Toyota Motors and and the US National Highway Traffic Safety Administration (NHTSA) recently announced a recall of 55,000 floor mats for various Toyota and Lexus models following an investigation into a crash of a Lexus ES 350 on 28 August 2009 that killed a 45 year old police officer and three family members. The car sped out of control and finally hit another vehicle. Suspicions center on an All-Weather floor mat that could have got entangled with the accelerator pedal.

Floor mat problems in cars are not that uncommon. My previous car was an Audi A4 and there the mats provided by the dealer (it was a second hand car) did not fit the nipples meant to match up with holes in the mats. As a result, the driver side mat always kept sliding forward under heel pressure and sometimes ended up under the pedals (luckily never over the pedals!). One day my wife and I wondered why the car suddenly had a hard time accelerating (it felt really sluggish), until I realized what had happened and pulled the thick mat back out from under the accelerator pedal, which could not be depressed properly (presumably braking would have been similarly affected, but luckily I did not have to find out).

The opposite problem of unintended acceleration tends to be far more severe than a lack of acceleration, of course. An accelerator stuck on full throttle is something that happened to me once while driving a VW Santana, back in the 1980s.

That 5 cylinder engine still used a carburetor, not fuel injection. At some point the cold start enrichment mechanism of the carburetor developed a problem. I reported this issue to the dealer before a service and asked them to fix it. I don’t know what they did about it, but the next weekend I took the car on a German autobahn and had a very unpleasant incident where it got stuck at full throttle while I was doing over 160 kph (there was no speed limit there).

I found that the car kept accelerating even if I took my foot off the pedal and reached about 180 kph. I then pushed the brake pedal as hard as I could and at least it didn’t accelerate any further and slowed down a little. I depressed the clutch pedal (the closest manual equivalent of putting it into Neutral). Without any load on the engine, the RPMs shot up to the max but were limited by the engine management. With the engine roaring at maybe 5800rpm, the car coasted on the straight road and the brakes could slow it down. Worried about the engine I then switched off the ignition, which did stop the engine but also disabled the brake servo and power steering. I quickly turned the ignition back on, but not far enough to engage the starter, just so the steering lock could not be activated by turning the steering wheel as it would with the ignition completely off. I finally brought the car to a complete stop on the hard shoulder, without brake servo assistance.

After opening the bonnet, I opened the carburetor with a screw driver from the emergency set in the back and found the butterfly valve stuck. After some prodding with my fingers, it went back to the proper position and I could resume my journey, with no further incident.

I can see that using the brake pedal, my first reaction in that situation, would not have been as effective in a 3.5 litre V6 Lexus as it was in my 2 litre VW, because the engine was far more powerful. Switching the car to Neutral, my second reaction, would have been quite effective in the Lexus though. My third reaction, switching off the engine, would have been quite impossible for the average driver in that loan car. Turning off an ignition key is one thing, but having to hold down the power button for three seconds (as in that Lexus and some other cars that use a start button instead of an ignition key) rather than just pushing it once is anything but intuitive. Yes, mobile phones and computers can be shut down that way too, but most of us grew up with cars that behaved differently from today’s computers. Actually, even when I started with computers they switched off as soon as you pushed the power button once. I can see how the driver would push the power button once to turn it off and nothing would happen. Then what?

If the driver missed his chance to shift the gear into N, he would likely run out of time in that car before hitting another vehicle in front, before he would have figured out how this is supposed to be done.

Most accidents occur due to a combination of errors. In this case it appears to have been the floor mat problem, combined with the driver missing the chance to respond effectively by shifting the gearbox to N.

Problems with stuck accelerators could have been ameliorated in the engine management by having the brake pedal override accelerator input. If for example the brake pressure was high enough for an emergency stop (the ABS has a sensor for that) then the fuel injection quantity could be limited to idling or at least significantly less than full acceleration would demand, effectively ignoring the accelerator, floor mat or no floor mat. The only side effect I can see is that it would make drag racer starts impossible.

When I explained about the recent Toyota problem to my wife, she replied that she had tried to remove the mats from our 2008 Prius for cleaning but didn’t manage to because the hooks held them in place too firmly. Even without the hooks I imagine the left foot rest in the Prius would stop the mat from moving much.

Whatever car you drive, make sure the floor mat is secured against sliding around. If you’re not sure, better get rid of it altogether. And if you should ever find yourself with a car accelerating out of control for whatever reason, remember that the N position of an auto box or clutch pedal of a manual will always disconnect the engine from the gearbox, not matter what causes the engine power to surge.