Congestion assessment according to DIN 18009-2
When many people need to exit a building quickly, it can cause congestion. This increases the time it takes for people to get to safety. People flow simulations are a useful tool to simulate such situations in the planning phase. But what exactly is a traffic jam? What criteria define it? And how should the results be evaluated?
In places that are used by many people at the same time, the risk of critical situations increases. Disasters such as the 1989 Hillsborough stadium disaster, in which 96 spectators died as a result of a bad decision by security staff, or the Love Parade disaster occur time and again.
How did this happen? The dynamics that arise when large numbers of people gather are complex. It becomes particularly dangerous when the density of the crowd becomes high and does not disperse. In such situations, the risk of injury increases because of fear, anxiety and jostling. Pressure waves and domino effects can also occur, leading to disasters.
Two factors play a critical role: the geometry of the site and human behavior. Are there obstacles such as columns in main corridors or bottlenecks that cause congestion? Socio-psychological aspects such as risk psychology, which deals with the behavior of people in emergency situations, also play a role.
Pedestrian flow simulations can be used to consider and analyze human dynamics in the design of buildings and events. Before this tool can be used, it is necessary to define what crowding is and how it can be identified and categorized.
What is a congestion?
Based on traffic science definitions, a traffic jam is characterized by the following factors
Capacity overload: The inflow to the location is greater than the outflow.
High densities: Density exceeds a level at which travel is comfortable.
Speed reduction: I can no longer move at my desired speed
Time loss: Because of waiting or queuing, it takes me longer to get where I need to go than it would normally take.
A traffic jam does not have to be a critical situation. In museums, traffic jams can form in front of individual exhibits, but this is not critical. Some traffic jams at events may even be intentional. As long as the congestion can be avoided or accepted by an individual, it is generally not considered critical.
The question now is when an accepted traffic jam becomes critical. This is done by translating the influencing factors into metrics.
What are the key parameters?
There are a number of criteria that can be used to identify jams. They can refer to the jam as a whole (macroscopic) as well as to the individuals within a jam (microscopic). The goal is to define characteristics that can be used to identify and evaluate jams. The following characteristics are considered in the selection of criteria
Comprehensible: A criterion must be understandable and comprehensible - not only to experts.
Clearly definable: It is important that the criteria do not leave much room for interpretation and ambiguity in order to remain comparable.
Easy to evaluate: The criteria should be practicable so that, at best, they do not lead to time-consuming and error-prone additional work.
Criteria that characterize the congestion:
Congestion duration: describes how long a traffic jam exists. This parameter is not very meaningful per se, as it depends on the people and the situation: a traffic jam can last several minutes, but each person passes through the jam in a few seconds.
Congestion size: refers to the number of people in a traffic jam. Since the number of people in a traffic jam changes over time, it is difficult to measure and should at least be related to time.
Extent of congestion: describes the spatial extent of a traffic jam. It can give an indication of the importance of a traffic jam, e.g. if the jam blocks another escape route. However, this criterion is very situation- and model-dependent (depending on the simulation model, individual people may be represented as circles, ellipses, or square cells, which can lead to different spatial extents). It should also be linked to another criterion such as duration.
Jam growth: describes the growth of a jam. It is a value derived from the number of people over time. Congestion growth can be used to make statements about capacity (congestion).
Flow through congestion: describes people per minute per meter depending on speed and number of people. It is a derived value and only makes sense if you know what the optimal flow is.
Criteria based on individuals:
Speed: describes the speed of each individual. It has proven to be useful for determining congestion: By setting a threshold speed, you can identify those individuals who are in a congested situation.
Individual Queuing Time: describes how long each individual is in the queue. This can be the individual queue time per queue, or the total queue time for consecutive queues, and is a good measure for evaluating queues.
Delay: describes the time lost by a person. This value is interesting because it indicates how much the travel time increases due to congestion. In practice, this value is difficult to determine because the travel time without congestion has to be calculated for each individual in order to have a basis for comparison.
Density: describes the (minimum, average, maximum) density of people in a traffic jam. Does not make sense from a simulation point of view, because some models store maximum densities and therefore the results are not comparable. In addition, the density measurement would need to be standardized.
Ratio of jam duration to total clearing time: describes the ratio of maximum jam duration to total clearing time per person. The meaning of this parameter depends strongly on the scenario. For example, if there are safe areas at different distances, this can distort the analysis.
As the parameters are listed, it becomes clear that not all variables can stand alone, but that the planner should consider them in the context of his scenario. The next section uses an example to illustrate how such an analysis can be performed.
Methods for Identifying & Assessing Congestion
The speed criterion should be used to identify congestion. However, it is necessary to agree on how high to set the limit. One way to determine this threshold speed is to use a fundamental diagram. A fundamental diagram describes the relationships between (people) flow, speed and density and has its origins in traffic analysis. The higher the density in a crowd, the slower people move; above a certain density, the flow rate is at its maximum; above the density, the flow rate begins to decrease and a traffic jam occurs. (more on the fundamental diagram here)
The congestion situations defined by the speed criterion must be checked for criticality in the analysis. The simulation expert is responsible for judging whether a traffic jam is critical or non-critical. For example, the following criteria can be used for the assessment
Duration of the congestion: How long will the congestion last?
Location of the incident: Where is the congestion located?
Individual congestion times: How long is a person in a traffic jam?
Congestion size: Number of people involved
In addition, boundary conditions such as the reason for the evacuation, the motivation of the people, the type of use, and other relevant factors should be included in the assessment.