ROAD DESIGN FACTORS AFFECTING DEATH AND SERIOUS INJURY

February 2002
EUROPEAN ROAD ASSESSMENT PROGRAMME
ROAD DESIGN FACTORS
AFFECTING DEATH
AND SERIOUS INJURY

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Road design factors affecting death and serious injury – February
2002

EuroRAP rates roads outside built-up areas for the risk of traffic accidents
that cause death and serious injury. It also highlights improvements that
could be made to the road to reduce the likelihood of an accident, or make
those that do happen survivable.

• EuroRAP does this by showing where these accidents happen
and the design factors associated with them.

• Most deaths occur outside built up areas, and most of these
on single carriageways;

• Roads with high accident rates are single carriageways, those
with junctions joining the major road at the same level, low
traffic flow and national roads;

• There are similarities between accident distributions on British
roads outside built-up areas and those in the Netherlands and
Sweden;

• High-risk sections of the British primary route network have
been identified and it has been shown that 2400 fatal and
serious injury accidents (6 per cent of the national total) could
be saved if below-standard sections of road are brought up to
the standard of the average.


1
Where do fatal and serious injury road accidents occur?

• The majority of road deaths in Europe occur outside built-up areas;

• The majority of deaths on roads outside built-up areas are on single
carriageway roads;

• In Britain, 9 per cent of deaths on major roads outside built-up areas are
on the motorways, 19 per cent are on dual carriageways, 38 per cent
are on single carriageways of national and regional importance and 34
per cent are on other single carriageways;

• The best indicator of overall risk for an individual traveller is the number
of accidents per unit of traffic (accidents per billion vehicle kilometres).
On this measure, the fatal and serious accident rate of the “A” road
network in Britain is about four times that of the national motorway
network;

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• In overall terms the average fatal and serious accident rate on the
British national “A” road network is significantly lower (about 13 per cent
lower) than that on the regional (local authority) “A” road network.


2
What road features are associated with high accident rates?

• Single carriageways – there is significant difference between the fatal
and serious accident rates for British dual, single and mixed dual-single
road sections, with the dual carriageway rate averaging about half that
of single carriageways, and mixed sections (having both single and dual
carriageway) about three-quarters of the single carriageway rate;

• Junctions at the same level as major roads – British dual
carriageways with split-level junctions have significantly lower fatal and
serious accident rates than those with same-level junctions, averaging
about half the rate for the latter sections;

• Low traffic flow roads – there are significant differences in fatal and
serious accident rate between roads with less than 10,000 Average
Annual Daily Traffic (AADT), roads with 10,000 to 20,000 AADT, and
roads with more than 20,000 AADT. (The ratio of these differences is
about 2.3:1.8:1);

• Many national A-level roads – lower-flow roads in Britain, regional “A”
roads and Scottish “A” roads may have lower fatal and serious accident
rates than the national A roads. (More research will examine why this
should be so. It will take into account features such as spacing
between junctions, level of conflicting flows, and the quality of road
protection and layout.)

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Accident rates by road type
80
70
60
50
40
30
20
10
0
KSI accidents per veh km
All dual
Dual
Dual
Dual at
Mixed
Single
grade
mixed
grade
dual/
separated junctions
single


3
How does the picture in Britain compare with elsewhere in
Europe?

EuroRAP has made a detailed analysis of accidents on roads in Britain and
a preliminary analysis of the accident rates and distributions on different
types of road in the Netherlands and Sweden. The findings to date show
that, where comparisons can be made, the same general results are to be
found in the Netherlands and Sweden, although the detail may differ;

Research for the OECD has shown that most death and serious injury
accidents on European inter-urban routes can be explained by defining
impacts as just four main types: “head to head”; those at junctions; impacts
with objects close to the road; and impacts involving vulnerable road-users.
Subsequent parts of the EuroRAP programme will provide detailed
analysis in each country of accident type by standard of road, but the table
below shows the protection different roads provide.

Protection from four accidents types afforded by different roads


Dual
Dual
Mixed dual


carriageway,
carriageway,
and single
Single
Motorway
junct’s grade-
at-grade
carriageway
carriageway
separated
junctions
Head-to-head High
High
High
Medium
Low
Junctions
High High Low Low Low

Roadside
High High High
Medium
Low
objects
Vulnerable
High High
Medium
Low Low
road-users
GB Fatal and
26.4 36.6 48.2 49.1 68.0
serious/bvkm

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4
Where are the British road sections with high accident rates?

• Sections of non-motorway roads in Britain with higher than average fatal
and serious accident rates for the amount of traffic that they carry
(although not all necessarily have a large number of fatal and serious
accidents) are to be found as No-star and 1-star roads in the “GB Risk
Rate ranking spreadsheet”

• Motorway sections in Britain with higher than average rates included the
southern section of M1, and part of the M4, M6, M11, M20 and M25.


5
How much of the British road network needs to be brought up
to the standard of the average?

• Class-A road sections in Britain with higher than average rates are
distributed throughout the national and regional networks. About
600kms of regional road and 650kms of national road have rates at
least twice the average rate, representing 7.5 per cent and 5.6 per cent
of the network lengths respectively.

• About 131kms of motorway sections have rates about twice the average
rate for motorways, representing 4.5 per cent of the motorway network.

• The potential for fatal and serious accident reduction from
improvements in road network design and management can be
identified. If those road sections with accident rates above the group
average were improved to the group average, an annual total of some
2400 fatal and serious accidents (ie, 6 per cent of the national total)
would be prevented.

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