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A similar European Union mandate is<br />
set to take effect in 2011.<br />
The UMTRI study was specifically<br />
designed to estimate the potential benefit<br />
of the two distinct safety systems, RSC<br />
and ESC. The former senses vehicle lateral<br />
acceleration in a curve and intervenes to<br />
slow the vehicle in accordance with an<br />
algorithm. The deceleration interventions<br />
are graduated in this order: de-throttling;<br />
engine brake; and foundation-brake application.<br />
The ESC system contains all the<br />
attributes of the RSC system plus yaw<br />
sensing and thus the added capability of<br />
seeing and then controlling vehicle<br />
understeer and oversteer, which are<br />
directly related to loss of control. The<br />
loss-of-control intervention strategy uses<br />
selective braking of individual wheels on<br />
the tractor.<br />
One of the key issues in the study<br />
involved the paucity of real-world crash<br />
data to work with because stability systems<br />
haven’t been around all that long and<br />
just aren’t widely used yet. So the study<br />
was based on the analysis of independent<br />
crash datasets using engineering and statistical<br />
techniques to estimate the probable<br />
safety benefits of stability control technologies<br />
for five-axle tractor-semitrailer<br />
vehicles. It’s complicated stuff, to say the<br />
least, but the researchers examined two<br />
distinct accident databases and isolated<br />
crashes that fit certain criteria, namely<br />
those that suggested a given crash could<br />
have been affected by the use of RSC<br />
or ESC.<br />
They also examined the comprehensive<br />
records of one un-named for-hire fleet<br />
that has used some variation of these<br />
technologies in significant numbers for<br />
quite a few years. There’s some interesting<br />
stuff in there, including the fact that icy<br />
roads mean you’re 30 times more likely to<br />
see a jackknife. That risk hasn’t been<br />
quantified before, and 30 times is a heck of<br />
a lot.<br />
We’ll just quote the study itself to give<br />
you the basic results, noting that we’re<br />
only talking about the U.S. here and the<br />
dollar figures are in American currency:<br />
“The findings of the study indicate that<br />
stability control systems provide substantial<br />
safety benefits for tractor-semitrailers.<br />
Assuming that all existing five-axle tractor-semitrailers<br />
operating on U.S. roads<br />
were fitted with RSC, the expected annual<br />
rollover relevant safety benefit is a reduction<br />
of 3489 crashes, 106 fatalities, and<br />
4384 injuries. Alternatively, assuming that<br />
all existing five-axle tractor-semitrailers<br />
operating on U.S. roads were fitted with<br />
ESC, the expected annual combined<br />
rollover and directional (yaw) instability<br />
relevant safety benefit is a reduction of<br />
4659 crashes, 126 fatalities, and 5909<br />
<strong>In</strong> <strong>Gear</strong><br />
injuries. Because ESC addresses both<br />
rollover and yaw instability crashes and it<br />
is more effective in mitigating rollover<br />
crashes (through additional braking capabilities<br />
over RSC), the net annual expected<br />
benefit for an ESC system was found to be<br />
greater than for RSC.<br />
“Assuming ESC was fitted to all tractorsemitrailers,<br />
savings from rollovers<br />
prevented by ESC are estimated at $1.527<br />
JANUARY 2010 45
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