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52
CHAPTER 4
Safety Issues and Their Treatment
This chapter provides information about how data and root causes and contributing factors and how to conduct a
observation can help to indicate why collisions happen and RCA can be found on the NASA website at www.hq.nasa.gov/
how that information can be used to determine treatments. office/codeq/rca/rootcauseppt.pdf. The procedure recognizes
The discussion starts with a general description of the con- the distinction between root causes and contributing factors.
cepts of safety analysis, including root causes and contribut- Root causes are one or more fundamental flaws or problems
ing factors. in a system that lead to the undesirable outcome, and without
After this initial overview, the chapter explores the major the root cause(s), the undesirable outcome (in our case a colli-
categories of root causes and contributing factors for collisions sion) could not have occurred. It is noted that root causes are
on LRT alignments, as suggested by the literature, the survey, not the proximate (or direct) causes of a collision, but lead to
the data analysis, the transit agency consultations, and the site those causes that in turn lead to the collision. This is an
visits. Such information can help practitioners understand important distinction because direct causes often vary from
their safety issues and identify how their concerns align with incident to incident, and are beyond the reasonable control of
those of other systems. the system. For example, a collision may be directly caused by
The final section outlines four treatment strategies that a pedestrian walking in front of an LRV. A root cause could be
address the causes and contributing factors for LRT collisions. a lack of sufficient warning to the pedestrian, but it may not
These strategies are based on the cumulative judgment of the be reasonable to expect that the system could have stopped that
project team after compiling anecdotal and statistical informa- particular pedestrian and avoided that particular collision. It is,
tion with safety engineering experience and in-situ observa- however, reasonable to expect the system to address the root
tions during the site visits. cause that people in general do not have sufficient warning in
the location or circumstances involved.
Contributing factors influence the occurrence or severity of
Root Causes and
a collision but are not actually root causes and if eliminated
Contributing Factors
would not have prevented the collision. Contributing fac-
Safety initiatives are often intended to respond to one or a tors provide a context for the collision. As an illustration, con-
series of crashes. To reduce the number of future collisions, it sider the case of a fatality on an interstate highway facility.
is necessary to determine the root causes. In most cases, there The collision occurred on a straight portion of roadway in
are many factors that influence an incident. These factors come good driving conditions. Police determine that the driver was
together in a specific way to influence the likelihood of colli- fatigued, fell asleep, and left the right-of-way. Driver fatigue was
sion, to result in the collision, and to affect the resulting sever- a causal factor. Police also determine that the vehicle was trav-
ity of the collision. eling at 70 mph when it left the right-of-way. The speed was a
Root cause analysis is a structured procedure for examining contributing factor. It did not on its own cause the collision, but
the reasons an undesirable event occurred, and for identify- would have reduced the time available for the driver to recover
ing ways to prevent the event from happening again. RCA is control upon leaving the pavement (e.g., if awoken by a rumble
employed in the industrial and medical communities for strip), and it affected the severity once the incident occurred. In
safety and quality management, and it is particularly well doc- the same case, the driver would not have left the roadway had
umented by NASA. (See NASA Procedural Requirement NPR there been barriers, but the lack of barriers did not cause the
8621.1.) A presentation explaining the relationship between crash. The absence of barriers is a contributing factor.
