Introduction

This analysis attempts to understand and quantify the impact that population density has on traffic congestion. Extensive research indicates that larger cities benefit from a scaling advantage, meaning that increases in population density do not impact traffic congestion as significantly as in smaller cities. This is crucial for West Michigan, which falls into the smaller city category and may face higher severity of transportation-related growing pains.

Current State of Traffic Density in Metro GR

The dynamic map below (only in HTML format) shows the current state of average daily traffic congestion based on data from MDOT/AADT. Target locations for high-density housing builds are highlighted with light blue circles. A visual analysis of these intersections reveals an important discovery: the target locations already experience moderate to high traffic congestion.

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Modeling Density’s Impact to Traffic Congestion


Numerous factors contribute to a community’s traffic density, including location of destinations, income, and availability of transportation modalities. Using research from Sustainability’s article “Population Density or Population’s Size: Which Factor Determines Urban Traffic Congestion?”, the following model was derived and applied to our research:


\[ Y = 0.327 \times \text{Density} + 0.209 \times \text{Population} + 0.165 \times \text{Income} - 4.447 \]
Given that this is a log-log model, this equation indicates that for every 1% increase in population density, there can be an expected 0.327% increase in traffic congestion, holding all other factors constant.

Note: This model was used due to it’s inclusion of 164 global cities (including Grand Rapids, MI) and it’s use of Hansen Threshold techniques to differentiate models between calculated city groups.

Traffic congestion can be difficult to quantify, so concurrent with the Sustainability model we will be using TomTom’s Traffic Index to understand traffic congestion and the average drive times in the GR Metro Area. According to this tool, the average time for a driver in Grand Rapids to drive 6 miles is 9 minutes at any time, or 10 minutes 27 seconds at peak rush hour. The table below shows how these factors are impacted as density increases, based on the model above and starting with Grand Rapids’ current density of 1715 individuals per sq/km.

Density’s Impact to Drive Times (Minutes) in Metro GR
Iteration Density (sq/km) Avg Drive Time Rush Hour Drive Time
0 1715.0 9.00 10.45
1 1732.2 9.03 10.48
2 1749.5 9.06 10.52
3 1767.0 9.09 10.55
4 1784.6 9.12 10.59
5 1802.5 9.15 10.62
6 1820.5 9.18 10.66
7 1838.7 9.21 10.69
8 1857.1 9.24 10.73
9 1875.7 9.27 10.76
10 1894.4 9.30 10.80


In this table, each iteration accounts for a 1% increase in density. After ten 1% increases (slightly different than a flat 10% increase), the average time only increased by 18 seconds while the rush hour average time increased by 21 seconds. This model indicates that increases in density have a minimal impact on traffic congestion. Density in Metro Grand Rapids would need to increase to 2,266 sq/km to see a full minute increase in rush hour average traffic congestion, holding all other factors constant.



Assumptions

This analysis uses generalized density and does not account for small, targeted influxes of high-density zones. For example, at 28th and Eastern, an increase of 200 drivers is unlikely to complicate traffic congestion in Metro Grand Rapids or even that specific section of the road. However, it is possible to create an acute bottleneck that the current intersection is not designed to handle (think of a popular fast-food restaurant at lunchtime and its impact on a road).

This analysis uses one particular model to assess the impact of density on traffic congestion, holding other factors in the model (population and income) constant. This helps to understand the marginal impact of density on traffic congestion only and should not be used to predict traffic congestion.

Conclusions

  1. Increasing density in specific corridors should not dramatically impact traffic density. The areas highlighted in Metro Grand Rapids already face daily traffic that will be negligibly impacted by a few hundred new drivers.
  2. Although density has a measurable impact on traffic congestion, Metro Grand Rapids can increase density for some time before noticing dramatic impacts on drive times.

Sources and Other Research