Just one more lane!

Our roads are more congested than ever, and to fix the problem we just need to build more roads and widen existing ones. Or so the thinking goes...

  1. Argument

    To fix car traffic, the most effective solution is to expand road capacity by adding more lanes. This approach directly addresses congestion by increasing the volume of vehicles that can be accommodated, leading to smoother traffic flow and reduced travel times. By adding more lanes to raods, cities can meet growing transportation demands and improve overall mobility.

  2. Rebuttal

    On the surface, this argument makes sense. After all, if you want to move more water through a pipe, you can simply replace it with a bigger pipe, increasing its capacity. In the same way, if we want to increase the capacity of a road, all we have to do is add more lanes. But what this analogy ignores is that people do not behave like water. People are not just governed by the raw laws of physics, but psychological effects and other social factors play a huge role in how we live and make decisions.


    When roads are widened, this creates something called “induced demand”. This is a term from the standard supply and demand model taught in introductory economics classes. The idea behind road widening projects is that by increasing the supply of the good (i.e. lanes), the price (i.e. travel time, inconvenience, etc.) will fall. And this is what actually happens shortly after a road has been widened: the roads are clearer and travel times are reduced, but because of the lower cost, people now start to change their travel patterns. For example, people who used to take a different route because of traffic may now choose to use the faster road, people who used to use public transport may now switch back to using their cars to get to work, and so on. These changes in behaviour are commonly referred to as “induced demand” and take a few years to develop. As a result, travel times return to their pre-development levels or get worse. This effect on motorways has been known for at least 60 years, and is called “Downs’ Law of Peak-Hour Traffic Congestion”, which states: On urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity

    (Downs, 1962)
    . Down’s Law has been confirmed by various papers since (
    (Duranton & Turner, 2011)
    ,
    (Hsu & Zhang, 2011)
    ).


    In fact, other researchers have attempted to quantify the induced demand effect, and one paper found that the amount of additional traffic must depend very much on the context, size and location of road schemes, but a reasonable average value is given by an elasticity of traffic volume with respect to travel time of about -0.5 in the short run and up to -1.0 in the long run

    (Goodwin, 1996)
    . This means that if travel time increases by 1%, traffic volume decreases by 0.5% in the short term and 1% in the long term. Conversely, if travel time decreases by 1%, traffic volume increases by 0.5% in the short term and 1% in the long term. This lockstep relationship between travel time and traffic is the reason why road expansions do not actually reduce traffic in the long term
    (Hymel, 2019)
    .

  3. Solution

    To understand the solution, it is important to realise that “induced demand” applies to all modes of transport, and that induced demand also has benefits. It means more people visiting shops, getting together to enjoy life and so on. So the question is not how to curb this demand, but how to deal with it most effectively, and to answer this question the so-called “corridor capacity” of a mode of transport tells us how to make the most effective use of our limited resources such as labour, construction materials and space. The corridor capacity of a mode of transport is typically expressed in terms of “passengers per hour per unit of space”, which quantifies the maximum number of passengers that can be transported through a transit corridor in an hour given a given amount of space. For example, as shown in , a cycle lane has a capacity factor of , so 12,000 cyclists can be moved in the same space that would normally move only about 2,000 cars (i.e. mixed traffic). These figures of course depend on a huge number of variables, but in general cars use a lot of space while only moving on average about 1.7 pax/car

    (Fiorello et al., 2016, p. 1108)
    .

    figure 1 Corridor capacity of each mode of transport assuming a wide lane. Data:
    (TUMI, 2021)

    Cities and villages that are mainly car-dependent can benefit from the so-called “road diet”. This is one of the techniques that transportation planners can use to improve the quality of life along a road and its users. It can also typically increase the total corridor capacity. shows a typical road undergoing a road diet. The design before the diet has an estimated corridor capacity of 36,000 pax/h with a 2x2 road layout and narrow bicycle lanes in each direction next to parked cars. There’s also space for pedestrians on each side.


    The proposed redesign involves reducing the street configuration from a 2x2 with parking spaces on both sides to a 2+1 configuration with only parking space on one side and a shared wide turning lane. The driving lanes are kept at the same width of as in the original design. The extra space from the diet is used to widen the bike lanes from to just about which just barely meets the latest minimum bicycle lane width requirement of

    (CROW, 2022, Table 1)
    in the Netherlands. The bicycle lanes are also moved next to the sidewalks and out of the dooring zone to reduce the risk of collisions with car doors, but kept at street level so cyclists do not accidentally get on the sidewalk. Some extra greenery is added, which provides shade in the summer and physically separates the heavy cars from the unprotected cyclists and pedestrians. All of this leads to a final estimated corridor capacity of 57,000 pax/h.

    image 1 - This is a typical wide road before and after a potential road diet.Source:
    (Streetmix, 2024)

    Road diets are not only an effective tool to improve the corridor capacity and to make the spaces more pleasant for residents, but also a number of studies has demonstrated that they make the streets objectively safer (

    (Boyle et al., 2023)
    ,
    (Venegas et al., 2023)
    ).


    While this proposed solution has focused on road diets and tackling the intra-city traffic problem, there are many other tools that can and must be used to create a well-functioning road network. But in general, the same principles apply to intercity networks as well: a wide variety of transport options is better than forcing people to be dependent on a single mode of transport. For example, instead of widening a motorway from 6 to 10 lanes, the money could be spent on building dedicated lanes for BRT systems or even a rail system. These solutions are also subject to induced demand, but because of their superior efficiency they can cope with the extra demand much better and for longer than roads (see ). And to be fair, a lot of things about this design could be changed, and I encourage everyone to go to Google Earth and use the measurement tool to first recreate the street in its current form on Streetmix, and then redesign it to see what is possible.

  4. Bibliography