I’m sure you’ve thought about the effect your genetics, diet, and level of exercise have on your health, but what about where you live? A recent study of 24 cities in California looked at how the design of street networks affected levels of obesity, diabetes, heart disease and high blood pressure.
Cities such as New York and Barcelona are arranged in a grid – small blocks with highly connected streets make it easy to walk to work, school, and the shops. The typical American suburb is very different however – sprawling and tree-like, with large plots to accommodate luxuries such as pools making them near impossible to traverse on foot. Previous research had found that compact grid cities killed a third of the number of people as tree-like ‘hierarchical’ cities, so the aim of this study was to better understand the exact reasons why this was happening.
Three street network characteristics were measured: connectivity, network density, and configuration. Network density was measured by dividing the number of nodes (intersections) and links (stretches of road between intersections) by the area, and network connectivity was measured by calculating the ratio of nodes to links. Configuration was harder to measure, as intersection density and link-to-node ratio tell you nothing about the actual layout of the streets. Therefore, streets were categorised as either major (primary, secondary, and connecting streets) or minor (all other streets) and networks were classified according to a chart adapted from the book Streets and Patterns by Stephen Marshall. Using Google Street View, major streets were then assessed for characteristics including number of lanes, and presence of bike lanes and pavements.
Using a statistical model, the researchers found that cities with a higher intersection density, i.e. grid-plan cities, had reduced rates of all four diseases. Reducing the intersection density of a city from 144 per square mile to 81 would lead to obesity levels rising 33.4%, diabetes rising 42.4%, high blood pressure rising 12.9%, and heart disease rising 19.7%. The same reduction in intersection density applied to a single block was found to only increase obesity levels, and only by 2.8% – suggesting that compact street networks need to be city-wide in order to improve health.
Conversely, measures of network connectivity found that a better connected road network (again, the kind found in grids) was most effective at block level, significantly reducing levels of obesity (by 8.6%) and heart disease (by 6.7%).
When compared to ‘TT’ cities (those with tree-like major and minor roads), it was found that fully-gridded cities had 15% less obesity, 10% less high blood pressure and 6% less heart disease. Cities with radial major roads and gridded minor roads also showed lower heart disease and high blood pressure rates than TT cities, suggesting that if commodities are easy to get to people are more likely to walk than to drive.
The characteristics of the streets also had an effect on disease rates. Wider streets with more lanes were associated with higher obesity and diabetes, probably because they discourage pedestrians as they are difficult to cross and often do not have pavements. Diabetes increased a massive 366.6% when major roads had an average of six lanes instead of two.
Of course, these results were obtained using a statistical model, so they are not necessarily an accurate measure of what is happening in the real world, where all sorts of other factors influence whether we get fat or ill. These results do however give us a good idea of the problems caused by living in cities where you have to drive everywhere, and thus have to go out of your way to exercise – and with 68% of Americans and 64% of Brits classed as overweight or obese, the more we know about how to avoid obesity the better.
Marshall, W., Piatkowski, D., & Garrick, N. (2014). Community design, street networks, and public health Journal of Transport & Health DOI: 10.1016/j.jth.2014.06.002