Over the weekend, I had a conversation with some friends on a recent accident involving NJ governor. It was reported by news media that the SUV was going at 91 MPH prior to the crash. My question was how they found out the speed, and one answer was that a speedometer would stuck at the speed the moment of the impact. I find it very hard to believe that any speedometer would stuck at the correct reading considering the violent nature of the collisions, unless the speedometers are very specially designed to record speeds and to sense the crash. Can anyone shed some lights on how accident investigators figure out the crash speeds?
it was likely the black box. in the old school, they would formula it out. weight of vehicle, length of skid marks, impact damage, etc. although unless there's a full on proper investigation, they're wrong. my girlfriend was in a crash once, the cops said they hit that fence at
80 MPH. hmmmmm...
1982 K10 @ 80mph + fence seems like they should have done more than just knock the fence over from 80 miles per hour. i don't care what anyone says, 80 is fast, technically speaking, and that truck is very heavy. also considering that not wearing a seatbelt in a 30mph crash has been known to cause death. oh and plus no one was wearing seatbelts. so sometimes they just throw numbers around, i think they think they're cooler when they do that shit.
That vehicle has a crash data recorder that records the last few seconds before and at the moment of a crash. The GM modules in some cases can record near events. There is a great deal of controversy over who owns the stored data and whether it can be used as evidence against the owner of a vehicle since we are assumed to have a right to privacy in out papers and records. The big question is "who owns the data?" and if it belongs to the owner of a vehicle who happens to be driving, is it not protected under the 5th amendment? There is also a discussion of whether the data may actually belong to whoever is in control of the vehicle at the moment. It seems many people have an interest in staking claim to and getting control of any incriminating data. There are also many accident reconstruction techniques for determining vehicle speed. The CDR seems to be pretty good but, should be verified by other means. It is not nearly as sophisticated as the CDR installed in aircraft.
Easy to do IF you have access to the proper software. They just plug into the OBD port and query the airbag module for the vehicle parameters at the time it deployed. It saves the information just like the freeze frame data that gets stored when the MIL comes on. It will tell them the speed, throttle position, engine RPMs, in what order the sensors for the bag(s)tripped,if you had the brakes applied (also shows if ABS was functioning at the time), seat belt use, exterior temperature. Plus a few other items.
No belt and 90 mph, I'd like to get a ticket for either in Jersey and have my say in court. Tricky as to what agencys have access to the SIRS module info after a deployment. Good info., lugnut.
Hopefully as a mere matter of getting a ticket -- actually stuff the car at 90 with no belt, "you should have such problems."
To get back to the original poster's question: I doubt that they would find a speedo stuck at 90 (in the car, that is; if your Speedo gets stuck at 90; well, you and all your buddies at the senior center should have such problems). Needles getting frozen at some reading is an ancient theme from fictional and apparently some factual accounts of aircraft crashes, violent disruptions of one's person in the era of mechanical wrist watches, etc. I am thinking that that takes extreme g's, especially in the case of an instrument where the position of the needle is a dynamic rather than a static affair (stopping a mechanical clock might be a lot easier to arrange, John Cameron Swayze notwithstanding).
A quick glance through the literature of a field not my own indicates that a slap mark on the *face* of a speedometer is what investigators are usually looking for after a motor vehicle crash, not the needle's actually being frozen. Mythbusters could doubtless have a lot of fun with some old jalopies and an abandoned runway in this regard.
As others have pointed out, there are several other ways of determining the crash speed through forensics at the general crime scene, perhaps onboard computers, and occupants coming clean about the situation.
keep in mind - it was a state police owned and operated vehicle, and the occupant was the governor of the state - I would assume that both the vehicle and the governor are tracked by GPS most of the time.
The comments above have been interesting to read. We, however, need to reme mber that modern vehicles no longer have speedometers that work by attachin g a cable to a vehicle wheel and the amount of revolutions that a wheel tur ns is transfer to the speedometer that works with internal weights that mea sure how far the separate the higher the revs. Modern speedometers are electronic that receive a signal from a sensor on t he vehicles gearbox and the analog needle moves to the speed that the vehic le is moving. There are small electromagnets in a servo that move the needl e around. If the power to the vehicle is shut off due to a violent crash th ere is no residual power left to return the needle to its resting place. Th at is why the speedometer can give a good indication of what the vehicles s peed on impact was. This method is accepted by Police in USA, Canada, UK an d Australia. I hope this explains why this method is used. Black boxes are not fitted in all vehicles in other countries.
i mostly agree with the above but will point out " no longer have speedomet ers that work by attaching a cable to a vehicle wheel " // was only true fo r bicycles and motorcycles. Car speedometers have always read thru a cable that attaches to the transmission, the modern difference is the magnets and sensors, the originals were simply spun by a gear in the transmission.
Not always, VWs and some other imports used a cable to a front wheel for the speedo drive. So did cars that used multi-speed rear axles as did many early cars.
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