City Simulator tool – temperature impacts on Atlanta’s carbon footprint for travel

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Markets

Buildings & places

Transportation
Location

Atlanta, GA, USA
Client

Atlanta Regional Commission
Completion date

April 30, 2020

Stephen Bourne

Project Director, Miami, United States of America contact form+1 678 247 2581

As temperatures rise globally, commuters are faced with the choice of comfort over cost.

As part of a recent study, AtkinsRéalis was commissioned to work with the Atlanta Regional Commission to understand the impact of increasing temperatures on travel mode choice, and the resulting impact on travel-related carbon footprint.

The team used AtkinsRéalis' City Simulator, which is a decision support tool that creates a digital twin of an area (city, county, transportation corridor, port) and then evolves it into the future. The simulation includes both economic-driven urbanization and climate change influenced disasters like storms, heat waves, and droughts. The tool is agent-based, so it simulates a population of avatars that statistically match the real population, so that disruption to people’s lives can be quantified and foreseen.

The study assumed that at the beginning of each year, each agent would review how hot it was in the previous year (specifically the summer maximum temperatures) and decide whether they would switch from transit (specifically buses) to using a single occupancy vehicle to avoid heat exposure when traveling to and waiting at bus stops.

Client objectives and challenges

Assess the impact that increased temperatures would have on the carbon footprint of Atlanta city travel;
Understand how modes of transport are used differently when city temperatures rise; and
Explore mitigation measures that would counteract any increase in carbon emission levels as a result of higher temperatures.

AtkinsRéalis solutions and added value

The City Simulator tool allowed the commuting routes of Atlanta citizens to be simulated in detail, documenting each street segment used by an agent and the chosen mode of travel (walk, single occupancy vehicle (SOV), carpool, bus, or train).

To understand how mode choice might change given increasing temperature, each person in the simulation was assigned a decision model that considered multiple factors including:

The percent of their salary they spend on transportation,
The travel time required to commute to work,
The amount of heat exposure they’re likely to encounter as they walk to and from as well as wait at bus stops.

The decision model assumed the factors would be considered trade-offs for each person, where for example a person may elect to pay more of a percentage of their salary if they get shorter travel times and lower heat exposure as a result.

Of course, each person may make this choice differently based on personal preference and available funds. To calibrate to real world conditions, the population’s preferences were set in the 2019 base year so that the percentage of people taking bus, using single occupancy vehicles, and so on matched known statistics in the Atlanta area.

To include the influence of increasing heat in the simulation, projections of future temperatures from global climate models were used. In each year, each person was simulated making a decision of which mode choice they’d used in the coming year based on how high temperatures reached the previous year. If potential heat exposure was too high, a person may choose to switch from bus to SOV, even if the cost as a percentage of their salary was significantly higher.

The resulting shift in mode choice over the whole study area showed a steady shift from transit to single occupancy vehicles as temperatures increased into the future. Figure 1 (below) shows the percentage of trips among walk, transit, and SOVs over the 2019 – 2050 simulation timeline. The upper line chart gives an example of a maximum temperature projection from a global climate model. The stacked bars display the percentage of trips.

Following the initial analysis, a second scenario was created incorporating adaptation and mitigation actions like air conditioning bus stops and increasing shade through planting campaigns. This showed the shift to SOV could be slowed down significantly.

Services provided

Training on using City Simulator via a Google Classroom with step-by-step video walkthroughs and an example study;
Projection of max temperatures in the Atlanta Area up to the year 2100;
Construction of a methodology for simulating the choice of switching to single occupancy vehicles based on salary, travel time, and heat exposure; and
Review of mitigation options available to counter impacts of temperature on carbon emission levels.

As the upper line shows, global climate models project steadily increasing maximum annual temperatures from 2020 to 2050. The result is that more and more people use single occupancy vehicles (auto) to get to work to avoid walking and waiting for bus and transit services in extreme heat.

Key benefits and success factors

The simulation was able to identify that the share of single occupancy vehicle trips increases from 77% to 80% over the 2019 – 2050 timeline, and that transit share trips would fall from 14.5% to 12% over the same period. This allowed the client to estimate the expected rise in transport carbon emission levels;
We were able to assess the congestion effects that increased single occupancy vehicle trips would bring about, thus highlighting to ARC the higher-stress and higher-risk driving environment that would exist for their city residents;
Analysis supported ARC in exploring options for adaptation and mitigation such as air-conditioned bus stops and increasing tree canopy to reduce heat exposure to residents taking transit; and
Following initial exploration of mitigation options with ARC, a next step in the study is to simulate the effects of potential counter measures and identify which are the most effective and cost-efficient.