Contradictions resolved: Why more density means less traffic

Writing about Baltimore, cities in general and smart cities in particular makes it evident that much of what is smart seems counter-intuitive on first glance. From this insight flowed the idea of a series of articles addressing those seeming contradictions head on.

District of Columbia transportation statistics: VMT is going down
(Source: DOT)

One of the more glaring "contradictions" where reality is counter to public opinion is that denser development actually means less traffic and not more.

How is that possible?

The answer has two parts and is  really very simple:

1. The closer things are together (the definition of density) the less travel is needed to get to places. 
2. The closer things are together the more alternatives to driving a car become plausible

The evidence can be found in many research papers and studies in which metrics like "vehicle miles traveled" (VMT) are used to track how far the average person drives in a car per year. In the US as a country, VMT is about twice of what it is in a typical European country. To be clear, that isn't because the US is bigger ( Los Angeles is 2,600 miles from Baltimore whereas Munich is only 491 miles from Hamburg), most annual VMT comes from  commuting and local trips for errands, and leisure. Long distances over 300 miles or so are usually not driven in a car in any country. 

District of Columbia: 100,000 new residents in 15 years

Of course, traffic is going to be much less congested if many people use transit, a bicycle or walk. Density encourages all of those "alternative" modes of transportation. Dense cities have the highest transit usage because where density is high transit is worth the investment because the cost benefit is much better than in low density areas where buses or trains have to go much further to fill up with enough riders to run. 

These simple facts come as a shock to most Americans who are deeply density averse even though they love to travel to Venice, Paris or Copenhagen, all eminently denser than most American Cities. 

Mode split in DC: 38.5% transit , 13.6 walk, 4.5% bike

All this is pretty logical, isn't it? Still, almost every neighborhood association fights development and density coming near them using traffic as the favorite argument why. 

People may say that the national average is not meaningful when they are concerned about the nearest intersection. While there is some validity to the argument that one dense project won't reduce congestion from decades of sprawl, it may be comforting that even on a local scale density reduces congestion. Washington DC is certainly still congested, but its growth of about 100,000 people between 2000 and 2015 has actually reduced local VMT. To judge the impact of development one always has to look at the alternative. 200 households of a mid-size apartment building spread over 200 large lot single family homes will cause vastly more traffic, not only where these houses are but also where those residents live, shop and play. 

Ok, people may say, that's all fine and well, but we in America don't like to live in dense cities. 

An explanation why the seeming contradiction that a denser city is also a more pleasant and attractive city would be another article, so would be one that proves that if everybody wants to live in a green rural setting, nobody will get it.

As long as people fight density, we won't have smart cities or protected landscapes.

Additional articles in this series Contradictions resolved will address additional popular misconceptions such as:

why bus and bike-lanes relieve congestion even while reducing road capacity, 

why open space protection inside growth areas still promotes smart growth, 

why affirmative action is appropriate to create a level playing field, 

why black lives matter is the right slogan even when all lives matter, 

why private public partnerships cost more than plain public investment
why reducing Baltimore’s taxes could result in a growing tax base, 

and why legalizing drugs could reduce drug addiction to name just a few.

Klaus Philipsen, FAIA  

International comparison : US VMT: 13,000 km, France 6,200 km