Network Screening

Network screening is the first step in the process of safety management. It sounds a little scary to me! reminds me of the hospital and health screening!! But in fact, it is the same thing, you try to screen the health of a transportation network based on the diseases it might have or the symptoms of the disease. For our case, however, the disease is the occurrence of accidents. 

Well, if you assume that the process of safety management is a six story building with no elevator, to go to the penthouse where lies the goal of interest (higher safety), you need to pass by each level of this building one after each other. Network screening is the first floor of this building. Within this floor, there are certain steps to be taken before taking the stairs to the second floor what is named the diagnosis floor. When we are in this floor we need to know the following items followed by an example for each item.

1- What our purpose this screening is, what are we trying to find by screening.

For example, we are interested to do a screening in order to find the locations with the highest problem on angle crashes, or run-off-road crashes.

2- What type of facility or what population we are screening.

For example, we are interested to conduct the screening on the intersections in a metropolitan area, or the interstate ramps within the state of Nebraska.

This is to narrow the domain of our network, so that we can more specifically look at the population in which we are interested in. This makes things easier because when we say only ramps, they are similar in characteristics so we do not include the characteristics of other types of facilities such as freeway segments. Think about fishing trout, you never go to a lake to fish trout; in fact, you only go to rivers with certain characteristics. Similarly, if you want to examine angle crashes, you don't want to consider freeway segments; you only take the intersections into consideration. It all depends on the first step: what the purpose is.

3- What performance measures we want to use for screening.

Again, think of purchasing a car; what is important to you? Maximum speed? Engine? Power? Body? These are performance measures that you may use to compare different types/models and select what's best for your pocket. For transportation safety screening you use what is important as a measure to select or prioritize the most dangerous spots for the State or Federal pocket.

For example, we say if the average yearly number of fatal crashes in an intersection is higher than, say, 3 (as a threshold), it can be considered as a dangerous place. Here the performance measure we used was the fatality rate. We could as well say that if the average overall number of crashes is more than, say, 20 (as a threshold), then we need to do something about that roadway segment. So we need to know what criteria we are using to evaluate the network. A relatively good threshold can be the statewide average observed value of the selected performance measure for the selected population. Section 4.2.3 of HSM provides and defines a comprehensive list of potential performance measures with their strengths and limitations. I have tried to walk through some performance measures in here.

4- How do we do screening; what method we are going to use.

Now that we know what we want and have chosen our performance measure to use for screening the selected population, how do we do it? Do we just simply find the location with the worst measure? Do we rank them based on the performance measure values? This step is about choosing a way to come up with the final prioritized hazardous locations for safety treatment. There are several ways to prioritize the hazardous locations: 

    - Sliding window

    - Peak searching

    - Simple ranking

5- Final evaluation and conclude the screening.

After the potential sites are listed and prioritized based on the selected performance measure, we need to further investigate the most hazardous locations, i.e. the locations on top of the list. Now it is time to see how we can improve those locations by thinking and planning about the different types of safety measures that are most beneficial for the selected locations. For this purpose we first to need to conduct a diagnosis of the problems which means we need to see what is actually causing all the crashes that made these locations so dangerous. 

That takes us to the next step in the process of safety management: Diagnosis.