Highway Safety Manual

I have started to review Highway Safety Manual (HSM) for my job and decided to create this space for my notes as a summary of each step I take in the process of educating myself in highway safety. The HSM was first published in 2010 by American Association of State Highway and Transportation Officials (AASHTO). HSM presents guidelines for analyzing and estimating traffic crash frequency and severity at various locations. The purpose of HSM is to "assess the safety impacts of transportation project and program decisions". Detailed information can be found at the official website of AASHTO's HSM.

The HSM is divided into four parts, but why? Couldn't they just have everything in one part, dissected in chapters? well I guess this would have been an easier way to make it understandable and legitimate to an amateur like me! They arranged in four parts, so that the related issues can be presented in the same part, however, these parts are related to each other as a whole. For example, if I want to develop a new project, say adding a new lane to a rural two-lane minor arterial highway in an urban area with specific properties what do I need to keep in mind? I would probably want to know the effects of this change on the safety of the area. for that I would probably go to Part B of the HSM which talks about safety management process such as screening, diagnosis, evaluating alternative implementation approaches, and prioritizing them so that I finally can decide on the most appropriate approach and proceed to next phases such as construction. 

But then what are the other parts about? Well, when I am in Part B and following the guideline I come across a step in evaluation that requires me to find the expected number of traffic crashes that might happen after the construction of my project (which can be done in different approaches or alternatives). I need to know the frequency and severity of different types of crashes that may happen in the future and then compare it to the current or past situation and basically evaluate different alternatives to find the best one. But how can I do that? That is whole another story which is included in Part C of the HSM. This part talk about the method to find the expected crash frequency for my project alternatives. 

Sometimes, we need to modify the expected number of crashes we found according to the improvement plans. Let's assume that I already know how many crashes may occur in the spot where I want to make a change. Now I am interested in the change in crash frequency after the improvement carried out. Do I have to go to Part C and estimate the crash frequency and go through much trouble to find out what is going on? Well, I may as well go to Part D which talks about modification factors, so that I can adjust the expected crash frequency based on the type of improvement planned. This is included in Part D because it is only talking about crash modification factors, however, as a whole it is connected to other parts of the HSM. 

Alright, now we have only missed Part A of the HSM, which, generally speaking, is an introduction to the principles and fundamentals of traffic safety and human factors involved in this field.

So,

What is this whole manual about? In my opinion, this manual is mainly used for safety management. So what is safety management now? In simple words it means to take care of the safety problems that exist on the road and enhance safety of the road users. And how does that work? For safety management there is a process, or a plan, just like any other management operation. There are certain steps that needs to be followed after each other so that we can achieve the results we want. 

Okay, here is how it works. In order to reduce the problems and save lives on the road, which basically means more safety first we need to know what the problem is, and where is it so we can resolve it. The process of safety management is like multi-level building on top of which lies the goal. in order to reach the goal, we take the stairs to each floor and go up level by level. Each level of the building represent a step that has to be accomplished before you go to the next level. That is the process of safety management.

Level 1 - Network Screening

Level 2 - Diagnosis

Level 3 - Selecting Countermeasures

Level 4 - Economic Appraisal

Level 5 - Prioritizing Projects

Level 6 - Evaluating Safety Effectiveness