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Logging Your Every Driving Moment

Some airbag controllers do more than just trigger the bags!

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Did you know that the airbag control module in your car could be logging a whole lot of factors about your driving - including your speed? If the proliferation of speed cameras, redlight cameras and radars isn’t enough to make you drive carefully, perhaps that piece of news just might!

Think about it. If in a moment of inattention you collide with the back of another car, you won’t be able to say that you were braking hard – not if the electronic record shows that in fact you never even started to slow until the time of impact. Convicted by your car? – it’s more than just a possibility, with one such case having already occurred in the US. There a driver involved in a double fatality claimed he had been travelling at about 100 km/h. But the electronic record showed that in fact the speed of the car five seconds before impact was 184 km/h...

So what data is logged and why is it recorded? Do all airbag-equipped cars have this facility? How can you read it? And who owns the information?

The implications - not only for drivers but also for insurance companies, the police, rental car companies and fleet owners - are profound.

But if the thought of your car logging your driving behaviour horrifies you, here’s a let-off. At this stage General Motors in the US appears to be the only car company wholeheartedly embracing the technology, publicly releasing details on their systems and also working with a third party provider to make available for general purchase a dedicated data reader. However, the benefits of Event Data Logging to crash researchers mean that the US Government is strongly supporting the development of universal standards and techniques of implementation. In other words, with the influence that US legislators have on world car developments, it’s probably only a matter of time before all cars have Event Data Logging in a format that allows easy reading.

Automotive Logging

About 25 years ago the fuel and ignition control technologies in cars started a move from mechanical systems (carburettors and points) to electronic systems (EFI and electronically controlled ignition). These systems use sensors to measure various car parameters, such an engine airflow, engine speed and throttle position, with an Electronic Control Unit (ECU) then making decisions about fuel injection pulse width and ignition timing. Additionally, most of these systems have the ability to detect and store faults, allowing their later diagnostic reading.

The increasing sophistication of automotive electronic systems has led to the adoption of ABS, Traction Control, Stability Control and Climate Control (amongst others), with each of these systems also able to log fault codes. Additionally, many car systems share sensors – for example, the road speed sensor is read by the engine management system, cruise control system and traction control system. This ‘sharing’ is made easier by the use of communications buses.

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It comes as no surprise then that the airbag control system has (a) the ability to store data, and (b) uses a wide variety of car sensors as part of its decision making. However, the use of the controller as an Event Data Recorder (EDR) goes a step further – not only are fault codes stored, but in some systems the input of a variety of car sensors is also continually logged.

How did this extra step come about?

In the early 1970s the US National Transportation Safety Board recommended that vehicle manufacturers gather information on vehicle crashes using on-board collision sensing and recording devices. Since 1974 General Motors (GM) systems has recorded data for impacts that resulted in the triggering of the airbag (a “deployment event”) while other systems were also introduced that could additionally record “near deployment” events. In 1999 GM implemented the capability to record pre-crash data, that is, data is recorded to a buffer on a continuous basis, the overwriting ceasing if a crash occurs. Ford in the US started installing EDRs in one model in 1997 and by 1999 nearly all its US models were so equipped. A range of other manufacturers either admit to some data recording or are looking to implement such strategies.

Rather than use airbag control systems to record crash and pre-crash data, in US-manufactured heavy trucks the engine’s ECU is used instead. On their diesel engines Cummins, Detroit Diesel and Caterpillar all use electronic control systems which log driving data.

The precedent for recording data on a continuous basis is well established in other areas of transport: in the US, regulations requiring the use of Event Data Recorders are in place in aviation (from 1958), marine (2000) and railway (1995) applications. The Transportation Safety Board of Canada draws an interesting relationship between the presences of the EDRs in different forms of transport and the number of fatalities occurring. “Motor vehicle accidents on highways accounts for 93 per cent of all transportation fatalities,” it points out, suggesting that it makes more sense to have EDRs in cars than in any other form of transport.

The GM System

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The information recorded by GM airbag systems consist of data for both deployment and near deployment events.

A near deployment event (ie one where the airbag doesn’t inflate) is defined as an event severe enough to ‘wake-up’ the algorithm within the control unit. (An algorithm is used to analyse the severity of the crash pulse, ie the control unit uses the shape and magnitude of the deceleration pulse it is undergoing before deciding whether or not to fire the airbag(s).) Two different systems are used by GM; one stores data on the near deployment event which had the greatest change in road speed, and the other the most recent near deployment event.

For both deployment and near-deployment events the following are recorded:

  • Driver’s Seat Belt – this is recorded as buckled or unbuckled; however, this status may be recorded incorrectly if power to the unit is lost during the crash.

  • SIR Warning Lamp – the on/off status of the Supplemental Inflatable Restraint warning lamp is recorded.

  • Change in Forward Velocity – this is determined by the integrating the average of four 312 microsecond acceleration samples and is recorded in RAM every 10 milliseconds. Depending on the module, either 300 milliseconds or 150 milliseconds of record is available.

  • Time to Deployment – the time in milliseconds between the start of the event (ie enabling of the algorithm which requires two consecutive acceleration samples of over 2g) and the command of the airbag deployment.

  • Time Between Events – the time in seconds between a deployment event and a near deployment event, if that time is less than 5 seconds.

  • Vehicle Speed – the pre-crash speed, recorded every second for 5 seconds prior to any event. This information is derived from the vehicle speed sensor.

  • Engine RPM – engine speed, as derived from the engine management system. As with vehicle speed, it is recorded every second for 5 seconds prior to any event.

  • Throttle Opening – the percentage that the throttle is open, where 100 per cent is wide open. This information is sent by the engine management system along with engine and vehicle speeds, so again is recorded every second for 5 seconds prior to any event.

  • Brake Status – brakes on/off, as derived from the ABS or engine management unit every second for 5 seconds prior to any event. Braking intensity is not shown.

  • Data Validity – a check that none of the four pre-crash parameters (vehicle speed, engine rpm, throttle opening or brake status) is out of range or has logged faults.

In addition, the number of ignition key cycles at the time of the events and at the time of download is logged, as is whether the passenger-side front airbag has been manually switched off.

One of the two GM EDR units is designed so that 150 milliseconds after the deployment algorithm has been enabled, all the data stored in the memory is permanently written to EEPROM. It then cannot be erased, cleared or altered – this type of device must be replaced after an airbag deployment.

(The Ford system records longitudinal and lateral acceleration, deployment strategy of dual-stage airbag, seat belt use, pretensioner operation and driver’s seat fore-aft position.)

Vetronix Data Reader

In 1999 GM licensed the Vetronix Corporation to build a data retrieval tool for their EDR. (Ford has more recently followed suit.) The Vetronix Crash Data Retrieval (CDR) tool consists of both hardware and software. The system costs about US$2500.

The GM airbag controller contains a full Event Data Recorder. The data logged just before and during the crash can be read either directly from the module or if the wiring in intact, from the car’s diagnostic port.

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This is a sample of the pre-crash data that is logged by the GM system, as read out using the Vetronix Crash Data Retrieval tool. Throttle opening, engine and road speed, and the on/off status of the brake switch are logged at 1-second intervals for the 5 seconds before the crash.

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During the crash the change in speed is logged every 10 milliseconds, allowing a detailed examination of the impact behaviour. The airbag system’s accelerometer is used in this process.

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The GM data is logged in hexadecimal form and needs a dedicated reader to make sense of the data.


As indicated above, heavy truck manufacturers are implementing EDR in the engine management ECU. Detroit Diesel’s DDEC IV system, for example, records vehicle speed, engine speed, throttle position and brake and clutch switches for a period of time before and after a “hard braking event” is detected. A hard braking event is user-adjustable but is normally determined by a wheel deceleration of greater than 7 mph (~11 km/h) per second. The time span and intervals between data collection vary with the different systems, but data collection each second for the minute before the rapid deceleration and for 15 seconds afterwards is typical.

Caterpillar engines manufactured from 1996 are equipped with EDRs and later models use an internal lithium back-up battery with an expected service life of 4-6 years. A truck involved in a crash should therefore retain the data for as long as six years after the event. The Detroit Diesel system uses non-volatile memory which does not require a back-up battery to retain its data.

Unlike car systems where - as one commentator bemoans - “there are almost as many event data recorders as there are Original Equipment Manufacturers and aftermarket suppliers”, all truck systems use the same 6-pin Deutch connector and a common communication adaptor to work with the PC.

Data Usefulness

The evidence appears strong that EDRs improve crash analyses. The reconstruction process is not only simplified but the accuracy of the reconstruction is improved, resulting in more detailed conclusions. The so-called Haddon Matrix has been used to show the information available with and without EDRs.

Information Available Without EDR





Skid Marks


Calculate change in velocity


Crash damage

Environment after crash

Information Available With EDR





Seatbelt use

Throttle input


Road speed

Engine speed

Conditions during crash


Airbag data

Seatbelt pretensioners

Crash pulse

Measured change in velocity

Airbag inflation time



Automatic crash notification*

Automatic crash notification*

Automatic crash notification*

*Automatic crash notification refers to systems with the capability of automatically alerting authorities (eg by mobile phone) where and when an accident has occurred.

When you consider that a crash investigator primarily has only vehicle damage and obvious physical signs like skidmarks (less likely to be present with ABS) on which to make major judgements, it can be seen that logged data on vehicle speed and other parameters can be enormously useful.

Data Validity

So how good is the data collected via an EDR? The answers to that are surprisingly broad; certainly there is plenty of information available for someone who wants to fight the EDR evidence in a court of law. However, on the other side of the fence, if used carefully, the data gained post-crash from an EDR is of great use in helping determine the events that occurred before and during the crash.

So what are some of the potential problems?

  • Vehicle speed, engine rpm, throttle opening and brake status are logged only once per second – much too slow a sampling frequency to be optimal when analysing many types of crashes. For example, did the driver brake at 3.1 or 3.9 seconds before impact? The difference is major. Additionally, these data are not synchronised with the start of the crash data, so are potentially offset from crash data by up to one second.

  • The recorded data goes back only five seconds before the algorithm enable event occurs. There is no record of vehicle behaviour earlier than this – behaviour which might shows erratic driver inputs, for example.

  • The use of only five data points for each of the parameters of speed, rpm, throttle opening and brake status can give a false impression that the behaviour of these parameters can be validly shown by a graph with these points connected by a straight line – but of course these data might have been behaving quite differently between the discrete points.

  • Most EDRs record speed only in a longitudinal direction. Many accidents involve lateral and as well longitudinal movement, and so the speed recording may give a false impression of the events that occurred. No current Original Equipment EDRs are known to record vertical accelerations.

  • Where the crash does not involve a major deceleration – for example when a truck hits a car or when a pedestrian is run over by either a car or a truck – the EDR is likely to not record the event at all.

  • Vehicle speed, engine rpm, throttle opening and brake status are all dependent for their accuracy on car sensors and/or switches. Vehicle speed sensor and throttle position sensors are often in error - variations in accuracy of these parameters by up to 10 per cent is not at all uncommon. This is a point that seems to have been overlooked by some researchers in the field.

Much crash test work has gone into testing the relationship between data gathered from EDRs and that gained through other logging techniques. One approach is to measure the vehicle’s change of velocity using the EDR and compare that figure with the crash test impact speed.

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A series of Canadian tests showed that generally there was fairly good agreement between the calculated and actual speeds – for example, an actual impact speed of 40.3 km/h and a EDR-calculated speed of 42.4 km/h. Typically the EDR showed a slightly higher speed because it took into account the bouncing back off the barrier that the car underwent after the collision.

However, one test involving a 2000 Ford Taurus had a significantly greater difference between actual (47.8 km/h) and EDR (53.6 km/h) speeds. The testers suggested that this discrepancy had been caused by a spike in the acceleration/time curve caused by structural deformation in the region where the EDR was mounted. A major discrepancy also occurred in another test, this one where a 1988 Chevrolet Cavalier’s EDR lost power during the crash. The independently measured test speed was 64.8 km/h but the EDR showed 56.8 km/h.

In low speed tests another study found that the shape of the crash pulse had a major affect on the accuracy with which the EDR reported the change in speed that the car had undergone. Because it takes a deceleration of over 2g before the algorithm is even enabled, a crash pulse with a sinewave shape on the acceleration versus time graph gave different calculated speeds to a crash pulse that looked more like a square wave. In other words, the car is decelerating for a moment before the EDR starts to log – and so calculate the speed change. This study found that the EDR underestimated nearly all the collision speeds. The researchers reported that “since real vehicle-to-vehicle collision pulses are probably shaped more like sine or haversine pulses than square pulses, the speed change reported [by an EDR] likely defines a lower limit for a vehicle’s speed change during a specific collision.”

However, away from the laboratory the usefulness of the data – even with these reported inaccuracies – can be clearly demonstrated.

The 83-year-old male driver of a 2000 Buick Century was negotiating a right-hand curve when he ran off the road, travelled down an embankment into brush and tall grass, then crossed a level section of lawn and a gravel driveway before colliding with two large rocks. The car came to rest approximately 140 metres from where it had first left the road.

Time Before Algorithm Enable

Vehicle Speed


Engine Speed


Throttle Position


Brake Switch Status


























The pre-crash data obtained from the EDR (shown above) indicated that the driver was operating neither the throttle nor the brakes for at least 5 seconds prior to impact with the rocks. At the crash scene the driver was lethargic; in hospital he failed to respond to treatment. An autopsy showed that he had died from the results of a brain haemorrhage that had occurred while he was driving – a diagnosis well supported by the EDR data.

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Another example of the usefulness of EDR data is in the crash of a 1998 Chevrolet Malibu, which ran under the back of a parked truck. The car sustained severe damage to its bonnet and windscreen area, resulting in a long crash pulse. The crash simulation software used by the investigator estimated the change in vehicle speed to be 23 mph (37 km/h) however through experience the investigator thought this figure to be low. A reading of the EDR data showed a change in speed of approximately 50 mph (80 km/h), which to the investigator appeared much more reasonable.

The latter case shows that when other investigative techniques might give speed indications in error by a very major amount, discrepancies of a few km/h in actual versus EDR speeds are in many cases of little importance.

Social Implications

While the potential societal benefits of the universal fitting of EDRs are highlighted by many road safety researchers (see the ‘Potential Benefits of Event Data Recorders’ breakout box below), many drivers and some vehicle manufacturers are more concerned about the personal privacy implications than the common good.

The US Federal Motor Carrier Safety Administration has stated that it believes that the following standards should apply to controlling access to EDR data:

  • The vehicle owner at any given time should own the EDR data

  • Only the vehicle owner, or another party having the owner’s permission, may access the EDR data. Exceptions would include instances where a law enforcement official has a warrant in connection with a crash investigation.

  • One method of assuring only owners have access is through the use of an EDR password.

  • The storage and retrieval of EDR data must protect the privacy rights of the individual in accordance with law.

At this stage none of those points has been implemented, although truck owners can deactivate the EDR by means of setting the deceleration threshold inappropriately, giving them some measure of control over the data being collected. In a 2001 NHTSA report, Volkswagen is quoted as stating that “due to high impact on privacy issues [vehicle speed] recording would be [the] owner’s choice at [the time of the] new car purchase or by dealer programming”.

Certainly there needs to be more public debate about the privacy issues involved.


If the US success at causing Onboard Diagnostics to be built into many of the world’s cars is repeated with EDR (and what government could resist implementing the requirement for such a device, especially when the financial cost to the consumer will be zero?), it’s very likely that in 5-10 years time all cars will have accident crash logging.

So next time you’re involved in a car crash and there is debate or uncertainty about the circumstances, think about the implications of accessing the EDR – or at least making some serious enquires as to whether one is fitted.

Potential Benefits of Event Data Recorders

- Real Time – Use of EDR data in conjunction with Automatic Collision Notification systems would aid in quickly locating crashes and despatching emergency personnel with better crash information in advance

- Law Enforcement – Obtaining impartial EDR data from a collision would help in more accurate determination of facts surrounding an incident

- Government – Collection of EDR data facilitates government in further regulatory initiative to help reduce fatalities, injuries and property loss

- Vehicle Design – EDRs allow manufacturers to collect better real world data to monitor system performance and improve vehicle design

- Highway Design – The use of EDR data can assist in assessing highway roadside safety and managing road systems

- Insurance/Legal – Additional objective data provided by EDRs advance quicker and fairer resolution of insurance and liability issues

- Research – EDR data could provide objective databases of driver behaviour and performance, as well as other research related topics

- Owners/Drivers – EDRs can help fleet owners and drivers monitor vehicle and driver performance to ensure the safe and efficient movement of people and cargo

Canadian Multidisciplinary Road Safety Conference, 2001

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