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Swedish TCT observers manual

Bogota, Colombia
Observers’ Manual
for the Swedish TCT
May 2-6, 2016
Prepared by:
Christer Hyden (Lund University)
Annie Chang (WRI)
Natalia Lleras (WRI/Despacio)
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Road safety analysis has traditionally relied solely on traffic crash data. The greatest
drawback of crash-based approach is that it is reactive – a significant number of crashes
need to occur and be recorded before action can take place. With the crash-based
approach, a minimum of three to five years of crash data before and after implementation
is needed for intervention evaluation. The following paradox ensues: road safety
practitioners have to wait for crashes to occur in order to prevent them 1. The
shortcomings of crash-based approach hamper safety analysis at a greater scale in
developing countries due to the poor quality and reliability of crash data.
In recent decades, surrogate safety has emerged as a proactive approach to road
safety analysis. Surrogate safety offers reliable analytical methods that utilize observable
non-crash traffic events and other surrogate data. Recent progress in sensing technologies
and statistical modelling has made widespread adoption of surrogate safety feasible and
To be useful for traffic safety applications, a surrogate measure should satisfy two
conditions2: (i) a surrogate measure should be based on an observable non-crash event
that is physically related in a predictable and reliable way to crashes; and (ii) there exists
a practical method for converting the non-crash events into corresponding crash
frequencies and/or severities.
The most developed surrogate measure is traffic conflicts. A traffic conflict takes place
when “two or more road users approach each other in space and time to such an extent
that a crash is imminent if their movements remain unchanged.” Traffic conflicts are
more frequent than traffic crashes and are of marginal social cost. As the process of a
serious conflict is almost identical to that of a serious crash, traffic conflicts provide
insight into the failure mechanism that leads to road collisions. The intuitive notion that
conflicts are potential crashes, but of a lower degree of danger suggests that a continuum
representation between conflicts and crashes can be assumed. In 1987, Hyden introduced
a pyramidal visualization of the hypothesized continuum for road user interactions from
normal, undisturbed passage, to a fatal traffic crash, as shown in the figure below. Hyden’s
Safety Pyramid illustrates that non-conflicts form the majority of observed encounters
while the observed frequency (or alternatively, probability) of occurrence decreases with
increasing severity3.
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Saunier, N, N. Mourji, and B. Agard. 2011. Using data mining techniques to understand collision processes.
Conference proceedings: Canadian Multidisciplinary Road Safety Conference, organized by Canadian Association of
Road Safety Professionals, Halifax, Nova Scotia.
Tarko, A., G. Davis, N. Saunier, T. Sayed, and S. Washington. 2009. Surrogate measures of safety. White paper:
Transportation Research Board ANB20(3) Subcommittee on Surrogate Measures of Safety, ANB20 Committee on
Safety Data Evaluation and Analysis.
Chin, H. C. and S. T. Quek. 1997. Measurement of traffic conflicts. Safety Science 26 (3): 169-185.
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The Swedish traffic conflict technique consists of a team of human observers who have
been trained to recognize, assess, and record the frequency and severity of traffic
conflicts. It focuses on situations where two road users would have collided had neither
of them made any kind of evasive maneuver. In the Swedish TCT, the severity scale (in
the continuum hierarchy) is established by applying two values: time-to-accident (TA)
and conflicting speed (CS). TA is defined as the time remaining from when the evasive
action is taken until the collision would have occurred if the road users had continued
with unchanged speeds and directions. The TA value can be calculated based on the
observer’s estimates of distances (D) and conflicting speed (CS), where:
CS =
distance of the relevant road user (evading road user) to the potential point
of collision at the moment when the evasive action is taken
speed of the relevant road user at the moment when evasive action is taken
It is important to note that the speed of the road user who takes the evasive action is
taken into account in determining conflict severity. TA values can be determined using
TA Table.
Together with estimated values of CS and TA, observers can determine whether the
conflict was ‘serious’ or ‘non-serious’ using the Conflict Severity Diagram. Qualitatively,
a serious conflict is characterized by suddenness and harshness in action of at least one
of the road users involved.
The relevant road user is the road user relevant for the defining of TA and CS values.
This can be decided by either:
 If only one road user takes evasive action, then, the road user who takes the
evasive action is the ‘relevant’ road user
 If both road users take evasive action, then, the road user whose combination of
TA and CS produces the value with the lower risk is the ‘relevant’ road user
Adopted from “The Swedish Traffic Conflict Technique Observers’ Manual” by Lund University, Dr. Lars Ekman
and Dr. Sverker Almqvist. 2001.
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The following table can be used to determine time-to-accident (TA) values from
observers’ estimates of conflicting speed (CS) and distance (D). TA is defined as the time
remaining from when the evasive action is taken until the collision would have occurred
if the road users had continued with unchanged speeds and directions. The TA value can
be calculated based on the observer’s estimates of D and CS, where:
CS =
distance of the relevant road user (evading road user) to the potential point
of collision at the moment when the evasive action is taken
speed of the relevant road user at the moment when evasive action is taken
The determined TA values will be used in determining the severity of the observed
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The severity of the observed conflict can be determined by using the Conflict Severity
Diagram below. Observers can determine whether the observed conflict is ‘serious’ or
‘non-serious’ using the previously derived values of TA and CS.
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The conflict study has to be conducted corresponding to an observation plan with
especially trained observers. When studies are carried out with several observers at the
same site the observation areas must be well defined in advance. It is not recommendable
that the observer is the same person who has planned the countermeasures because of
the risk of self-fulfilling results. It is also recommendable to change observer or
observation teams with some intervals to keep their attention.
Every conflict situation is recorded on a conflict recording sheet, by using the definition
based on the estimates of speed and distances from where a TA-value can be calculated.
Distances at the location should be recognized by either measuring or stepped out for
making the estimations easier.
Also speed estimations should be carried out in connection the actual location to
familiarize the observer to the vehicle speeds on the site. This can be done with a hand
held radar gun.
According to the definition a serious conflict occur when two road users with crossing
courses would have collide if they have continued with unchanged speed and directions,
but some of the road users make an evasive action.
All conflict situations over and to the left of the “border line” in the TA-graph have to be
recorded. In order, not to miss any serious conflicts, situations just below the borderline
should also be recorded.
When the observer arrives to the study location, one should familiarize oneself with it. Is
this the intersection to be studied? Which part or how is the observation area to be
decided on for the conflict study.
A good observation point is selected next. The main objective is to select a location that
offers the observer a clear view of the decided observation area. For conflict observations,
a location approximately 10 - 25 meters from the inter-section area is usually the best. If
there is some elevation from the ground level it is favorable for a better overview. The
location of the observation point should be noted on the conflict form, especially if the
study will be used in a before and after study for the intersection.
To obtain accurate conflict recordings, it is essential that the observers not influence
passing road users. Ideally, the observer and the recording folder should be invisible. The
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best practice is to blend into the natural life of the intersection to become inconspicuous,
i.e. to look like the observer is waiting for someone.
It is recommended that the conflict observations are conducted at the spot in the free air,
(not from inside a vehicle or a café) because a lot of useful information and
communication as gestures between road users otherwise can be lost.
Observer comfort and, of course, safety are also important factors. For this reason the
observation can be made of a team of observers. Suitable clothing and for example coffee
or water for the breaks can be brought in personal equipment.
For uniformity in the field study, observations should always start at the pre-scribed
times, and observation periods should always be of the prescribed length. To start on time,
the observer will have to arrive at the site at least 15 minutes before the observation period
start. An important preliminary activity is to watch the traffic for a couple of minutes or
more to be familiar with the major traffic movements, the signalization or traffic control
characteristics, and any unusual activities.
Whether the survey lasts for several hours or several days, the observation process
normally consist of 1-hour observation periods. It is determined during the planning of
the study how many days and observation periods per day are to be used for the conflict
For each conflict situation should be recorded:
1. Time of day
2. Road user I and II, the primarily involved in the situation. These are stated and
described in the sketch. If there are some other road users secondary involved they
should also be mentioned.
3. Speed at the moment of the evasive action is taken. The speed can be estimated for
both of the road users or only one, the one who make the evasive action.
4. Distances to the imaginary collision point, where the road users would have collide
if they continued with unchanged speed and directions.
5. TA-value, calculated from the speed and distance via the table.
6. Evasive action: The most common evasive action for vehicles is breaking or
swerving and for pedestrians stopping or running.
7. Sketch in which the road users movement and directions are shown. It is important
to give such details in the sketch that it easily can be recognized in the evaluation
phase. Some extra identification of the site is always worth-wile, typical
geographical landmarks or localization of different shops or professional activities
around in the intersection.
8. Description of the course of events. Besides the sketch the observer should give a
short description of the events and how it was grasped. From the sketch and the
description it should be evident the causes to the conflict situation.
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During the observation periods the observer should make notes of violence’s to the rules
of the site, hazardous behaviors, long waiting times and other factors of interests from a
safety point of view.
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