Analysis of increasing number of casualties Between 1995 and 2007, the real number of severely injured pedestrians
and cyclists in a collision with a motor vehicle on 30 km/h access roads has
increased from 27 to 590. This is an increase by more than a factor of 20.
The number of fatalities rose from 3 to 16. The increase was larger for
cyclists than for pedestrians: the number of casualties among pedestrians
increased from 13 (in 1995) to 173 (in 2007), among cyclists the increase in
casualties was from 14 to 417. The increase of the police registered number
of casualties, from 16 in 1995 to 346 in 2007, was somewhat smaller. This is
due to a small decline in the registration rate of this crash type during that
period.
The increase of the number of casualties on 30 km/h access roads can to
some extent be attributed to the increase in road length. The length has
increased by a factor of six. The number of casualties among pedestrians
has approximately kept pace with this increase in road length. During the
past ten years, the increase in the number of casualties among cyclists, from
2.4 to 11.7 severely injured per 1,000 km road length has been faster.
It is customary to compare developments of the number of casualties over
time with the changes in mobility. The mobility of drivers on 30 km/h roads
was found to be in almost equal pace with the increase in length: both
increased with a factor of six. The pedestrian and cyclist mobility increased
with a factor of five during that period, and therefore remained somewhat
behind the increase in road length. This means that especially the cyclists'
risk has seen an extra increase. This cannot be explained by a shift in
mobility between different age groups which have different risks.
The conversion from 50 km/h to 30 km/h is indeed responsible for a
substantial reduction of the number of casualties. The many uncertainties in
the analysis make it difficult to accurately calculate the size of this reduction,
but the value is approximately the same as the value that can be expected
on the basis of the literature, namely more than 70%.

Analysis of the contribution of the road layout
The influence of road layout on the occurrence of a crash depends on the
crash type. Crashes that occur during special manoeuvres like reversing or
parking, or as a result of the sudden opening of a car door, are not strongly
related to the road layout. This is the case for about 20% of the crashes on
30 km/h access roads. The 30 km/h layout (i.e. the desired motor vehicle's
speed) does play a role in three-quarters of the crashes: half of the crashes
occur on intersections and a quarter of the crashes are caused by children
unexpectedly crossing a road section. In addition there are a small number
of crashes that can be attributed to silly antics and stupid actions, and in
which the road layout may have played a role.
Photographs showing the environment of houses that are offered for sale on
the Funda real estate website were used for the analysis of the crash
locations. This way the environmental features could be determined for a
major part of the crash locations. These features were used to determine the
Sustainable Safety level and the credibility of the speed limit in force.
Particularly the credible speed limit turned out to be a distinguishing feature:
in approximately only a quarter of the investigated crash locations the 30
km/h limit was a credible limit. This was determined by investigating to which
extent the road features had a decelerating or an accelerating effect.

Most significant shortcomings of the layout of 30 km/h access roads
In 70% of the crashes in which the road layout probably played a role there
was no credible speed limit. For crashes in which the road layout is
unimportant the percentage is 57%. This suggests that an incredible speed
limit does indeed increase the risk of a crash. If all 30 km/h access roads
were to have a credible speed limit, this would annually save approximately
200 severely injured casualties, especially among cyclists. This is a
decrease of approximately one third. Half of the extra increase (as compared
to the increased due to road length increase) of the number of casualties
among cyclists can be explained by the increase in roads with a non-credible
speed limit. This study was unable to find an explanation for the other half of
the extra increase in cyclist casualties.
Approximately two-thirds of the crashes involving children between the ages
of 0 - 11 years old are crossing accidents. Suitable measures aimed at safe
crossing locations are lacking in three-quarters of these situations. At
present, this kind of measures can annually save a maximum of 50
casualties.
The large number of crashes at priority intersections involving cyclists is
remarkable. Such intersections do not really fit in a Zone 30. Especially
drivers are found to obey the traffic rules better at intersections without any
designated priorities than at priority intersections. The crashes that were
investigated show that cyclists obey or ignore the priority regulations to the
same extent on any type of intersection. A credible speed limit also reduces
the speeds driven and hence the number of casualties. It is impossible to
make a reliable calculation of the possible reduction, because there is no
data available about the numbers of priority intersections and intersections
without any designated priorities in Zones 30. A thorough investigation
requires reliable data on the road features. At present, this is not available.