Highlighted terminology is explained at the bottom of the page.

There is a broad spectrum of motoring incidents that our clients request our professional opinion on, either in a non-disclosable or CPR-compliant report. In the main, they relate to excess speed and road user perception response, with an increasing amount of cases involving a driver's chosen response actions. Ordinarily many of these incidents do not result in criminal prosecutions being taken against the accused driver. However, when internal organisational action is sought, it is essential that not only are all investigative options explored, but that consideration is given to the robustness of the evidence they choose to rely on. Furthermore, there is an inherent failure to account or even be aware of human factor related elements. We are well versed in the field of what investigative opportunities exist yet are not taken. This leads to inconsistent and integrity lacking evidence for which disciplinary matters are sought. We methodically work through the chain of evidence and critique its robustness through either a desktop analysis service or through an investigative service.

Our clients who use this motoring incident analysis service have been rewarded with identifications of weakness in chains of evidence, evidence collaboration, calibration of electronic equipment, areas of ambiguity and the sheer volume of human factors that often go unnoticed.

A concise introduction to the two main areas is given below.


Excess Speed

For many years offences for speeding have been challenged in the UK courts, some with success and others without. While CI Forensics does not seek to undermine prosecution cases based on Home Office approved technology, there is an alarming rate of employees being individually targetted within their own organisation, whether governmental or private. These incidents rarely make it to the Criminal Justice System, but at times they do make it to the Courts and Tribunals Judiciary. CI Forensics does not just offer its clients a critical review of any evidence but provides a thorough analysis of the management of the investigation, often highlighting many integrity issues within its chronological chain.
While the under the Road Traffic Regulation Act 1984 s.89 a person charged with an offence of excess speed cannot be convicted solely on the opinion evidence of a single witness, many organisations are increasingly relying on telematic devices installed in vehicles to discipline their employees. A telematics device is generally a system installed in a vehicle to record information about driver behaviour, such as speed, the rate of acceleration/deceleration, yaw rates (how quick the vehicle turns) and the number of miles driven. A wide variety of in-vehicle enablers for data collection exists to support numerous connected vehicle and telematics-based programs. For the general motorist, a telematics device can simply be plugged into the Onboard Diagnostic Port (OBD-II port), whereas others are hard-wired into the vehicle electrical system. Once the device is installed, it starts recording information about a driver's behaviour. This information is relayed over a cellular network to whoever requires the information. The majority of telematic devices acquire their speed from the Global Navigation Satellite Systems (GNSS), of which the Global Positioning Systems (GPS) is the most common. Furthermore, some telematic systems also incorporate Micro-Electro-Mechanical Systems (MEMS) accelerometers that work in conjunction with the GNSS. Some of these systems are very accurate, but the danger comes when there is no proven credibility to a device's accuracy and precision. These telematic devices do not undergo Home Office approval, so they have not been sufficiently tested for evidential use in speeding incidents yet organisations often use their data for disciplinary matters. CI Forensics can accurately test speedometers and telematic data using high-end GPS data acquisition equipment accepted and used extensively in the forensic collision investigation industry. We often challenge proceedings on a device's repeatability and reproducibility for similar circumstances. Furthermore, as is more commonly requested by our clients, regularly just a critical review of the management of an investigation reveals a failure to account for measurements of uncertainty, systematic and random errors and reveals numerous missed investigative opportunities to provide corroboration. In failing to consider or adequately investigate, organisations leave themselves struggling to prove beyond the balance of probability the evidence for their initial proceedings. Occasionally organisations produce digital video evidence (CCTV) in attempt to corroborate telematic data, while correct analysis of any footage may well provide a robust not less than speed, it is the verifiability of the proper analysis that we critique. CI Forensics can analyse digital video evidence for the integrity of any calculated speeds and often produces a comprehensive list of variables that have failed to be discounted.

Of interest, the majority of speedometers in use are non-calibrated, and while there are regulations produced by United Nations Economic Commission for Europe (UNECE)* in relation to the accuracy of vehicular speedometers, these regulations only give set tolerances. For the EU, the indicated speed must never be less than the true speed. Therefore, it should not be possible to speed inadvertently as a result of an incorrect speedometer reading. Furthermore, the indicated speed must not be more than 110 % of the true speed plus 4 kph (2.5 mph) at specified test speeds. For example, at 80 kph (50 mph), the indicated speed must be no more than 92 kph (57 mph). The UNECE regulation relaxes the requirements for vehicles mass-produced following type approval. At Conformity of Production Audits the upper limit on the indicated speed is increased to 110% plus 6 kph (3.7 mph) for motorcars, buses, trucks and similar vehicles; and 110% plus 8 kph (5 mph) for two- or three-wheeled vehicles that have a maximum speed above 50 kph.

* (European Commission, 1997) (European Commission, 2000) (European Commission, 1975)


Perception Response Times

A road user's perception response time is the time interval that commences when some object or condition first becomes possible for a reasonably alert road user to detect and ceases when they initiate a discernible response; it is not a reaction time which is the time interval for a person or system to respond to a given stimulus or event. It is essential the distinction is understood as there is a great possibility of those tasked to judge the actions of others to incorrectly apply their own understanding of reaction times, which are fundamentally incorrect in motoring incident analysis.

As an immediate insight, a road user’s perception response time is complex and wide-ranging and Paul has authored over 18,850 words on this subject from his extensive research of this human factor field. Only a concise selection of some of research is given to raise awareness of the subject.

Perception response time is a world away from reaction time. During critical incidents, a road user’s perception response time and any decisions occur during a highly stressful and life-threatening event. As expert witness, there is an inherent risk of offering opinion evidence with the benefit of hindsight. Any forensic collision investigators with a background in traffic-related duties will likely know the dangers of hindsight from their involvement in vehicle pursuits. Pursuits are situations where the circumstances, driver input information, decision-making processes and many other factors change at such speed that any incorrect decision made by an officer within such a wildly differing and dangerous environment to that of any reviewer, requires extreme deliberation and such careful consideration when the benefit of hindsight is used.

With perception response times, there is a limit to how quick the road user can respond to a given hazard; no road user will have a perception response time of zero seconds. Depending on the circumstances, there may be no upper limit with a road user never actually perceiving the hazard.

In almost all incidents there is a necessity to consider a road user’s performance and, therefore, their perception response time. The vast majority of road users have many years of collision or incident-free miles but in each of those miles will undoubtedly have successfully dealt with many hazards, ranging from the very basic to more the complex. When a hazard confronts a road user, which ultimately results in a collision or incident, we need to consider the road user’s performance. It is during this consideration where difficulties arise, as each road user will respond slightly differently to others, some better, and some worse. Some of the factors contributing to a road user’s response are:

  1. Age
  2. Sex
  3. Experience
  4. Eyesight
  5. Demeanour
  6. Distraction
  7. Impairment

The last factor is an important one, a road user’s response is very susceptible to the effects of impairment through intoxication, drug usage, fatigue, daytime/nighttime conditions and so on.

The question is, what should we use as a benchmark to determine and compare a road user’s performance to? The UK government suggests a reaction time derived from their use of the phrase thinking time of around 0.67 seconds in the HMSO publication of the Highway Code. This 0.67 reaction time is commonly referred to by people when discussing a road user’s performance, but is a very simplistic view and CI Forensics strongly argues against its use. While there are experimental research studies in which perception response times are of this lower magnitude, they are rarely universally so short for all test subjects.We must continually remind ourselves that we will rarely establish the true perception response time of a road user.Even if we did establish a road user’s true perception response time, how and to what do we compare it to? Is it fair and proportionate to say that all road users, regardless of the hazard, should have a perception response time of 0.67 seconds? In general, forensic collision investigators either give a specific time, for example, 1.48 seconds quoting research studies and set road user specifics or a general range of 1 to 2 seconds.

Often when forensic collision investigators offer a specific perception response time, it is based on experimental research data. For example (Quimby & Watts, 1981) gives a driver under the age of 25 a median perception response time of 1.88 seconds. We should be wary from the outset if anyone uses such a perception response time to 2 decimal points. While the median of the tests conducted may well be 1.88 seconds, it assumes a high degree of accuracy throughout the test. Whether that degree of accuracy in any reconstruction is appropriate, when the factor under consideration is unknown, is worthy of thought. Is it right that reconstruction calculations are based on such accuracies? Does the experimental research data apply to the collision scenario under investigation? Even if it were to, would the number of subjects in the study be a fair reflection of the population? The issues surrounding road user perception response times in nearly all research papers confirm themselves a high degree of variance exists. Paul suggests that a degree of accuracy to these levels is unsafe unless there is a high degree of verifiable external and independent corroborative evidence.

The nature of the hazard likely influences a road user’s perception response time. Hazards include changing traffic light sequences, traffic signs, vehicles braking ahead, curved paths and road user interactions. Crucially, many of these hazards occur on a regular basis and are dealt with by road users without any additional cognitive stress. In the main, road users appropriately respond to these hazards and the road geometry is designed to take into account the majority of road user’s perception response times as road design engineers consider the stopping sight distance.

An immediate unexpected potentially life-threatening hazard rarely confronts a road user. While road users may deal perfectly well with general and common hazards, they may not fare so well in high-pressure situations. Here the road user is performing not only with a degree of urgency but with the knowledge that the consequences of making an inadequate or incorrect response will likely have serious implications. Cognitive stress relates to the level of urgency that road users experience when confronted with a hazard. The main factors for this stress are the severity of the hazard and the available time to respond to it.

Critical to any judgements on a road user's perception response time is a consideration for its factors:

It is vital the likely factors involved in a specific collision scenario or incident are considered before an opinion on a range of values for a road user’s perception response time is given. While some forensic collision investigators may give a specific time, Paul does not agree with this method after hours of research. While the majority of experimental research studies may indicate a similar perception response time range, how we apply them to the collision or incident under investigation is the fundamental consideration.

Leading experts have conducted many research studies in this field and what follows is an introduction to the various factors of a road user’s perception response time.

The two popular research authors in this field are (Green, 2000) and (Summala, 2000), an analysis and review of their work was conducted by (Muttart, 2001) and further discussed by (Krauss, 2015). We now discuss the key points that came from their research:

(Green, 2000) identified five key factors of a perception response time, those being:

  1. Expectation
  2. Urgency
  3. Age
  4. Gender
  5. Cognitive load

The most prominent factor placed on a road user’s perception response time was expectation, with further subdivision into expected, unexpected and surprise.

Expected – Hazards where the road user has foreknowledge of their occurrence.

Expected hazards are predominantly only applicable under research experimental conditions where the test subject is aware that they are participating in a research experiment. In these scenarios, the main elements of a road user’s perception response time are already consciously preprogrammed. As ‘expected’ a road user’s perception response time is short, the reported 85th percentile being between 0.7 to 0.75 seconds.

Unexpected – Hazards familiar to road users but with little expectation.

For example, many road users have to react to a vehicle slowing down in front of them, the stop lamps illuminating giving the first visual stimulus or cue that the vehicle ahead is reducing in speed. Although they are unexpected, they are by no means rare, and we could even class them as expected hazards. (Green, 2000) offers a mean road user’s perception response time for unexpected hazards as 1.25 seconds.

Surprise – Hazards uncommon to road users with no expectation.

For example, the sudden appearance of a lifeless body in the middle of the carriageway on an unlit rural road. Not many road users would expect to face such a hazard and will have almost certainly never have been confronted with the same hazard before. (Green, 2000) offers a mean road user’s perception response time for a surprise hazard as 1.5 seconds.

(Muttart, 2001) who reviewed the work of the other experts suggests six key factors for a road user’s perception response time:

  1. Contrast – The difference in the degree of colour or shading between an object and its surroundings. For example, a bright object is more visible when it contrasts against a dull background.
  2. Anticipation – Green separated this anticipation into expected, unexpected or a surprise.
  3. Strength of Stimulus – The consideration of the number of stimuli and their associated motion.
  4. Eccentricity – The angle between where the road user is looking compared to where the hazard is located. As this angle increases, the perception response time is likely to increase.
  5. Decision Making – A road user has to make a decision before responding, if there is more than one potential choice then the perception response time will likely increase.
  6. Response Complexity – A road user not only has to make a decision on the circumstances they face, but they require time to consider what other factors are involved in that decision. For example, just deciding to change lanes requires both a mirror and blind spot check. Clearly, some road users will omit these depending on the urgency of the developing situation.

After considering many research articles, Paul suggests that a range of 1 to 2 seconds is in good keeping with most of the experimental research. However, it is best suited where there is evidence of no complicating factors. Where evidence suggests other influential factors, for example, visually complicated surroundings and an unexpected or surprise hazard, then an extended range is preferable.


Fees & Lead Times

Our service level agreements for costs and lead times for our motoring incident analysis vary due to the complexity and investigative routes chosen by our clients. We can combine many of our services together to offer significant savings.

Please contact us to discuss your requirements.



Accuracy - The accuracy of a measurement is how close a result comes to the true value.

Cellular Network - A communication network where the last link is wireless.

Expected – Hazards where the road user has foreknowledge of their occurrence.

Global Navigation Satellite System - A generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage.

Global Positioning System - A satellite-based radionavigation system owned by the United States government and operated by the United States Air Force

Measurement Uncertainty - A parameter associated with the result of a measurement that defines the range of the values that could reasonably be attributed to the measured quantity.

Micro-Electro-Mechanical Systems (MEMS) - A process technology used to create tiny integrated devices or systems that combine mechanical and electrical components.

Onboard Diagnostic Port (OBD-II port) - A vehicle's self-diagnostic and reporting capability.

Perception Response Time - The time interval that commences when some object or condition first becomes possible for a reasonably alert road user to detect and ceases when they initiate a discernible response

Precision - The precision of a measurement refers to how well it agrees with each other in multiple tests.

Random Error - An error caused by unknown and unpredictable changes during measurement.

Reaction Time - The time interval for a person or system to respond to a given stimulus or event.

Repeatability - The variation arising when all efforts are made to keep conditions constant by using the same measuring instrument and operator and repeating during a short time period.

Reproducibility - The variation arising when using the same measurement process among different instruments, operators and over longer time periods.

Stopping Sight Distance - The distance travelled during a perception response time and associated manoeuvre time.

Surprise – Hazards uncommon to road users with no expectation.

Systematic Error - An error that under the same conditions will always be of the same magnitude and polarity.

Unexpected – Hazards familiar to road users but with little expectation.



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