INTERURBAN ACCIDENT RATES BY ROAD TYPE AND

INTERURBAN ACCIDENT RATES BY ROAD TYPE AND GEOMETRIC
ELEMENTS
Dr. D. O’Cinneide
J.C. Murphy
T. Ryan
University College Cork, Ireland
1.
INTRODUCTION
The identification of accident rates by road type enables a comparison of road
types and is important in a time where information on the best allocation of
investment funds in roads is vital in order to utilise the resources of the
European Union and each country to best advantage.
This paper summarises the findings of a four year detailed investigation into
road geometric elements and accident rates which was part-funded and
supported by the Irish National Roads Authority 1. The resulting national
accident rates are the first results of analysis carried out on the Irish National
Road Authority (NRA) Needs Study Database. Previously, accident rates in
Ireland were mainly based on default UK accident rates. The derived
accident rates are compared to published International accident rates.
The NRA Database allows research which previously could not have been
conducted on the entire network to take place. The purpose of the database is
to store detailed information of the network and provide data for the
assessment of investment needs and the development of national transport
policy.
This computerised National Roads Database / Geographical
Information System database contains detailed information on each uniform
section of the National Road Network such as geometric details, accident
data, and traffic volumes.
A comparison of the relative safety of the various road types in the interurban
(speed limit > 40mph) part of the network was carried out by calculating and
comparing accident rates. These rates were expressed in accidents per 100
million vehicle kilometres of travel.
Geometric design standards are largely based on logically derived
relationships rather than the results of accident studies or database research.
Road type is the predominant factor in geometric design choice which affects
accident rates. The choice of road type affects not only the speed of vehicles,
the number of junctions or interchanges, the type of passing manoeuvre
permitted as well as various geometric design features. In order to provide the
highest feasible standard of road, the cost of the various road types are
balanced against the expected benefits. The accident savings are vital part of
such an economic assessment.
© Association for European Transport 2004
2.
ACCIDENTS – WHAT THEY ARE & HOW THEY ARE RECORDED
Standardisation of accident definitions is an essential step in the process
towards internationally comparable accident rates. The accident definitions 2
in this study and in the NRA Database analysis NRA (2001) are consistent
with Grime (1987) and the Hopes Report (1993). Incongruities arise due to
the differing international definitions and reporting procedures. For example, a
serious injury accident in Austria is defined as one which results in a person
being hospitalised and unable to work for at least 24 days. Yet some
countries do not even require hospitalisation for an accident to be described
as a “serious injury accident”. (Berns & Bruhning, 1998).
In Ireland, accidents are recorded by the police and entered into a database
by the National Roads Authority. The management of such an accident
database is an arduous task with many incongruities arising. For example;
“either a lack of data or imprecision in the grid coordinates” (NRA, 2002).
Human error in data recording and transferring is minimised by the use of
unique accident ID numbers assigned by the police. The accident analysis
carried out in this study was based on accident data from the National Road
Needs Study Database. This database only includes injury accidents. Data
checks performed include those to ensure no multiple entries occur and to
identify accidents occurring at change points of road sections.
This paper examines the results of two studies of the NRA Database; the first
study concluded in 2002 and examined accident data for the period 1990 to
1998, the second study concluded in 2003 and examined accident data for
the period 1996 to 2000. The results produced by these two studies permitted
comparisons of the derived accident rates. Table 1 shows the lengths and
number of accident by road type for the Interurban 3 sections of the National
Network. The variations between the road lengths and the years for which
accident data was available in the two studies was as a result of the updating
of the National Road Network database, i.e. sections that were eligible for
inclusion, or the road type to which sections were assigned, may have been
altered.
Interurban Length –
National Network (km)
Road Type
Number of Accidents
Study 1
Study 2
1990 - 1998
1996 – 2000
3020
5102
3459
1021
869
2719
1621
Improved Wide 2-Lane
226
369
690
809
All 2-Lane Undivided
4189
4258
8511
5889
Motorway
103
114
68
149
Dual Carriageway
150
157
697
592
3-Lane (All)
87
87
244
147
4530
4616
9520
6777
Study 1
Study 2
Unimproved 2-Lane
2943
Improved Standard 2-Lane
TOTALS
Table 1 - Interurban Lengths and Accidents occurring by Road Type, unadjusted for
underreporting.
© Association for European Transport 2004
Table 2 shows the percentage of reported accidents by accident type
occurring on each of the different road types on the Interurban National
Network as identified by Study 2. This table also shows the values contained
in the newly published Irish Cost Benefit Analysis Guidelines for COBA
11(NRA 2004), which took into account the findings of this study.
% Fatal
Road Type
Study
2
NRA
2004
% Serious
Study
2
NRA
2004
% Minor
Study
2
NRA
2004
Unimproved 2-Lane
8.2
24.8
67.0
Improved Standard 2-Lane
12.8
23.6
63.5
Improved Wide 2-Lane
9.5
26.0
64.5
All 2-Lane Undivided
9.7
9.7
24.6
24.6
65.7
65.7
Motorway
6.7
6.0
13.4
13.3
79.9
80.7
Dual Carriageway
6.8
6.8
14.9
14.9
78.4
78.4
3-Lane (2 lane side)
14.8
11.0
27.2
24.2
58.0
64.8
3-Lane (1 lane side)
13.6
11.0
30.3
24.2
56.1
64.8
ALL ROADS
9.4
23.6
66.9
Table 2 - Percentage of Fatal, Serious and Minor Injury Accidents occurring by Road Type,
unadjusted for underreporting.
From the percentages in Table 2, it is clear that there are few major variations
in the composition of the accident totals for each of the three categories of
Undivided Two-Lane road. The main variation is the percentage of accidents
proving fatal on Improved Standard Two-lane roads. At almost 13%, this
appears high in comparison with both the Improved wide and Unimproved
Standard categories (almost one third higher). Serious and minor injury
accidents exhibit roughly the same distribution pattern, with approximately
25% of accidents involving serious injury, and 65% involving minor injury in
the cases of all three road types. It is likely that the actual percentages are
affected by underreporting rates. It can also be seen that the results of Study
2 have been taken into consideration in the newly published Irish Cost Benefit
Analysis Guidelines.
© Association for European Transport 2004
3
INTERURBAN ACCIDENT RATES FOR THE IRISH NATIONAL
ROAD NETWORK
The carriageway types examined in this paper are Motorway, DualCarriageway, Three-Lane, Undivided Improved Wide Two-Lane 4, Undivided
Improved Standard Two-Lane 5 and Undivided Unimproved Two-Lane 6.
Interurban road accident rates were calculated by severity, non-injury
accidents were not included in the study. The units of the accident rates were
accidents/100 million vehicle kilometres travelled. This unit was chosen after
the examination of the international statistics and a regression analysis on the
most significant variables affecting accidents. The network was divided into
homogenous sections on the basis of Carriageway Type, Traffic Volumes,
and Speed Limit (Urban /Interurban) for the purpose of analysis.
Accident Rate = Total (No. of Accidents)x 108
Total(veh.km)
Table 3 and Table 4 show the results of the analysis on accident rates for
Study 1 and Study 2 respectively. When viewing these rates, it is important to
bear in mind the relative lengths and vehicles km of each road type available
for analysis. It is also important to note that accident underreporting has a
large effect on the accident totals, and therefore on the accident rates
presented here.
Road Type
Length
(km)
Vehicle Km
2943.2
Accident Totals
Accident Rates*
Fatal
KSI
ALL
Fatal
KSI
ALL
34354352930
465
2069
5102
1.35
6.02
14.85
1020.6
20466979552
334
1198
2719
1.63
5.85
13.29
225.6
5297259391
91
303
690
1.72
5.72
13.03
4189.3
60118591873
890
3570
8511
1.48
5.94
14.16
Motorway
102.7
6774919891
3
28
68
0.10
0.41
1.00
Dual Carriageway
150.2
9259379575
68
235
697
0.73
2.54
7.53
87.3
1937031081
38
110
244
1.96
5.68
12.60
4529.5
78089922420
999
3943
9520
Unimproved 2-Lane
Improved Standard
2-Lane
Improved Wide 2Lane
All 2-Lane Undivided
3-Lane (All)
TOTAL
Table 3- Study 1 - Interurban Accident Rates by Road Type (*Accidents per 108 Veh. km of
travel) unadjusted for underreporting.
© Association for European Transport 2004
Accident Totals
Fatal
KSI
All
Accident Rates *
Fatal
KSI
All
Road Type
Length
(Km)
Vehicle Km
Unimproved 2-Lane
3019.9
23025641926
284
1141
3459
1.23
4.96
15.02
869.3
12444924688
208
591
1621
1.67
4.75
13.03
369.2
6270711306
77
287
809
1.23
4.58
12.90
4258.5
41741277920
569
2019
5889
1.36
4.84
14.11
Motorway
113.9
5231463495
10
30
149
0.19
0.57
2.85
Dual Carriageway
156.6
6569210644
40
128
592
0.61
1.95
9.01
49.9
773051607
12
34
81
1.55
4.40
10.48
Improved Standard
2Lane
Improved Wide 2Lane
All 2-Lane Undivided
3-Lane (2 lane side)
3-Lane (1 lane side)
37.4
600852747
9
29
66
1.50
4.83 10.98
TOTAL
4616.3 54915856414
640 2240 6777
Table 4- Study 2 - Interurban Accident Rates by Road Type (*Accidents per 108 Veh. km of
travel) unadjusted for underreporting.
If the rates of accident underreporting are assumed to remain constant,
regardless of road type and year, then valid comparisons can be made
between the relative safeties of the individual road types on the basis of the
accident rates. It can be seen from Table 3 and Table 4 that there is an
increasing trend in safety as the road type is upgraded. Divided roads are
significantly safer than undivided roads. Motorways have by far the lowest
accident rates (the short length of Motorways and the small number of
Motorway accidents in both studies should be noted), followed by Dual
Carriageways, then Three-Lane roads (roads with climbing lanes) and TwoLane roads.
Dual Carriageways are the second safest road type, showing the second
lowest rates in all three categories of accident. The median barrier of both
Dual Carriageways and Motorways may have contributed significantly to their
safety when compared with Two-Lane and Three-Lane Undivided roads. The
control of access, presence of hard shoulders and consistency of design are
also likely to make telling contributions.
Although Two-Lane Undivided roads proved the most dangerous in terms of
the All-accident rate, Three-Lane roads showed a high Fatal accident rate,
both for the one-lane and the two-lane sides. The short length of the ThreeLane road sections used in the calculation of the rates must be noted, as it
may have contributed to the high Fatal accident rate when compared to the
other road types. This high rate may be due to a higher percentage of head
on collisions and right turn accidents on Undivided Three-Lane roads.
Perhaps the most surprising feature of the All-accident rates is the seemingly
small reduction in the All-accident rate when going from Three-lane roads to
Dual Carriageways. This increase in safety is more pronounced when the KSI
and Fatal accident rates for both are compared.
© Association for European Transport 2004
Study 1 (see Table 3) showed an increase in Fatal accident rates on TwoLane Undivided roads as the road is improved; from Unimproved (the safest)
to Improved Standard to Improved Wide (the most dangerous). This differs
from the results obtained in Study 2, which showed improved wide Two-Lane
roads as the safest Two-Lane road type in terms of Fatal and All accident
rates. Both studies showed a decrease in the All accident rate as the quality
of Two-Lane Undivided roads is improved. When compared with Unimproved
Two-Lane roads, Improved Two-Lane road types appear to have a
significantly higher Fatal accident rate, a similar KSI rate and a lower All
Accident rate. The higher Fatal rate may result from higher speeds on
Improved roads.
The principal difficulty when examining accident numbers and rates is the
level of underreporting accidents. Reporting rates of accidents increase with
increasing accident severity. It is reasonable to assume the need of larger
adjustment factor for minor injury accidents than serious injury accidents.
Inconsistencies and a lack of clarity in many cases make it difficult to compare
the underreporting rate of injury accidents for different countries. It is
estimated, on the basis of evidence from a variety of countries and sources,
that the rate of road accident reporting in Ireland for serious injury accidents
could be anywhere between 50% and 75%. The reporting rate of Minor injury
accidents could be as low as 25% and as high as 50%. Therefore there is a
large margin for error in applying any corrective factor to minor injury accident
totals. Table 5, below, shows the estimates of reporting rates in Ireland as
well as the relevant adjustment factor. These underreporting adjustments are
then implanted in Table 6 for Study 1 and for Study 2.
Accident
Severity
Reporting Rate Estimate
Reporting Rate
Adjustment
Factor
Fatal
100%
1.00
Serious
75%
1.33
Minor
50%
2.00
Table 5 - Summary of Accident reporting estimates.
Road Type
Unimproved 2Lane
Improved Standard
2Lane
Improved Wide 2Lane
All 2-Lane Undivided
Motorway
Dual Carriageway
3-Lane Undivided
Adjusted Accident
Totals (Study 1)
Fatal
KSI
All
465 2598 11273
334
1483
5833
Adjusted Accident
Rates *(Study 1)
Fatal KSI
All
1.35 7.56 32.81
1.63
28.50
208
717
2777
Adjusted Accident
Rates *(Study 2)
Fatal KSI
All
1.23 6.18 26.32
5.77
22.32
91
373
1480 1.72 7.04 27.94
77
356
1400 1.23 5.68
890 4454 18586 1.48 7.41 30.92
569 2498 10238 1.36 5.98
3
31
145 0.04 0.46
2.15
10
37
275 0.19 0.70
68
290
1611 0.73 3.13 17.40
40
157
1085 0.61 2.39
38
134
517 1.96 6.91 26.69
21
77
245 1.53 5.59
TOTAL
999 4909 20859
640 2768 11842
Table 6- Interurban Accident Rates by Road Type – Adjusted for underreporting (see Table 5)
(*Accidents per 108 Veh. km of travel)
22.33
24.53
5.25
16.52
17.82
© Association for European Transport 2004
7.25
Adjusted Accident
Totals (Study 2)
Fatal
KSI
All
284 1424
6060
1.67
4
HOW DO THEY FIT IN WITH RATES FOUND ELSEWHERE?
4.1
Comparison of study results with the accident rates used in
Ireland
“Motorways are considered a safe but expensive road option, requiring a high
traffic volume to be considered cost effective. Dual Carriageways appear the
next safest carriageway type after Motorways. Although Dual Carriageways
require higher investment costs than single carriageways, the additional cost
can be outweighed by the increased benefits associated with the higher travel
speed and better accident record than single carriageway roads” National
Road Needs Study, NRA (1998).
Accident rates were generated by this study for the four road types in the
National Road Network in Ireland. These rates were compared with default
accident rates used in the economic assessment of road schemes in Ireland
(NRA, 1998) and the UK (DMRB, 2004). The accident rates for each road
type are shown in Table 7. The accident rates found in the University College
Cork studies were also adjusted for underreporting based on estimates made
from a comprehensive review of literature pertaining to the problem of
accident underreporting. The recently issued NRA (2004) COBA 11 figures
reflect the results found by the University College Cork Studies.
Road Type
DMRB,
2004
COBA
11
10.7
NRA
Technical
Report,
1998
7.0
Study 1
Unadjusted Adjusted
Accident
Accident
Rates
Rates
1.0
2.1
Study 2
Unadjusted Adjusted
Accident
Accident
rates
rates
2.8
5.2
NRA,
2004
COBA
11
Motorway 2 lane
3.7
Dual Carriageway
12.1
12.0
7.5
17.4
9.0
16.5
9.4
2lane
Modern 2-Lane
26.0
Roads
Modern 2-Lane
17.0
13.3
28.5
13.0
22.3
21.2
14.2
Roads (HS)
Older 2-Lane
28.8
Roads
15.0
26.3
20.0
14.9
32.8
Other 2-Lane
44.5
Roads
Modern Wide 217.0
18.0
13.0
27.9
12.9
22.3
Lane Roads
Three-Lane
12.6
26.7
10.7
17.8
12.5
Undivided
8
Table 7 – Comparison of Irish National Interurban Accident Rates (*Accidents per 10 Veh.
km of travel)
The relative safety of road types in terms of safety is the same for the
University College Cork Studies and the recently issued NRA (2004) COBA
11 figures. It can be seen in Table 7 that the default accident rates are higher
than the unadjusted rates found by this study. The NRA (2004) COBA 11
figures are similar to the unadjusted accident rates found by both Study 1 and
Study 2. The accident rate for Motorways is more in keeping with the actual
situation in Ireland than the UK DMRB (2004) COBA 11 values and the NRA
(1998) Technical Report. For almost all road types examined, the default
© Association for European Transport 2004
rates presented by the DMRB and in the Technical Report of the Needs Study
are lower than the rates obtained in this study adjusted for underreporting.
While the results presented here show that the default rates seem to
underestimate the rate of accident occurrence on Irish National Roads, it is
difficult to be definitive, as so little is known on the extent of the problem of
accident underreporting in Ireland, or indeed internationally.
The NRA (2004) COBA 11 guidelines produced accident rates which are in
keeping with the finding of Study 1 and Study 2. The lower rates for
Motorways found in both Study 1 and Study 2 could be attributed to the short
length of motorway in the network and the relatively small number of
accidents occurring.
4.2
Comparison of Accident Rates in Different Countries
A list of “safest” countries cannot easily be identified, due to the various rates
which could be used to identify “safeness”. A list drawn up by the number of
road deaths per motor vehicle may yield a totally contradictory answer to a list
based on the number of road deaths per million vehicle kilometres. Also, if
only interurban sections were examined further results may be found.
Correction factors can sometimes be applied to international figures in order
to make standardised comparisons. It is important to include the lengths,
vehicle kilometres and various accident totals in the same table as the
calculated accident rates, to keep the rates in context.
Comparisons of accident data on the IRTAD website provide an international
road traffic and accident database which contrasts rates in various countries.
From these values and the detailed notes that accompany them, it can be
seen that accident rates from different countries cannot simply be compared
without question; variations in definitions and underreporting are important
factors when contrasting accident rates. Figure 1 shows the injury and fatality
rate for 2002 various countries on all roads expressed per 108 vehicle
kilometres of travel. Ireland can be seen to be mid-ranking in terms of fatality
rates and fourth in terms of injury accidents. It must be noted however that
underreporting of injury accidents would influence these figures greatly.
Injury Accidents per 1 billion Veh.Km
Fatalities per 1 billion Veh.Km
90
80
70
60
50
40
30
20
10
0
8
Figure 1 – Fatalities and Injury Accidents per 10 Veh. km of travel (IRTAD, 2002)
© Association for European Transport 2004
It is very difficult to compare accident rates in different countries by road type
because of differences in geometric layout, intersection types, traffic
composition, etcetera. The absolute volume of traffic on the different types of
road is also relevant. Direct comparisons are most relevant for motorways.
Figure 2 shows and fatality rates for Interurban roads and Motorways reported
by IRTAD (2002) expressed per 108 vehicle kilometres of travel. Again
Ireland can be seen to be mid-ranking in terms of fatality rates. It should also
be noted that these results cannot be directly compared with the number of
fatal accident occurring.
In t e r u r b a n F a t a l i t i e s p e r 1 b i l l i o n v e h . k m
Motorw ay Fatalities per 1 billion veh.km
20
18
16
14
12
10
8
6
4
2
0
Figure 2 – Interurban and Motorway Fatalities per 108 Veh. km of travel (IRTAD, 2002)
Comparing the results found by the University College Cork Studies to Figure
1 and Figure 2 it can be seen that the Injury accidents are slightly
underestimated, whereas the fatality accidents are overestimated.
In
particular for Motorways, Table 6 showed the fatality rate in the order of 0.19
and Table 7 showed the accident rate at 5.2. The fatality rate for interurban
roads is again overestimated in Figure 2 as can be seen from Table 6.
Similar results were also found by EuroRAP (2002). “The safest roads
everywhere in Europe are undoubtedly the motorways - though again
Portugal shows the worst record with 14.1 deaths per billion vehicle-km. Italy
is next with 10.2, followed by Austria (8.9), Belgium (7.2), France (5.4) and
Finland (5.0). Lower-rate countries are Germany (4.5), Denmark (4.3), Ireland
(4.0), Switzerland (3.3), Netherlands (3.3), Sweden (3.2) and Britain (2.0)”.
These fatality rates for Ireland are closer to the Motorway all accident rate
(Table 6).
© Association for European Transport 2004
5
THE RELATIONSHIP BETWEEN GEOMETRIC ELEMENTS AND
SAFETY
A regression analysis was also carried out into the relationship between
geometric elements and safety using the NRA database. This mirrored that
carried out in 1975 on a similar database in Ireland. (Hearne, 1976). As with
Hearne’s analysis, this work was not intended to result in the creation of a
predictive equation, rather to examine the extent to which each of the
variables included influence the occurrence of road accidents. The geometric
elements considered were hard shoulders, medians, intersections, verges,
grade, curves and passing sight distance. Other influencing factors on
accident rates such as volume, speed and length were also considered.
The results of the stepwise linear regression on Two-Lane Undivided roads
showed that the coefficients obtained indicated that vehicles kilometres of
travel was the most important of the variables examined in terms of the
number of accidents which occur on a given two-lane undivided section of the
Irish National Primary or Secondary Network. Higher values of vehicle
kilometres of travel on a given road section appear to increase the likelihood
of accidents occurring on the section, as would be expected. The variable
‘lane plus shoulder width’ was the second most important variable, indicating
that, as the carriageway width increases, the number of accidents tends to
decrease. Neither lane width nor hard shoulder width was found to be
statistically significant when examined individually in the case of this model.
Roadside events (the number of junctions, developments and lay-bys present
at the roadside) were shown to be slightly less important than the combined
width of the travel lane and hard shoulder.
A second regression analysis dealt only with unimproved or standard rural
sections of Two-Lane Road. This identified vehicle kilometres as the most
important feature followed by the number of roadside events. Lane width was
the third most important feature, with the accident number increasing with
increasing lane width (to a maximum lane width of 3.75m). Hard shoulder
width was the fourth most important of the variables analysed. As the Hard
shoulder width is increased, the accident numbers on the section decreased.
Further detailed analysis was carried out into the impacts of volume and lane
and shoulder width on accidents, resulting in the conclusion that as hard
shoulder width is increased, there is a reduction in the rate of accident
occurrence. Lane widths of 3.25-3.50m should be avoided on Undivided
Roads (Just below minimum lane width recommended by the NRA) 3.503.75m seems the optimal range for lane widths.
© Association for European Transport 2004
6
CONCLUSION
Any database, such as the NRA Needs Study Database used in this study,
which is continuously updated, results in problems in the analysing of the
impact of geometrics on accident rates since the details of the accidents are
linked to sections with details that do not describe the actual road section on
which the accident occurred. Details of the road sections are constantly
updated, with Traffic Volume and Carriageway Type details, etc. of upgraded
sections removed as the sections are physically altered. Care must be taken
to ensure that this risk is minimised and information is checked thoroughly.
Further work needs to be carried out to determine accident reporting rates so
that more reliable accident statistics can be calculated. Also, the recording of
accident details such as the direction of travel and accident type (e.g.
opposite direction) would be of considerable use. International comparisons
require the standardisation of accident reporting and the formalisation of
principal definitions as well as a more detailed breakdown of vehicle
kilometres travelled and lengths of sections used.
Table 8 show a comparison of road safety when road types are compared
with motorways, using the results of Study 2. It can be seen that in general
Motorways are 5 times safer than Undivided Two-Lane Roads and 3 times
safer than Dual Carriageways and Three-Lane Undivided roads. Looking at
Fatal accidents it can be seen that Motorways are 7 times safer than
Undivided Two-Lane Roads and 3 times safer than Dual Carriageways but
only 8 times safer than Three-Lane Undivided roads. It is clear that road
safety is improved as the road is upgraded.
All
Accidents
Motorway 2 Lane
1
1
1
Dual Carriageway 2 Lane
3
3
3
Three-Lane Undivided
8
8
3
All 2-Lane Undivided
7
9
5
Improved Wide 2 Lane
6
8
4
Improved Standard 2 Lane
9
8
4
Unimproved 2 Lane
6
9
5
Table 8 – Comparison of safety to Motorways (Study 2)
Carriage Type
Fatal
KSI
A detailed examination into the impacts of lane width and shoulder width on
accidents shows that lane widths of 3.25 to 3.50m should be avoided on
undivided roads while a lane width from 3.50 to 3.75m is optimal for safety.
To minimise accidents on undivided roads, the hard shoulder width should be
in the range 2.50 to 3.0m. It was also found that lane width and road type (e.g.
motorway) were the most influential factors on accident rates. Hard shoulder
presence reduces the rate of All Accidents on Two-lane Undivided roads. The
results were similar to those obtained by Rohloff (2000), Hedman (1990),
Hearne (1976), and the Transportation Research Board (1978).
© Association for European Transport 2004
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© Association for European Transport 2004
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© Association for European Transport 2004
Notes
1
Acknowledgement: The University College Cork project was part funded and supported by the
National Roads Authority, particular assistance was given by F. Crowley and F. Trace.
2
Accident definitions: Slight/Minor Injury – no death or serious injury occurs, injuries occur such as
sprains, bruises etc. Serious Injury - no deaths but serious injury occurs. A serious injury is defined as
an injury for which a person is detained as an ‘inpatient’ in hospital or where any of the following
occurred; fractures, concussion, internal injuries, crushing, sever cuts or lacerations, severe shock
requiring medical treatment. Death/Fatal – involves one or more fatalities as a result of the accident and
death occurs within 30 days. Killed or Seriously Injured (KSI) – involves one or more fatalities or one or
more serious injuries.
3
Urban sections are defined as sections with a speed limit of 40mph (64kmph) or less, Interurban
sections as those where the speed limit exceeds 40mph.
4
Improved Wide Two-Lane roads are defined as having a carriageway width of greater than 4.0metres
and a hard shoulder width of greater than 1.0metre.
5
Improved Standard Two-Lane roads are defined as having a carriageway width of less than or equal
to 4.0metres and a hard shoulder width of greater than 1.0metre.
6
Unimproved Two-Lane roads are defined as having a hard shoulder width of less than or equal to
1.0metre.
© Association for European Transport 2004