Crashes Increase with Safer Cars

God Bless the Child Them that's got shall have Them that's not shall lose Why the highway fatality rate might increase when new safety equipment is added to some cars.

Technology is being sold in cars today that makes those cars less likely to be involved in crashes.  According to an analysis by Consumer Reports, this technology includes:

• Automatic emergency braking (AEB) - Brakes are automatically applied to prevent a collision or reduce collision speed.
• Forward-collision warning (FCW) - Visual and/or audible warning intended alert the driver and prevent a collision.
• Blind-spot warning (BSW) - Visual and/or audible notification of vehicle in blind spot. The system may provide an additional warning if you use your turn signal when there is a car next to you in another lane.
• Rear cross-traffic warning - Visual, audible, or haptic notification of object or vehicle out of rear camera range, but could be moving into it.
• Rear Automatic Emergency Braking (Rear AEB) - Brakes are automatically applied to prevent backing into something behind the vehicle. This could be triggered by the rear cross-traffic system, or other sensors on the vehicle.
• Lane-departure warning (LDW) - Visual, audible, or haptic warning to alert the driver when they are crossing lane markings.
• Lane-keeping assist (LKA) - Automatic corrective steering input or braking provided by the vehicle when crossing lane markings.
• Lane Centering Assist - Continuous active steering to stay in between lanes (active steer, autosteer, etc.)
• Adaptive Cruise Control - Adaptive cruise uses lasers, radar, cameras, or a combination of these systems to keep a constant distance between you and the car ahead, automatically maintaining a safe following distance. If highway traffic slows, some systems will bring the car to a complete stop and automatically come back to speed when traffic gets going again, allowing the driver to do little more than pay attention and steer.

If these technologies were adopted, it would certainly be reasonable to expect the overall fatality rate to decrease.  However, the National Highway Traffic Safety Administration currently is reporting an increase in the highway fatality rate.  How can cars be safer.... and yet the fatality rate be increasing? The problem may be because the crash rate is computed for all vehicles and the safety equipment is only in newer cars.  If the crash rate went down for newer cars with safety equipment AND the crash rate was unchanged for all other vehicles, then the overall average crash rate would indeed decrease. However, if the crash rate for new cars went down, but the crash rate for all other vehicles increased, even by a small amount, then the overall crash rate might go up until the market share of cars with this newer equipment was larger.

So how could the crash rate for vehicles that are NOT equipped with newer safety technology go up?  This is because the combined crash rate is determined not only by the vehicle, but also by the driver.  When no vehicles have improved safety technology, then all drivers behave the same way. However if some vehicles have improved safety equipment and others do not, this changes the dynamic of defensive driving behavior by the drivers.  

All drivers have to make judgments about the behavior of other drivers.  That is the definition of defensive driving.  If no safety technology is available, when a driver is being tailgated, that driver could correctly assume that the tailgater was driving unsafely and chose to change lanes or take some other defensive driving action.  As safety equipment becomes more widespread, that same driver might assume that the tailgater has safety equipment, and that driver might chose instead not to change lanes or take some other defensive driving action.  If that driver assumed incorrectly, the crash rate of that driver might actually increase.

Currently the fatality rate is about 1.1 per 100 Million Vehicle Miles of Travel (MVMT).  Let’s assume that the fatality rate decreased by 50% for all new vehicles with safety technology.  If the fatality rate for all other vehicles did not change, then the overall crash rate would go down.  However, let’s instead assume that a decrease in defensive driving causes an increase in the fatality rate due to the driver’s actions. Properly this change due to driver behavior should be applied to all drivers, both those with and without new safety technology.  Let’s simplify the combined rate and assume that that there is a 50% reduction in the cumulative effect of more safety technology and less defensive driving.  However, the other vehicles would only experience an increase due to less defensive driving.  Let’s simplify their combined rate and assume that that there is a 5% increase in their fatality rate due to less defensive driving.  

The combined fatality rate for all vehicles depends on the percentage of vehicles equipped with new safety technology.  As shown in the table below, if only 1% of the vehicles had the new safety technology, the overall fatality rate would only decrease if there was also no change in the fatality rate for all other vehicles.  If instead, the fatality rate of other vehicles were to increase as shown, there would be a 4% increase in the fatality rate, from 1.1 to 1.15 per 100 MVMT, even without any change in the miles traveled.  While there would certainly be a decrease in the number of fatalities involving cars with new safety equipment, this doesn’t offset the increase in the number of fatalities involving all other vehicles.  With these changes in fatality rates, it is not until the share of cars with new safety equipment increases to 10% of the fleet that the overall number of fatalities would actually decrease.

This is why, in the short term, improvements in the technology that produce safer cars could result in an increase in the crash rate.  Large improvements for a few, at the expense of a small decrease for the many, might result in a worsening overall condition.  This short term change does not offset the potential long term impact.  It just suggests a need for more widespread adoption of this technology has to be achieved before overall safety could be improved.

Fatality Rate ( fatalities per 100 Million VMT)
Base Rate		1.1
Safer Cars compared to the Base		50%
Other Cars compared to the Base		105%
% of cars with safety equipment	% of other cars with no safety equipment	Combined fatality rate if other cars have the same crash rate as the base	Combined fatality rate if other cars have a higher crash rate than the base
1%	99%	1.09	1.15
2%	98%	1.09	1.14
3%	97%	1.08	1.14
4%	96%	1.08	1.13
5%	95%	1.07	1.12
6%	94%	1.07	1.12
7%	93%	1.06	1.11
8%	92%	1.06	1.11
9%	91%	1.05	1.10
10%	90%	1.05	1.09