspective of the motorist, because it does not consider their willingness-to-pay for the asset they are using.

      I agree the right-hand photo in the graphic does not illustrate LoS F, but that’s not important to the main point, i.e. LoS is a fairly useless key performance indicator.

      It may have use in the short-run, i.e. are we making efficient (through-put maximising) use of the road asset, which occurs at LoS C/D. But in the long run LoS is not indicative of true economic value.

  3. A really congested, totally blocked up road, gets a rating from a pedestrian as very high – because then they can cross, at any point. But the same road, flowing freely, is a pain in the arse to the pedestrian, as it takes forever to find a gap to cross over in – or the need to walk half a block to get to the next pedestrian crossing. So, i guess it relates not just to whether you are an economist or an engineer, but also if you are a user – and what sort of road user you are. What we need are traffic engineers as focused on pedestrian traffic as they are on vehicular traffic…

    1. excellent point. Which I think is why value is a much better metric than throughput: Value benefits/costs to all users and allocate space in the road corridor accordingly. Easier said than done, of course, and I think some strategic/design considerations may need to be evaluated in a more qualitative fashion, but value provides a useful starting point. LoS tells you very little of interest.

      1. Here’s the simple metric I’m working on-

        (Size of vehicle and its spatial displacement/#people)/trip distance.

        Goal is to keep this number as low as possible, while providing the highest amount of trips.

        Trips include a whole range of activities not typically measured- walking to lunch, serendipitous encounters, accessing recreation etc.

        1. Interestingly your metric comes out in units of metres per person, or in plainer terms: how far do you have to travel to meet the next person?

          I’d think dividing by trip time, rather than trip distance, would make slightly more sense. To be fair to low-density sprawl, part of the point is that you (partly) make up for lower density with higher travel speeds. Perhaps we should measure population density in people per square minute, rather than per square kilometre?

        2. You have to be a bit careful with ratios as the answers are not always what you intend. The current North Shore District Plan when first notified had a trip generation rate limit. If your business generated more than 5.0 trips per 100sqm it became a discretionary activity and needed consent. Less that 5.0 was ok in any business zone. The intent was to protect existing centres from shops establishing outside centres. A man came to us as he had exceeded 5.0 and asked for advice on getting consent. We advised him to just build a mezzanine floor and don’t put anything on it and the problem would be solved! The Council’s intent was to reduce the numerator of the ratio but he could comply by increasing the denominator.

    2. Maybe if NZ drivers were a little more courteous to pedestrians… Adopt the California approach: if someone wants to cross the street, stop and let them.

  4. Even if you built your 20 or 50 or 100 lane motorway you would still experience a LOS of F at some point, unless that road was an infinite distance long. The reason is that *somewhere* you have to get down to one lane, and whether you have three lanes merging into one or 20 lanes merging into one, you have the same capacity constraint and therefore…congestion! That’s the problem with limited access highways in an urban area – they create a barrier to dispersal of traffic over the entire network and overload the entry and exit points. No urban motorway will ever relieve congestion in a meaningful way. Rural motorways are a different story, which is why the guy who came up with the idea knew and said they should never penetrate an urbanised area.

    1. Exactly. Stop trying to build urban motorways. But, because only the urban centres are growing and we have this clumsy system that HAS to build more state highways, and wants to respond to growth….

      So currently they build either underused duplicate rural highways decades too soon (Waikato, PuFord, Transmission) or ruinous and useless urban destructors (Basin Reserve, Auckland)

      Broken system.

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