Traffic Safety Criteria
Traffic safety on a road is related to the number of vehicles using a road section (SWOV
2006). The risk of accidents increases as more traffic is driving on a road section.
General statement: ‘Less traffic, same risk, less victims’ (Wegman 2004)
To assess the effects of a measure on traffic safety three indicators will be used. Based
on the general statement above, the combination of traffic (intensities) and risk (key
figures in number of serious accidents1), the accident density can be calculated. Thereby
two major influencing criteria from Dijkstra and Drolenga (2007) are taken, the number
of left turns and the junction density.
The traffic safety will be measured at the Waalbrug and at the Singels (directly after
Traianusplein, direction south). This is the main focus area and these two road section
could have different effects of the taken measures. For infrastructural designs the two
indicators from Dijkstra and Drolenga will be used to compare the new situation with
the old situation.
Accident Density
Accident density is the number of accidents per year per kilometer per road section.
This takes in account the number of accidents, the road length and the number of motor
vehicles that pass along it. (Janssen, 2005)
When the intensities (and road features) of the various road sections on a route are
known, the accident density can be calculated using a model.
First the level of exposure will be calculated, expressed in millions of vehicle kilometres
driven in a year.
VPi = Li * Ii * 365
year
VPi = Level of exposure of road section i in vehicle kilometres driven per
Li = the length of the road section I in km
Ii = is the daily volume for road section i
The expected number of serious accidents, Accident Density on road section i can be
estimated by multiplying the level of exposure VPi by the associated key figure Ki of the
road section. The key figures Ki are shown in Tabel 1.
LOi = Ki * VPi
LOi = Expected number of serious accidents (Accident Density)
Ki = Key figure of road section
Road with speed limit in kph
30
1
Key figures in number of serious accidents
per billion motor vehicle km
122
Serious accidents: accidents with fatalities and serious casaulties
50
70
272
12
Tabel 1 Key figures for three road types (Janssen, 2005)
In these key figures, number of serious accidents per one billion vehicle kilometre, is the
risk at junctions taken into account. This is why the figure for 50 km/h is 20 times
higher then for 70km/h.
This following graph of the above showed linear accident density function shows the
direct relation of intensity and traffic safety.
Picture 1 Expected number of serious accidents
Because of comparing different road sections with different lengths the density is used.
By calculating the density, the length of the road section is left out of the formula. The
accident density can be estimated as follows:
Accident Density = Ki * VPi * 365 / 1.000.000.000
The accident density is per kilometer, therefore everything is divided by one billion
because of the Ki value is per billion motor vehicle km.
The sensitivity of the accident density is low, a substantial change in intensity, 1.000
cars less, will effect the expected number of serious accidents with only 0,1. There the
estimated values for the VPi are averages from a model for 2020, the absolute changes of
the accident density of less then 1% are considered as marginal.
The ordinal scale to asses the accident density is:
% Change
% change > 1%
-1 %  % change  1%
% change < -1 %
Tabel 2 Ordinal scale accident density
Ordinal scale value
+
0
-
Infrastructure criteria
To quantify the traffic safety of different routes, Dijkstra & Drolenga (2007) came up
with nine criteria to asses these routes. These criteria are:
1. Number of transitions between road categories limited. An optimum sustainable
safety route diagram hast the right number of category transitions. The largest
part of the route must be made at a highest category, through road.
2. Nature of the transition is correct. The correct transition van Access road to
distributor road to through road and backwards.
3. As few missing road categories as possible. The number of road categories
encountered in a route, in relationship to the number of road categories present
in the network.
4. Proportion (in length) of access roads as low as possible.
5. Proportion (in length) of distributor roads as low as possible.
6. Travel distance. The smaller the total distance travelled on a route, the less risk
to which a vehicle is exposed.
7. Travel time. The free flow travel time, the shorter the travel time on route, the
better is the route.
8. As few turnings as possible across oncoming traffic. The number of left turns at
junctions. Because turning left is seen as the most dangerous manoeuvre
(Drolenga, 2005).
9. Low junction density on distributor road. The route’s potential for disruption.
Where Dijkstra and Drolenga (2007) these criteria applied to different routes, in this
case these criteria will be applied to different measures to compare the current and the
future situation. The measures will not affect the first seven criterions and so these will
be skipped.
The criteria are applied to the infrastructural designs, which are changed in comparing
with the current situation. In the other situation, measures without any infrastructural
change, this criteria will state the same.
An easy ordinal scale to asses the future will be applied for both criterion and compared
with the current situation.
Compared with current situation
Less crossings/Left turns
Same crossings/left turnes
More crossings/left turns
Tabel 3 Ordinal scale Left turn and crossing criteria
Scale
+
0
-
Measure 1
AC - Waalbrug
AC - Singels
# Left Turns
# Crossings
Current
Situation
6,19
4,42
Future
Situation
6,19
4,42
% Change
Value
0,00 %
0,00 %
0
0
0
+
Current
Situation
6,19
4,42
Future
Situation
6,17
4,39
% Change
Value
-0,32 %
-0,58 %
0
0
0
0
Current
Situation
6,19
4,42
Future
Situation
6,19
4,40
% Change
Value
0,00 %
-0,45 %
0
0
0
0
Measure 2
AC - Waalbrug
AC - Singels
# Left Turns
# Crossings
Measure 3
AC - Waalbrug
AC - Singels
# Left Turns
# Crossings