Posted by Damien Pierce, Software Engineer, and John Anderson, Senior Research Director, Google Research
Some cities or communities develop an evacuation plan to be used in case of an emergency. There are various reasons why city officials might implement their plan, with a primary one being a natural disaster like a tornado, flood, or wildfire. An evacuation plan can assist the community in responding more effectively to an emergency, potentially saving lives. However, evaluating such a plan can be challenging for a city since it is impractical to have an entire town or city rehearse a complete evacuation. For instance, Mill Valley, a city in northern California, created a wildfire evacuation plan but lacked an estimate of how long the evacuation would take. In this article, we present a case study in which we collaborated with the city of Mill Valley to test and enhance their evacuation plan. Our approach is outlined in our paper, “Mill Valley Evacuation Study”. We began by utilizing a traffic simulator to model a citywide evacuation. The research aimed to provide the city with detailed estimates of evacuation time and identify modifications to make the plan more effective by studying the egress pattern. While our previous work on this topic provided an estimate for evacuation time and demonstrated how time could be reduced through certain road changes, it turned out that the recommendations in that paper, such as altering the number of outgoing lanes on a main road, were not feasible. This current round of research improves upon the initial study by more accurately modeling the number and starting locations of vehicles, using a more realistic map, and working closely with city officials to ensure that recommended plan changes are deemed viable.
Geography and methodology
Mill Valley is located in Marin County, California, to the north of San Francisco. Many residences are situated on the steep hillsides of several valleys surrounded by dense redwood forests. Aerial views of Mill Valley, courtesy of the City of Mill Valley. Some of these residences only have one exit direction, leading towards the town center. From there, the recommended evacuation route is towards Highway 101, which is located in the flat part of the city and is the area least likely to be affected by potential wildfires. While some neighborhoods have alternative routes away from the city and Highway 101, these routes pass through hilly forested areas that could be dangerous or impassable during a wildfire. Therefore, the evacuation plan directs all vehicles west of Highway 101 to head east, towards the highway (see map below). The neighborhoods east of Highway 101 are excluded from the simulation since they are away from areas with a high fire hazard rating and are close to the highway. Mill Valley has approximately 11,400 households west of Highway 101, and most households have two vehicles. Evacuation times are influenced by the number of vehicles, so it is beneficial to minimize the number of vehicles used during an evacuation. To achieve this, Mill Valley has a public awareness campaign encouraging each household to evacuate using only one vehicle. While the exact number of vehicles used during an evacuation is unknown, it is safe to assume that on average, it is between one and two per household. The main evacuation challenge, therefore, is efficiently getting between 11,000 and 23,000 vehicles from various residences onto one of the three sets of Highway 101 on-ramps. The simulated part of Mill Valley west of Highway 101 is depicted inside the blue border. Highway 101 is shown in green. The red squares indicate the three sets of Highway 101 on-ramps. The pink area represents the highest fire hazard rating.
The current research employs the same general methodology as previous research, utilizing the open-source SUMO agent-based traffic simulator on a map of Mill Valley. The traffic simulator models traffic by simulating each vehicle individually, with detailed behaviors determined by a car-following model. Each vehicle is assigned a starting point and time, as well as an initial route. Most vehicles’ routes are updated throughout the simulation based on conditions. To account for potential changes in driver behavior during high-stress evacuation conditions, the impact of each car’s “aggressiveness” is also investigated, although the effects are minimal in this case. Some simplifying assumptions are made, such as vehicles originating from residential addresses and the roads and highways initially being empty. These assumptions approximate conditions that could occur during a nighttime evacuation. The main inputs in the simulation are the road network, household locations, average number of vehicles per household, and a departure temporal distribution. Assumptions must be made regarding the departure distribution, and after consulting with city officials, a distribution was chosen where most vehicles depart within an hour.
Four bottlenecks
Mill Valley has three sets of Highway 101 on-ramps: northern, middle, and southern. All vehicles must utilize one of these sets of on-ramps to reach their destination (either the northernmost or southernmost segment of Highway 101 included in our map). Given that only the majority of Mill Valley west of the highway is considered, there are two lanes leading to the northern on-ramps and one lane each for the middle and southern on-ramps. Since every vehicle has to pass over one of these four lanes to reach the highway, they serve as bottlenecks. Additional lanes cannot be added due to geographical constraints and existing infrastructure. The objective of this research, therefore, is to modify traffic patterns to maximize traffic flow on each of the four lanes.
Evacuation plan
At the beginning of this research, Mill Valley had a preliminary evacuation plan that involved modifying traffic patterns, such as disabling traffic lights and changing traffic rules, on certain road segments, as well as specifying the necessary resources (traffic officers, signage) to implement these changes. For example, a two-way road may be converted into a one-way road to double the number of outgoing lanes. Temporarily changing the traffic direction is known as contraflow. The plot below illustrates the simulated fraction of vehicles that have departed or reached their destinations over time, for 1, 1.5, and 2 vehicles per household (left to right). The dashed line on the far left represents the fraction that have departed. The solid black lines depict the results of the preliminary evacuation plan, while the dotted lines indicate the results of the normal road network (baseline). The preliminary evacuation plan significantly speeds up the evacuation. The cumulative fraction of vehicles vs. time in hours. The demand curve is shown as the dashed line on the far left. The solid lines depict the preliminary evacuation plan curves for 1, 1.5, and 2 vehicles per household (left to right). The dotted lines show the same for the baseline case. The effectiveness of the preliminary evacuation plan can be assessed by measuring the traffic rates at the bottlenecks. The plots below display the rate of traffic on each of the four lanes leading to the highway on-ramps for the case of 1.5 vehicles per household, comparing the baseline case (normal road rules; shaded in gray) to the preliminary evacuation plan (outlined in black). The average rate per lane varies significantly among the different scenarios. While the evacuation plan leads to increased evacuation rates, there is still room for improvement, particularly in the middle on-ramps. The rates of traffic on the four lanes leading to Highway 101 on-ramps for both the baseline case (normal road rules; shaded in gray) and the preliminary evacuation plan (outlined in black).
Final evacuation plan
After analyzing the map and exploring various alternatives, we, in collaboration with city officials, identified a minimal set of new road changes that significantly reduce the evacuation time compared to the preliminary evacuation plan (shown below). This is referred to as the final evacuation plan. It extends the contraflow section of the preliminary plan 1000 feet further west, to a main intersection. Importantly, this allows for one of the two outgoing lanes (normally) to become an incoming lane during the evacuation, effectively doubling the capacity of the middle on-ramps. The final evacuation plan also includes additional signage and traffic officers to facilitate the modified traffic patterns. The final evacuation plan for Mill Valley.
In conclusion, by utilizing a traffic simulator and collaborating with city officials, we were able to test and enhance the evacuation plan for Mill Valley. The final evacuation plan incorporates specific road changes and traffic management strategies to more effectively evacuate the city in case of a wildfire. This research demonstrates the importance of modeling and analyzing evacuation plans to ensure the safety and well-being of communities during emergencies.
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