Svensson and Hyden 2006

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Accident Analysis and Prevention 38 (2006) 379385

Estimating the severity of safety related behaviour

Ase Svensson , Christer Hyden

Department of Technology and Society, Lund Institute of Technology, Lund University, Box 118, SE-22100 Lund, Sweden

Received 14 October 2005; accepted 18 October 2005

Abstract

The aim of this work is to be a starting point for a more thorough description and analysis of safety related road user behaviour in order to better

understand the different parts forming the traffic safety processes. The background is that it is problematic to use analysis of crash data and conflict

data in the everyday traffic safety work due to low occurrence rates and the focus on rather exceptional and unsuccessful events.

A new framework must consider the following aspects: (1) The importance of feedback to the road users. (2) Inclusion of more frequent events,normal road user behaviours and the possibility to link them to a severity dimension. (3) Prediction of safety/unsafety based on the more frequent

events.

By constructing severity hierarchies based on a uniform severity dimension (Time to Accident/Conflicting Speed value) it is possible to both

describe the closeness to a crash and to get a comprehensive understanding of the connection between behaviour and safety by both considering

unsuccessful and successful interactive situations. These severity hierarchies would make it possible to consider road users expectations due to

feedback and estimate its safety relevance.

2005 Elsevier Ltd. All rights reserved.

Keywords: Traffic; Safety; Behaviour; Conflicts; Interaction; Feedback

1. Introduction

The aim of the work behind this paper is to extend the traf-

fic safety assessment concept to also include normal road user

behaviours, thus not only exceptional behaviours such as those

leading to crashes and/or serious conflicts. The goal is to pro-

vide a framework for a more thorough description and analysis

of road user behaviour in order to better understand what we

define as the traffic safety processes, i.e. the interactional pro-

cesses that define events of different severity.

2. Background

2.1. Interaction

Traffic is interactionall events in traffic contain some kind

of interaction but of course to varying extent. There is interac-

tion between road users and there is always interaction between

the road user and the road environment. In this paper the term

Corresponding author. Tel.: +46 46 2229125; fax: +46 46 123272.

E-mail addresses: [email protected] (A. Svensson),

[email protected] (C. Hyden).

interaction is restricted to the relation between road users. The

interaction between road users can be described as a continuumof safety related events (see Fig. 1).

This pyramid shows how few and exceptional those events

are that we usually base our safety estimates on, i.e. the crashes,

rarely also including the serious conflicts.

2.2. Need for surrogate measures

The traditional way of approaching traffic safety has mainly

been concerned with the occurrence of traffic crashes and their

consequences.There are, however, disadvantages with the use of

crash data analyses and these have been discussed extensively in

several reports, e.g. Englund et al. (1998), Grayson and Hakkert

(1987). (1) Crashes are rare events and are therefore associated

with the random variation inherent in small numbers. (2) Not

all crashes are reported and the level of reporting is unevenly

distributed with regard to, e.g. type of road users involved, loca-

tion, severity of injuries, etc. (Berntman et al., 1995). (3) The

behavioural or situational aspects of the events are not covered

in police crash data (Berntman, 1994). Crashes are also excep-

tional in the sense that they are a collection of events where all

alternatives to handle the situation safely, have vanished one by

0001-4575/$ see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.aap.2005.10.009


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380 A. Svensson, C. Hyden / Accident Analysis and Prevention 38 (2006) 379385

Fig. 1. The pyramidthe interaction between road users as a continuum of

events (Hyden, 1987).

one. This is indeed exceptional compared to most other events

that actually are handled safely (though to different degree).

Thus, we need to get a more comprehensive understanding of

the connection between behaviour and safety by both consider-

ing unsuccessful and successful interactive situations. The need

for surrogates or complementary methods for crash data analysisis consequently highthe Traffic Conflicts Technique (TCT) is

such a method.

A conflict is a situation where two or more road users

approach each other in time and space to such an extent that

a collision is imminent if their movements remain unchanged.

The development of TCT has shown that serious conflicts con-

tain most of the qualifications lacking with crash data analysis.

Serious conflicts do for instance possess the quality of being an

indicator of a breakdown in the interactiona breakdown that

could correspond to the breakdown in the interaction preced-

ing a crash. A serious conflict is also, like the crash, a situation

that nobody puts him/herself into deliberatelythe situation isperceived as being too threatening (Hyden and Stahl, 1979).

The relationship between serious conflicts and injury crashes

reported by the police has been elaborated on and established

through two validation studies. Hyden (1987) deals with three

samples of data. The product validation part produces a set of

conversion factors, i.e. establishes the relationship between the

number of crashes and the number of serious conflicts. In the

process validation part analyses are conducted regarding simi-

larities of the processes preceding crashes and serious conflicts.

Analyses showed big similarities between crashes and conflicts

when thecomparison was based on Time to Accident (TA) values

and Conflicting Speed (CS). It also showed that the distributions

of different types of evasive action were very equal for crashesandconflicts. In Svensson (1992) analyses on theproduct valida-

tion of the Swedish TCT show that at lower crash frequencies it

is preferable to use conflicts instead of crashes when estimating

the expected number of crashes. For further information about

the Swedish TCT and other TCTs see also, e.g. Grayson (1984).

Many of the shortcomings in crash data analyses are provided

for with the use of TCT, but not all. Sometimes also the serious

conflicts are too few to obtain statistically significant estimates

at assessment studies. The analyses of serious conflicts do also

have the same angle of approach as the crash data analysis, i.e.

the primary focus is set on rather exceptional and unsuccessful

events; unsuccessful in the sense that road users have to take

strong evasive action to avoid a crash. Experience with the TCT

has, nevertheless, shown that it is possible to include less severe

events than crashes, i.e. serious conflicts, and reach better under-

standing of the traffic safety process.

3. Extension of the concept

As the task here is to try to explain the relationship between

road user behaviour andsafety this implies an unambiguous need

for widening the scope of traffic safety and safety related events.

There are at least three fundamental issues that are important to

consider, to link and to interpret when structuring a new frame-

work.

The importance of feedback to the road users.

Inclusion of more frequent events, normal road user

behaviours andthe possibility to link them to a severitydimen-

sion.

Improve prediction of safety/unsafety.

3.1. The importance of feedback to the road users

What is it that makes one traffic environment more crash

prone than another? A basic hypothesis in this paper is that

the feedback to the road users can be an important explanatory

factor. The occurrence of crashes can be due to lack of feedback

but also due to the fact that existing feedback is misleading or

perhaps incorrectly interpreted.

The importance of feedback to road users has not least been

acknowledged when traffic education for children is discussed.

According to Thomson et al. (1996) referred to by Whitebread

and Neilson (1996) pedestrians require a range of fundamentalskills to interact safely in traffic. The pedestrian has to, among

many other things, make judgements of whether the crossing

place is safe or not by co-ordinating past experience, present

information and predictions about the future. Children lack this

cognitive ability and they have due to obvious reasons not yet

had the time to achieve feedback based on previous experiences

in traffic. Thus, practical training in traffic is crucial.

Crash statistics also clearly show the need for practical train-

ing and the importance of feedback. Young drivers with a fresh

drivers license is a road user group with high crash risks. Expe-

rience obviously plays an important role here. It is, however,

according to Evans (1991) reasonable to in addition assume that

the over involvement by young road users in traffic crashes mustinvolve more than lack of driving experience as the tendency is

the same for pedestrians as for car drivers at that age.

Feedback from interactions is most likely a very important

part of the learning process and the more obvious feedback the

road users have got, the more influence it has on their behaviour

in similar situations/environments.

3.2. Inclusion of normal road user behaviour

3.2.1. Criteria of events in the framework

We are looking for a framework that handles predefined

events that are much more frequent than injury crashes and seri-


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A. Svensson, C. Hyden / Accident Analysis and Prevention 38 (2006) 379385 381

ous conflicts; events that are not as exceptional as injury crashes

and serious conflicts but still have a logical link to crashes

and serious conflicts. The framework and the selected events

must make it possible to make analyses of the feedback effect

on an individual level as well as on an aggregated level, e.g.

of locations, of type of manoeuvre, etc. To be able to make

comparisons between different locations with, for instance, dif-

ferent geometrical design, the severity of the predefined events

and the distribution of these events must be described in a reli-

able and uniform way. This distribution of events, a severity

hierarchy, is a feasible solution where the distribution of events

over severity will provide us with information regarding the traf-

fic safety situation at a location. Another prerequisite is that

the events should describe behaviours that can be related to

feedback, i.e. to road users expectations of the prevailing sit-

uation occurring in a specific traffic environment. Finally, the

prediction of safety/unsafety should be improved by using these

events.

3.2.2. Severity and severity hierarchyA collision presupposes a collision course. It therefore seems

logical to define the common denominator for events to be

included in the severity hierarchy, as those events where the

road users move on a collision course. The aim must, further-

more, be to construct a severity hierarchy for traffic events so

that for each event a severity, i.e. the events closeness to a crash,

can be estimated. One feasible, and here chosen, possibility is to

describe an events severity by its Conflicting Speed and Time

to Accident value.

TheConflictingSpeedis thespeed of theroad user takingeva-

sive action, for whom the TA value is estimated, at the moment

just before the start of the evasive action.The Time to Accident is the time that remains to a crash

from the moment that one of the road users starts an eva-

sive action if they had continued with unchanged speeds and

directions.

These measures have been applied in the Swedish TCT. For

clarificationit might be added that situations with low PET(Post-

Encroachment-Time) values are included in the Swedish TCT,

however on the precondition that the involved road users behave

as ifthey were moving on a collision course. It has been shown

(Hyden, 1987) that serious conflicts, as defined in the Swedish

TCT, are events that represent a logical continuation of crashes

on a severity scale. It is therefore reasonable to assume that,

using the same severity definition, less severe events could log-ically follow on the serious conflicts. Severity, described by an

events TA/CS value, does not refer to the known outcome after

the evasive action but to the severity of the event an infinitesi-

mal unit of time before the evasive action. At this very moment

we can say that an unknown event with a certain location in the

severity hierarchy has a certain closeness to an injury crash, i.e.

a certain probability of resulting in an injury crash. The out-

come in the form of a crash or not then depends on the success

of the evasive action and of course on the local characteris-

tics.

In the work of identifying factors with regard to risk causa-

tion we would be much better off starting from the much more

Fig.2. TA/CS graph definingthe differentseveritylevels.There is a continuation

towards lower severity levels. Severity level 1(not shown) intersects the X-axis

at TA = 13.0 s (Svensson, 1998).

frequent normal behaviours, than the rare and unique crashes.This, however, presupposes that all events with a TA/CS value

are parts of the same traffic safety process as the crashes and

serious conflicts, i.e. that there is a link between normal road

user behaviour and critical events with crashes at the end of the

scale.

To conclude:

Relevant events in the traffic safety process are henceforth

called interactions.

Interactions are characterised by collision course and the

severity is described by the TA/CS values.

The severity distribution of the interactions is analysed by the

construction of severity hierarchies (see Figs. 2 and 3). The

understanding of different hierarchy shapes together with

information about absolute numbers on different severity lev-

els will increase the prospect of making reliable predictions

about safe/unsafe environments and make it possible to con-

sider road users expectations due to feedback.

Fig. 3. Severityhierarchywith severity levels corresponding to the ones defined

in Fig. 2 (Svensson, 1998).


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Fig. 4. Example of a possible hierarchy shape (Svensson, 1998).

Hyden (1987) indicated with his pyramid (Fig. 1) that there

would be a continuous increase in the number of events the

further away you come from the most severe events. This isobviously true when we look at allevents.If we,however,restrict

ourselves to interactions characterised by collision course, then

it is likely that the shape is different. As with accepted gaps,

speeds, etc. it is most likely that one will find a distribution that

will be similar to a normal distribution, i.e. the severity hierarchy

will be peaked on both sides (Fig. 4).

The hypothesis is that different hierarchies will have differ-

ent degree of accumulation of interactions allocated to different

parts of the hierarchy. The shape and position of the convexity,

i.e. the part of the hierarchy where most events are located, could

possibly reflect the most frequent interactive road user behaviour

for that location/manoeuvre/road user type. The convexity could

be interpreted as representing the road users optimisation oftheir desires to keep a high mobility standard, to maintain safety

margins and to maintain comfort.

3.3. Prediction

A new framework should improve the conditions of making

reliable safety/unsafety predictions. If we by safety mean lack

of crashes or serious conflicts then we would have to equate

unsafety with the presence of crashes and/or serious conflicts.

This definition is, however, not adequate for at least the two dif-

ferent reasons mentioned earlier, i.e. crashes are rare and they

do not offer any understanding of the causality. A more thor-

ough understanding could be achieved by applying a concept

where it would be possible to identify the preconditions for a

safe/unsafe environment. Such a concept prerequisites a broader

angle of approach than merely stating whether crashes occur

or not.

4. Model

Fig. 5 below shows what our model for the relation between

road user behaviour, shape of the severity hierarchy, feedback

and predictions for safety and unsafety looks like.

Fig. 5. Modelfor connecting theroad user behaviour to predictions about safety

and unsafety.

4.1. Relations in the model

4.1.1. Interactions with TA/CS value severity hierarchy

prediction

The information we get from each interaction and its sever-

ity, is how close this particular interaction was to a crash, i.e.

how imminent the danger was. Also the different parts of the

hierarchy (clusters of interactions) contain information, like;

the significance of the predominant behaviour when it comes to

describing theprobability for thelocation to produce safe/unsafe

interactive situations; or the significance of the absolute numbers

on the different severity levels. The uttermost interest, however,

lies in the understanding of different hierarchy shapes, i.e. dif-

ferent compositions of interactions with different severity. If

we were to understand the relations between the different lev-

els in the hierarchy and their relevance for safety we would be

in a much better position for exploring the pre-conditions for

safe/unsafe road user behaviour and safe/unsafe locations, i.e.

to make more reliable future predictions.

4.1.2. Interactions with TA/CS value severity hierarchy

feedback

The severity described by the TA/CS value can be inter-

preted as a reflection of road users expectations of the situa-

tion/location. These expectations are formed by earlier experi-

ences in similar situations/locations. Crashes and serious con-

flicts are probably too rare to produce efficient feedback to anindividual road user. How often will the road user be facing

this very specific situation again? The feedback is obvious but

not operational. For the individual road user it is important that

there are more events from which he/she can learn. There is of

course an interplay between feedback, expectations and predom-

inant behaviour. Road users get feedback from being involved

in different types of situations; road users expectations are then

influencedby this feedback;theseexpectations then setthe scene

for the composition of behaviours at a location; and so on.

When the severity becomes too high it is primarily because

the road user(s) are not prepared to interact due to expectations

based on earlier feedback. A vehicle driver approaching a green


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Fig. 6. Interaction frequency (interactions per observation hour) for different

severity levels. Straight ahead driving vehicles versus pedestrians. The pedes-

trian is taking evasive action. A non-signalised intersection (DSp) and a sig-

nalised intersection (VSp). Please note that the severity level here is on the

x-axis compared to on the y-axis in Figs. 3 and 4 (Svensson, 1998). The severity

of an event increases with increased severity level.

signal at an intersection expects crossing pedestrians to stop for

the red signal; that is what pedestrians usually do. A situation

with a too high severity might occur at the rare occasion when

a pedestrian actually crosses on red. Garder (1982) showed thatthe proportion of jaywalkers at traffic signals increased with

increased city size, while at the same time the higher the pro-

portion of jaywalkers was the lower the risk of being involved

in an injury crash was. The vehicle drivers expectations are

violated and he/she is therefore not prepared to act.

By analysing all interactions with a TA/CS value, from the

lowest to the highest severities,at a location andby analysing the

shape of the severity hierarchy, the distribution of the road users

expectations the predominant behaviour can be discussed.

By analysing the different shapes of severity hierarchies based

on interactions at different types of locations, involving different

types of manoeuvres, involving different types of road users, it

would be possible to identify different groups of interactionsthat (1) give relevant feedback; (2) give no feedback or even

give misleading feedback. Thus, it would be possible to define

locationsandderivedbehavioursassafe/unsafeduetothequality

of the feedback to the road users.

4.1.3. Feedback shape of the severity hierarchy

prediction

In this section we will discuss the shape of the hierarchy

based on interactions collected at two different intersections,

one signalised and one non-signalised intersection. The aim is

(1) to try to understand what the different parts in the hierarchy

represent; (2) to try to identify if there are preconditions forrelevant feedback or misleading feedback to the road users; (3)

to try and estimate if it would be feasible to make predictions

based on the feedback at the two different locations.

The interactions, in Fig. 6 and accompanying Table 1, involve

straight ahead driving vehicles and pedestrians, where it is the

pedestrianwho is taking evasive action. The twodifferentcurves,

VSp and DSp in Fig. 6, represent interactions at a signalised

respective at a non-signalised intersection. Due to the fact that

these analyses only are based on two specific intersections and

for specific manoeuvres, the results are of course uncertain and

we have to be very cautious when trying to make any kind of

generalisation.

Table 1

Interaction frequency per hour per severity level for interactions involving

straight ahead driving vehicles versus pedestrians

Severity level Interaction frequency per severity level for different

category cells

VSp DSp

30

29

28 2.28E-05

27 2.28E-05

26 0 0

25 0 0.94

The pedestrian is taking evasive action. A non-signalised intersection (DSp)

and a signalised intersection (VSp). Only the severity levels with the highest

severities are included. The severityof an event increaseswith increasedseverity

level. The interactions on severity levels 27 and 28 are injury crashes reported

by the police.

For the highest severities, severity levels 2630, there are

difficulties to see any difference due to the low frequencies.

This is instead shown in Table 1 below.There are indications of great differences in the shapes of the

hierarchies, which obviously have to be due to differences in

the road user behaviour at a signalised and at a non-signalised

intersection. There is a difference regarding the frequency of

events at different severity levels. The location and the extension

of the convexity, i.e. the part of the hierarchy where most events

are located, is also different for the two hierarchies.

4.1.4. Events of the highest severities

With our chosen severity dimension the highest severity lev-

els are level 26 and more severe (see Fig. 2). The events at these

severity levels consist of serious conflicts and injury crashes.They do give very relevant feedback to the involved road users.

The question is, however, if they are frequent enough to produce

efficientfeedback. The indication is that the signalised intersec-

tion seems to produce more occasional events of the highest

severities than the non-signalised intersection.

4.1.5. Events of fairly high severities

The second group of interactions, those of fairly high sever-

ities, is located at severity levels 2025. Even these interactions

are characterised by closeness in time and space thus still having

a strong closeness to an unsafety dimension. At these levels the

safety margin is so small that the situation easily can turn into a

much more critical event.There are two possible interpretations of the fact that there

are many interactions at these fairly high severities, either (a)

somethinggood because there is behavioural feedback regarding

the necessity of increasing the awareness at interactions with

fairly high severity, or (b) something bad because interactions

at these high severities are always associated with an element

of surprise and they are so close that they easily can turn into

more serious events, i.e. crashes.

Interactions at fairly high severities are very frequent; they

constitute the predominant behaviour at the non-signalised inter-

section, while there are no crashes or serious conflicts. At the

signalised intersection on the other hand, those interactions with


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fairly high severity do not seem to exist while crashes do. This

supports the alternative interpretation that these interactions can

be positive because they are frequent and severe enough to pro-

duce efficient feedback to the involved road users, but not severe

enough to result in crashes.

4.1.6. Events of lower severities

For the group of interactions located further down in the

severity hierarchy the danger is not very imminent. We are now

talking about events located at severity level 19 or less severe

(see Fig. 2). The risk that one of these interactions results in a

crash is very small, or in terms of TA/CS values; there is no close-

ness. Nevertheless, if the majority of events are located at these

levels, it could have different interpretations and implications

on safety. One interpretation could be that this is good for traffic

safety as the normal road user behaviour is to divert from col-

lision course at an early stage. From a feedback point of view,

however, they can be considered as negative due to the lack of

other feedback than everything went fine as usual. These situ-

ations are not perceived as interactions. Road users are reactingon the signal not on the presence of other roadusers on collision

course. These situations, therefore, could place the road user

in a difficult position once there is an interaction that requires

a good preparedness for safe interaction, e.g. when somebody

violates a red signal. A consequent hypothesis is therefore that

a traffic environment where the majority of interactions con-

sists of these types of interactions with poor feedback, can be

an indicator of severe safety problems.

4.1.7. Convexity

The form of the convexity, i.e. the part of the hierarchy where

most events are located, can evidently range from being narrowwith regard to the extension over severity levels, as for the non-

signalised intersection, to being more widely spread over several

severity levels, the signalised intersection. The latter could be an

indication of road users difficulty to interpret and decide upon

signs of possible threat. It can also be seen as a confirmation of

the safety effect of signalisation; the priority rules are clear and

the intention of the pedestrians is to stop but due to differences

in the individual safety margins the evasive action is taken at

different closeness to the signal (or rather to the interacting flow

of motor vehicles).

4.1.8. The shape of the whole hierarchy

The total shape of the severity hierarchy can be interpretedas the distribution of individual safety marginssafety margins

thatdiffer due to eachindividuals unique acceptance of comfort-

able margins in time and space at interactions, and due to time of

detection. According to Naatanenand Summalas (1976) discus-

sions about the zero-risk theory, these margins can also depend

on other considerations than safety, such as the wish to maintain

a certain speed or the wish to conserve energy and comfort. In

addition to these motives behind the safety margins chosen, we

have to consider the complex relation to feedback.

The interactions at the signalised intersection (dotted line

in Fig. 6) do at first sight seem to be the result of safe

behaviourmost interactions are handled in due time before

they become critical. These margins are on the other hand not

the result of deliberate considerations but rather the result of

interactions with the signal itself. We do also have the informa-

tion that pedestrians do get killed and seriously injured at this

type of intersectionthe narrow peak up to thehighest severities

(Table 1). There is a safety problem at the signalised intersec-

tion at the same time as most interactions are located towards

the lower severities while there is a lack of interactions at the

fairly high severities. The shape of the interactions at the non-

signalised intersection (solid line in Fig. 6) gives certainly the

analyserthe feeling of unsafetymostinteractions are located at

fairly high severities. However, there are no indicators of safety

problems at this location even though most interactions take

place at fairly high severities. A reasonable assumption about

this latter group of interactions is that here the mobility and

safety desires of the involved road users have been balanced.

The analyses here point at the necessity of interpreting the

different parts of the hierarchy shapes in order to better under-

stand the preconditions for good and relevant feedback and their

safety implications. However, when it comes to the prospect ofmaking reliable safety predictions it is presumably an advantage

if it can be based on the shape of the whole severity hierarchy.

5. A need for a behaviour-based framework in

exploring traffic processes

By continuing working according to the outlined approach

in this paper we hope that the concept of analysing the shape of

severity hierarchies in the near future can be used:

in describing differences in road user behaviour;

to improve the understanding of road user behaviour;

for predicting the frequency of the most severe events from

information about less severe events;

to learn about the importance of feedback and different types

of feedback;

for formulating traffic safety strategies.

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Documents

Svensson and Hyden 2006