Of the importance of density: empirical proof

One of the main aims of proponents of a better urbanism is increasing density. Unfortunately, it's also one of the things that scare people used to low-density developments and who associate density with cities: congestion, crime, etc... This perception is pretty ridiculous for people who live in most cities, but it's still there. Some people ask: "Why are you so obsessed with density?", others are less polite and say "Why do you want to force people to live cooped up like chicken in cages?".

The reality is that what people like me want to do is not increase density for density's sake, the main goals are about:

• Creating more sustainable cities where there are economical and environmentally friendly ways to get around instead of just relying on private motorized vehicles
• Create systems with richer public realms so as to favor the creation of communities through regular social interactions
• Make cities where it's just plain better to live because of a vibrant culture and a lot of activities, economical and cultural

It just so happens that some density is required for this. Alternatives to cars largely don't go as fast as cars, rapid transit can beat them, but only with stops being widely spaced, so you need density around them. So for these alternatives to be viable, destinations need to be closer to residential areas. However, you can't build a supermarket every 1 kilometer, each supermarket, like any store, requires a certain pool of customers to be financially viable. For instance, if supermarkets require about 7 000 people to stay afloat and you have a city of 20 000 people, you can sustain only 3 supermarkets at once.

So you can approximate how many stores you can have in an area by looking at the population. Some proximity stores like corner stores need few people to sustain themselves, specialized stores need a very large pool of customers, so they will need to be concentrated at nodes of regional transport system, whether boulevards, highway interchanges or rapid transit stations, depending on what people use to get around.

Let's take this example of a suburb with a population of 20 000 people in a perfect circle. The density of residential areas is 3 000 people per square kilometer (7 500 per square mile), typical of Québec suburbs, with only 80% of the city area being residential areas, the rest being parks, commercial, industrial, etc... We'll suppose that density is spread equally around. This city can sustain 3 supermarkets. Even if all three were placed to maximize proximity, only 10% of the population would be within a 5-minute walking distance from one, and 40% within 10 minutes.

A 20 000-people suburb with three supermarkets, 5-minute walksheds drawn around each one

Increase density to a respectable but still middling 8 000 people per square kilometer (20 000 per square mile), and it already looks much better. As the population is the same, the number of supermarkets they can support is the same, 3, and 30% of people are within a 5-minute walk from the closest supermarket, and everyone would be within 10 minutes of walking from at least one:

A 20 000-people small city with 3 supermarkets

Now build a dense urban neighborhood of 15 000 people per square kilometer (38 000 per square mile) and nearly 60% of people are within the 5-minute walksheds of the nearest supermarket, and a lot of people are within 10-minute walking distance from the 3 supermarkets.

A dense urban neighborhood of 20 000 people with 3 supermarkets

Now, this is a simplification, in reality you can have greater densities around commercial areas and the walksheds may be smaller because of less walking-friendly street grids, but it illustrates the principle. As you build denser, more things can be within walking distance which helps walking, of course, but also transit as the greater density means increased ridership and thus higher frequency.

An example of how density increases active transport modes and transit, and reduces dependency on the car

OK, right now, it's a pretty basic article about a basic principle. A bit of a bore I wager. However, I have more to offer.

In Québec, we have so-called "origin-destination" studies which question thousands of people in major agglomeration about their trips, whether to work, to school, for leisure or else. These studies include where people start their trips (origins), where they end them (destinations), but also their motives and their mean of transport (mode).

The studies split up the metropolitan areas in a variety of small sectors, each one getting a page looking like this:

Example of an O-D sector and its info

This page offers all the info to establish the density of the area as they not only offer the population and the area, but they also show the street grid, which helped me approximate exactly how much of each area is inhabited and how much is not inhabited (absence of streets or very rare ones). Like in this example, around 50 or 60% of the area would be inhabited, the rest is a park.

By inputing this data in an Excel worksheet, I've been able to create graphs showing how mode share changes depending on population density of inhabited area in each sector. And the results are telling. But first, for people who may not know, some hints:

Montréal 

• Québec's metropolis, 1,6 million people in the city itself, around twice that including suburbs
• Has 4 subway lines that are very used and very frequent, and a complete bus system with many frequent lines in the city itself, which they call the "10 minutes max" lines.
• Commuter rail lines in the suburbs, but that only operate in the peak periods for the most part, and with pretty low frequencies

Québec

• Québec's national capital, around 500 000 people in the city, which has swallowed many suburbs over the years, 700 000 people including the suburbs that have not been merged.
• No subway, no tramway, no rapid transit. Just a few lines of "metrobus" which are essentially frequent bus lines with stops spread a bit further apart.
• A LOT of highways, very car-dependent suburbs, but a well-preserved dense inner city with European-inspired areas

Final note, "mode share" for those who don't know means what share of all trips are made in a specific mode. So a 10% transit mode share means that 10% of all trips are made in transit.

OK, so in the following graphs, the sectors from Montréal are in red, those from Québec are in blue. I looked at the trips of residents, meaning trips (excluding returns) that originate from the sectors, and not those that ended in them.

First, how does transit use change with residential density?

Transit mode share (y-axis) vs residential density (x-axis, people per square kilometer)

The relation is very clear here, transit mode share seems directly proportional to density in Montréal. The odd point that is way over the trend at around 5 000 people per square kilometer is actually the downtown sector, in which few people actually live because most of the space is commercial and offices. In Québec City, there is also a positive trend, but it seems to max out at about 12 to 15%. There are two factors that may influence this:

1. The absence of subway or other rapid transit line with high frequencies, which makes transit slow and much less useful. The subway seems to help the transit share even in relatively low densities, as sectors in Montréal with around 6 000 people per square kilometer (15 000 per square mile) have transit use about 50% higher than similarly dense areas in Québec City. Which shows the limited use of buses in mixed traffic to drive up transit use.
2. Montréal is a bigger city, so even in the dense core there may be times where people need to take transit to go a bit further, whereas all the attractions in the urban core of Québec City may be closer together and thus be closer to dense areas, not needing transit to get there.

 Now let's look at active transport, walking and biking:

Active transport mode share (y-axis) vs residential density (x-axis, people per square kilometer)

Interesting here how Québec City's trend is actually over Montréal's. Maybe due to the lack of good transit option, people in dense areas may walk more and take transit less than in Montréal. Alternatively, the poor performance of Montréal may be mainly about denser suburbs between 7 000 to 10 000 people per square kilometer which are not that walking friendly (I live in one, Lasalle, walking to stores isn't impossible but not really pleasant). Note that some high-density sectors in Montréal do match similarly dense sectors in Québec City for active transport mode share.

Anyways, let's look at the car mode share to finish:

Auto mode share (y-axis) vs residential density (x-axis, people per square kilometer)

In both areas, the trend line is clearly linear, which actually surprised me to see such a strong linear relationship between density and car mode share. I expected it to decline as density rose, yes, but to see such a strong effect was even a bit strange, it seems too perfect. Yet, that is the real result.

Anyway, Montréal is the winner here, in high-density sectors, its car mode share is up to 10 points less than in Québec City's high-density sectors. However, the car mode shares in the suburbs are very similar. Despite Québec City's sectors having higher active transport mode shares, the much higher transit use, largely due I think to the rapid transit lines in Montréal, means that Montréalers in dense areas use cars to get around even less than the people in Québec City's dense areas. But they are more alike than they are different.

And again, that red dot in its lonesome far below the trend is Montréal's downtown area, which has little population but sky-high density in everything else (jobs, stores, etc...).

Despite the trend being clear, sectors with identical densities can vary in car mode share by up to 15%, so density isn't everything, even if it's a very strong factor. I guess the other factors that may lead to differences include:

• If sectors have a lot of mixed use rather than use separation which increases distances
• If the area has highways cutting it in half and allowing fast car movements
• If sectors have good or bad transit service (in the case of Montréal, do they have access to the subway or not)
• If they have street grids that favor walking or street grids which makes walking more difficult
• If the population in the area is richer or poorer

I may do something like that later on, but for now, I think I'm quite satisfied with the results, which clearly show how density is a very strong factor reducing car use and increasing the rate of walking, biking and transit use.