Comparing US and European crash barrier protection standards highlights key differences in vehicle types.

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THEGLOBAL
REVIEW2015
THEGLOBAL
ROAD
REVIEW2015
IN THIS ISSUE:
TOWARDS VISION ZERO
PUTTING UP BARRIERS
RISING TOLL OF CRASH DEATHS
DELINEATION: THE SPACE RACE
RED MEANS STOP
TAKING CARE ON COUNTRY ROADS
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2. PASSIVE SAFETY STANDARDS
GLOBAL ROAD SAFETY REVIEW 2015 15
COMPARING
BARRIERSTANDARDS
Comparing US and European crash barrier protection standards highlights key
differences in vehicle types - *Luigi Grassia, *Mauro Corsanici
M
ost of the crash cushions available
on the market are designed
according to one of the major
international standards, namely EN 1317
and NCHRP 350. EN 1317 is the European
standard and it is still used to certify new
products, whereas NCHRP 350 is the US
standard and was used up to 2010 to certify
road safety products but now it has been
replaced by the MASH standard.
It is of interest to compare the
requirements of the two standards with
respect to their recommendations about the
design of crash cushions. In particular this
paper reports a comparison between the
requirements to design a non-gating TL3
crash cushion according to NCHRP 350
and to design a redirective 110km/h crash
cushion according to EN 1317.
NCHRP 350 prescribes to test crash
cushions with two different kinds of cars:
an 820kg small car and a 2tonne pick-up
truck. These vehicles are both used to certify
the crash cushions for three different levels
of velocity, namely 50km/h (TL1), 70km/h
(TL2) and 100km/h (TL3).
EN 1317 prescribes to run crash tests with
three different types of cars: a 900kg small
car, a 1.3tonne medium car and a 1.5tonne
large car. Crash cushions can be tested at four
different velocities, namely, 50km/h, 80km/h,
100km/h and 110km/h. The small car is used
in the test at 50km/h, with the small and
medium cars used for 80km/h and 100km/h
tests and the small and large cars used for the
110km/h test.
The capacity test is defined as the crash test
characterised by the maximum level of energy
in the test matrix for that level of velocity: it is
the head-on impact of the heavier vehicle for
the level of velocity under consideration. In
NCHRP 350 the heavier vehicle is the pick-up
at each velocity, whereas in the EN 1317 the
heavier vehicle depends on level of velocity.
In figure 1 the energy is shown for each
capacity test for each level of velocity in the
two standards. The red bars are for NCHRP
350, whereas the blue bars are for EN 1317.
FIGURE 1
Comparison of the energy in kJ involved in
the capacity test at each level of velocity: red
– NCHRP 350; blue – EN 1317.
From Figure 1 it appears that the energy
involved in the impacts are comparable for
TL2 (NCHRP350) and 80km/h (EN1317)
but also for TL3 (NCHRP) and 110km/h
(NCHRP). It is necessary to compare the
requirements of the two standards.
The capacity test for TL3 requires
absorbing 70kJ of energy more than the
110km/h. The nominal energy in the head-
on impact of the 2tonne pick-up travelling
at 100km/h is 770kJ whereas the nominal
energy involved in the head-on impact at
110km/h according to EN 1317 is 700kJ.
This means that crash cushions designed
according to the European standards absorb
10% energy less than crash cushions designed
according the US standards. This does not
mean however that 350 crash cushions are
safer than 1317 crash cushions. In order to
stress this point it is possible to compare the
impacts of two vehicles with the same kinetic
energy but travelling at two different speeds.
A train with a mass of 90tonnes travelling
at 4.7km/h will have the same kinetic
energy characteristic as a 200kg motorcycle
travelling at 315km/h.
But assuming that these vehicles were to
impact with a TL3 barrier, the results would
be very different. It is of note that the speed
of deceleration is crucial to the injuries any
passengers or drivers receive.
This example is useful to understand an
important aspect related to the difference
between the NCHRP 350 and EN 1317. It is
true that the energetic level of NCHRP 350 is
larger than that of EN 1317 by 70kJ. However,
the EN 1317 test deals with an impact at
110km/h, a speed of 10km/h greater than for
900
EN 1317
NCHRP 350
800
700
600
500
400
300
200
100
0
50km/h 70km/h 80km/h 100km/h 110km/h
TL1
TL2
TL3
Figure 1.
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3. PASSIVE SAFETY STANDARDS
NCHRP 350, and the higher speed results in
a greater deceleration.
In Figure 2, the crash tests required by
the two standards to test non-gating TL3
and redirective 110km/h crash cushions are
shown. The table shows the tests required
by NCHRP 350 on the left side and the tests
required by the EN 1317 are reported on the
right side. The frontal tests are shown at the
top and the lateral tests at the bottom.
FIGURE 2
Comparison of the crash tests required to
certify a TL3 non-gating crash cushion (left
side of the table) and a redirective 110km/h
crash cushion (right side of the table).
From Figure 2 it appears that the NCHRP
350 standard requires running only two
frontal crash tests, whereas EN 1317 requires
running four frontal impacts. On the other
hand, EN 1317 requires running only two
side impacts, whereas the NCHRP 350
requires running four side impacts. The
maximum kinetic energy involved in the
frontal impact is 770kJ for NCHRP 350 and
700kJ for EN 1317. Referring to the side
impacts the difference in terms of energy
between the two standards increases: the
kinetic energy calculated using only the
transverse component of the velocity is
90kJ required by NCHRP 350, whereas that
required by EN 1317 is only 47kJ. As a result,
the side impacts for NCHRP 350 are for
much greater forces than EN 1317. However
it is worth noting that for the NCHRP 350
standard, it is not mandatory to run the
head-on impact with a small car and the
angle impact at 15° on the front. The latter
is the most difficult crash test to pass from
a perspective of biomechanical parameters
due to the deceleration and occupant impact
velocity.
Evaluation of crash test data is crucial. But
before any kind of crash test, accelerometers
must be installed that correspond to the
centre of gravity of the vehicle. These
accelerometers measure deceleration during
the impact of the vehicle against the crash
cushion. Both standards are designed to
record the longitudinal (X), transversal
(Y) and vertical (Z) acceleration. The way
the data is recorded is the same in the two
standards, but there are differences in the
way this data is used to calculate the various
biomechanical parameters.
The parameters calculated are the occupant
impact velocity (OIV) and the occupant
ridedown acceleration (ORA) for the NCHRP
350 standard, and the theoretical head
impact velocity (THIV) and acceleration
severity index (ASI) for the EN 1317. The
OIV and THIV refer to the theoretical impact
velocity of the occupant inside the passenger
compartment, whereas ORA and ASI refer
to the average deceleration of the centre of
gravity of the car.
In both standards, the occupant of the
vehicle is considered as an object free to move
inside the passenger compartment. The head
will continue to move at the nominal velocity
of impact due to inertial effect whereas the
compartment of the vehicle around it will
begin to decelerate due to the opposing force
applied on the vehicle by the impacted safety
device. Consequently it is possible to define
a time at which the head hits the passenger
compartment, this is called the flight time
and is indicated by t*. Both the two standards
are designed to calculate t* as the time at
which the occupant (or the occupant’s head)
inside the vehicle has travelled either 0.6m
in the longitudinal direction or 0.3m in the
lateral direction.
The OIV is defined as the largest value
between the two components of the occupant
velocity (Vx and Vy) at t*. The THIV is
defined as the resultant of the occupant
velocity at t*.
At this point, two main differences
arise between the two standards: the first
difference is that NCHRP 350 considers the
components of velocity separately, whereas
EN 1317 considers the components of
velocity together. The approach followed
by the EN 1317 is more precise than that
used by NCHRP 350 because the occupant
feels the two components of the velocity
simultaneously and not separately as assumed
by the NCHRP 350 standard. Combining
these factors is important: for example,
assuming that Vx and Vy at t* are both
equal to 43.2km/h this gives an OIV of 43.2
compliant with the requirements of the
NCHRP 350 standard and a THIV of 61km/h
that is not compliant with the requirements
of EN 1317. This means that a crash test
compliant with the NCHRP 350 standard
would be not be compliant with EN 1317.
The second difference looks at the limits set
out by the two standards for OIV and THIV.
In this, the NCHRP 350 standard sets the
same limit of 43.2km/h for OIV in both the
frontal and side crash tests. However, EN
1317 sets two different limits for THIV in
the frontal and side tests. The limit for the
frontal test is 44km/h and in the side test the
limit is 33km/h. As a result the THIV and the
OIV limits are almost the same for the frontal
impact. But for the side impact, the EN 1317
limit is more conservative and safer than that
of NCHRP 350. In addition, NCHRP 350
only evaluates the longitudinal component of
the velocity.
Once the limit for the occupant velocity is
fixed, it becomes more difficult to stay within
the limits should initial velocity increase.
This means that from the THIV or OIV point
of view it is more difficult to pass a test at
110km/h for EN1317 than at 100km/h for
NCHRP350, even if the energy involved in
the impact is almost the same.
ORA and ASI are both a measure of the
GLOBAL ROAD SAFETY REVIEW 201516
NCHRP 350
TL3 Non-gating crash cushion
EN 1317
110 km/h redirective crash cushion
Test
Velocity
(km/h)
Mass
(kg)
Angle/Position Type Test
Velocity
(km/h)
Mass
(kg)
Angle/Position Type
TL 3.30 100 820C 0o
/Offset 1/4 Mandatory TL 2.1.100 100 900 00
/Offset 1/4 Mandatory
Frontalimpact
TL 3.31 100 2000P 0o
/Frontal Mandatory TL 1.3.110 110 1500 00
/Frontal Mandatory
NA TL 1.1.100 100 900 00
/Frontal Mandatory
TL 3.33 100 2000P 150
/Frontal Optional TL 3.3.110 110 1500 150
/Frontal Mandatory
TL 3.32 100 820C 150
/Frontal Optional NA
TL 3.38 100 2000P 200
/Side Mandatory TL 4.3.110 110 1500 150
/Side Mandatory
Sideimpact
TL 3.39 100 2000P 200
/Rev Side Mandatory TL 5.3.110 110 1500 150
/Rev Side Mandatory
TL 3.36 100 820C 150
/Side Mandatory NA
TL 3.37 100 2000P 200
/Side Mandatory NA
Figure 2.
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