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Mobility & Transport - Road Safety

Functional limitations, diseases and medication

As people age, functional limitations and disorders occur that can increase the crash rate of road users. This is particularly the case in the decline of motor functions like muscle strength, finely tuned coordination, and the ability to adapt to sudden changes in bodily position. There are few indications that a decline in visual and cognitive functions, as part of normal ageing, also has road safety consequences. Only in the case of severe sensory, perceptual, and cognitive limitations does the relation between functional limitations and crash involvement become visible. Examples are eye disorders such as cataract, macular degeneration, and glaucoma and diseases like dementia, stroke, and diabetes.

In considering whether to advise restricting or suspending driving or making prescribing decisions, the end functional capacity is the main concern, rather than individual sources of functional loss. That is, one needs to take into account the severity of impact of all combined effects, and importantly, one needs to consider how much compensatory potential the older driver may have to compensate for a particular deficit. The relative risks of being involved in crashes due to a medical condition like diabetes or a vision impairment have been calculated in the EU-project IMMORTAL. The results of these calculations are shown in the table below. Descriptions of the medical conditions mentioned:

  • Age-related functional limitations
  • Age-related disorders
  • Medication

Relative risks of selected medical conditions. Source: Vaa, 2003

Medical condition Relative risk* 95% Confidence Interval Number of results
Vision impairment 1.09** (1.04; 1.15) 79
Field of vision 0.90 (0.69; 1.17) 4
Progressive eye diseases 0.86 (0.50; 1.49) 4
Binocular visual acuity 1.13** (1.05; 1.22) 39
Hearing impairment 1.19** (1.02;1.40) 5
Arthritis/Locomotor disability 1.17** (1.00;1.36) 12
Cardiovascular diseases 1.23** (1.09;1.38) 48
(Serious) arrhythmia 1.27** (1.09; 1.47) 14
Abnormal arterial blood pressure 1.03 (0.86; 1.22) 8
Suffering from angina 1.52** (1.10; 2.09) 3
Myocardial infarction 1.09 (0.62; 1.92) 2
Diabetes mellitus 1.56** (1.31;1.86) 25
Neurological diseases 1.75** (1.61;1.89) 22
Diseases affecting central nervous system (incl. stroke, Parkinson's disease) 1.35** (1.08; 1.67) 11
Epilepsy/other seizures 1.84** (1.68; 2.02) 8
Mental disorders 1.72** (1.48;1.99) 33
Dementia 1.45** (1.14; 1.84) 18
Alcoholism 2.00** (1.89; 2.12) 3
Drugs and medicines 1.58** (1.45; 1.73) 68
Renal disorders 0.87 (0.54; 1.34) 3
Weighted average across all main groups 1.33** (1.28; 1.37) 298

* A relative risk of 1.09 means that drivers with the mentioned medical condition have a 9% higher crash rate than drivers without any medical condition.
** The relative risk is statistically significant at a level of α < 0.05


Age-related functional limitations and their relevance to road safety

As drivers age, information processing capacities generally decline. However, individual differences are large, not only in terms of the chronological age at which the ageing process manifests itself, but also in terms of the pace at which the ageing process continues. Several traffic related studies have given an overview of the specific sensory, cognitive, and motor skills that deteriorate with age. The most important functional changes that accompany normal ageing relate to:

  • Visual and perceptual abilities
  • Cognitive abilities
  • Physical abilities

Decline in vision and perceptual abilities

Visual functions that decrease as people age are: visual acuity, peripheral vision, visual acuity in poor light and sensitivity to glare, contrast sensitivity, detection of movement, and colour vision.

Decreases in visual acuity (the ability to see details) generally accelerate after the age of 50. The decrease of visual acuity often occurs so slowly, that people don't notice their visual acuity becoming worse and they therefore often overestimate their visual powers. In many cases, the visual acuity of drivers is below the legal standards without them knowing it. Good visual acuity is not only important for the perception of traffic signs and signals, but also for long distance view that is specifically needed for overtaking on secondary roads.

The visual field determines to what extent objects and events situated outside the fixation point are perceptible. Adequate peripheral vision is one of the most important prerequisites for safe driving. It is necessary to see cars in adjacent lanes when making a lane change, to keep the car centred in its lane, to see a pedestrian leaving the pavement or to see another car approaching an intersection. Defects in peripheral vision typically occur in patients with glaucoma (increased eye pressure), macular degeneration, diabetes, and after stroke or other cerebral diseases. These diseases are found more often amongst older drivers (see eye disorders).

Functional impairments relevant for driving in the dark are nighttime visual acuity and sensitivity to glare. Impaired nighttime visual acuity is the result of two age-related changes that reduce the amount of light reaching the retina: reduced pupil size and yellowing of the lens. A consequence of reduced retinal illumination is that sources must be of higher intensity to be seen at night. However, older adults' increased vulnerability to disability glare makes it important to assure that lighting is appropriately directed: sensitivity to glare, which increases between the ages of 40 and 70, leads to a slower recovery from headlights and other reflecting sources [Fozard et al. (1977) cited in Aizenberg & McKenzie. Cataract is one of the main causes of sensitivity to glare among older people.

As far as contrast sensitivity is concerned, older adults have more trouble discriminating small details and this becomes worse at low illumination levels. Contrast sensitivity is - even more than visual acuity - necessary for the perception of (the information on) traffic signs. Besides this, contrast sensitivity is also believed to play a role in distance perception and the estimation of the speed of moving objects.

The age-related changes in colour vision relevant to driving also concern contrast. Older adults have problems discriminating between blue, blue-green and violet: not important colours for the perception of traffic signs and signals. However, older adults also have problems discriminating between colours that are very much alike. Therefore, the differences between colours used in displays of Intelligent Transportation Systems should be as large as possible.

Detection of movement decreases with increasing age as a result of age-related changes to neural mechanisms and a less smooth movement of the oculomotor system that is also the result of the ageing process. Obviously, the ability to detect movement is very important for safe driving, not only for being able to detect vehicles driving on a crossing road and to estimate their speed, but also for being able to detect changes in the speed of vehicles straight ahead of them, i.e. stopping, slowing down, speeding up, and reversing.

Declines in cognitive abilities

Age-related declines in sensory abilities such as vision have an impact on the input the driver receives from other road users and the infrastructure. To select the appropriate information, interpret it and make decisions which must then be translated into an appropriate driving action, one needs perceptual and cognitive processes. Some of these processes decline as people grow older, including the ability to maintain attention over longer periods of time, the ability to separate important from unimportant information (selective attention), the ability to share attention between various tasks (divided attention), short-term memory and information-processing speed.

Speed of processing information is often crucial to making safe decisions in driving. Fundamental to this aspect of the driving task is the time taken by a driver to respond to the demands placed upon him or her by the traffic environment (often referred to as the 'perception-reaction' time). Research studies have generally found that reaction times to simple stimuli do not deteriorate dramatically with age. Reaction times of older drivers only increase when drivers have to make decisions in complex situations.

Physical changes

Physical abilities that decline as people age are joint flexibility, muscular strength, and manual dexterity. These age-related changes can influence the ability to get in and out of a car, operate the vehicle, and can influence injury and recovery.

An example of the influence of reduced joint flexibility is that reduced neck rotation can hinder the driver while checking for approaching traffic at intersections or before merging. This is especially detrimental to older drivers, since they rely on neck rotation to compensate for their restricted visual field. Decline in joint flexibility is not the same for all body parts. In a study by Kuhlman, older adults had approximately 12% less cervical flexion, 32% less neck extension, 22% less lateral flexion and 25% less rotation than the younger control group. Joint flexibility can be greatly influenced by degenerative diseases such as arthritis, which is experienced to some degree by approximately half the population over 75 (Adams & Collins, 1987; cited in Sivak et al. The crash risk associated with diseases such as arthritis can be found here.


Age-related disorders that can restrict fitness to drive

A number of diseases and disorders are found to be related to crash proneness. These are: eye disorders, dementia, Parkinson's disease, stroke, cardiovascular diseases, and diabetes. These diseases and disorders can occur at every age, but they are more common among older adults. Another thing that is more common among older adults is comorbidity, which means having more than one disease. Specific crash risks associated with age-related disorders can be found here. It should be noted, however, that increased risks can also be caused by the medications that are being taken because of one or more disorders.

Eye disorders

The eye disorders cataract, macular degeneration, glaucoma, and diabetic retinopathy are the leading causes of a significant decline in visual acuity and visual field while ageing. The crash risks associated with eye disorders can be found here. Cataract is characterized by a clouding of the eye lens and affects glare sensitivity, colour perception and night vision. Fortunately, it can be treated by replacing the lens with an artificial one. Age-related macular degeneration is a disorder of the central part of the retina and affects visual acuity and colour perception. This disorder, therefore, can lead to the inability to read road signs or to see cars. Glaucoma affects the peripheral part of the retina as a result of damages caused by high intraocular pressure. This condition is painless and the patient is often unaware of the deficit in visual field, a deficit which causes difficulties seeing cars or pedestrians approaching from the side. Persons with diabetes, a disease that affects between 10% and 20% of the older adults (Harris, Hadden, Knowler & Bennett, cited in Klein, are at higher risk of developing cataract, glaucoma and abnormalities that affect the retinal blood vessels (diabetic retinopathy). The latter often result in deficits in the peripheral visual field.


Dementia is a syndrome that is characterized by a progressive decline of cognitive functions as a result of brain damage or illness. The most common dementing illness is Alzheimer's disease, accounting for approximately half of all dementia cases. Alzheimer's disease is characterized by impairments in memory and at least one other cognitive domain, such as attention or judgement. The exact impairments depend on the areas of the brain that are affected.

Dementia often involves a poor understanding of one's own illness, as a result of which patients are often not capable of judging their own limitations and of adapting their behaviour accordingly. Thus, drivers with dementia are less likely to limit their exposure to high risk situations than drivers who have diminished visual and physical abilities, but intact cognitive abilities. The mere diagnosis of dementia is not enough to advise older adults to stop driving. According to an international consensus group on dementia and driving, drivers should be advised to stop driving when they are diagnosed with moderate or severe dementia. When continued driving is considered permissible, it is of great importance to ensure regular follow-up examinations. The crash risk associated with dementia can be found here.

Parkinson's disease

Parkinson's disease is a progressive, age-associated neurological syndrome that is primarily due to the insufficient formation and action of dopamine. Patients suffer from resting tremor, stiffness, the inability to initiate movements (akinesia), and impaired postural reflexes. In addition, Parkinson's disease is associated with depression and dementia at rates much higher than age-related norms. Estimates of the prevalence of dementia range from 30% to 80% (Kaszniak, 1986, cited in Holland, Handley & Feetam, whereas estimates of the frequency of dementia in the total group of people over 65 years of age vary from 5% to 15%.

Both the movement and cognitive effects of Parkinson's disease have potentially important implications for the patient as a driver. However, as in other chronic diseases, the level of function is a more important criterion for fitness to drive than the diagnosis of Parkinson's disease itself. Most people with severe Parkinson's disease give up driving.

One particular area of concern relating to driving and Parkinson's disease is the occurrence of excessive sleepiness that is common in this disease. A study by Frucht (cited in Holland, Handley & Feetam, showed that excessive sleepiness was prevalent in 51% of the study participants. This sleepiness correlated with severity and duration of Parkinson's disease and risk of falling asleep at the wheel. The use of anti-Parkinson (dopaminergic) drugs also seems to contribute to the excessive sleepiness (Fabbrini et al., 2002; cited in Holland, Handley & Feetam). The crash risk associated with Parkinson's' disease can be found here.


A stroke, also known as cerebrovascular accident (CVA), is a neurological injury whereby the blood supply to a part of the brain is interrupted, either by a clot in the artery or by a burst of the artery. Driving after a stroke is problematic if there are cognitive changes, even assuming that the patient is rehabilitated enough to manage the driving task. Many people do not resume driving after a stroke. Those who stop driving are generally older and/or have other sources of impairment or disability in addition to the effects of their stroke.

Little research has been done into the effects of a stroke on fitness to drive. In general, it is assumed that effects of a stroke on motor performance, such as paralysis, can be compensated for by vehicle adaptations and retraining. Other effects, such as apraxia (lack of ability to imagine, initiate or perform an intended action) and lateral neglect, have more severe consequences. In the case of lateral neglect, which means that the patient does not react to or look at things that are located on one side of the visual field (the side opposite to the affected hemisphere), people should be advised to stop driving. A study in which driving of left-sided and right-sided stroke victims and controls were compared, showed that the performance of those with right-sided brain damage was consistently poorer than that of those with left-sided damage. The former more frequently failed the driving test, and particularly performed more poorly at intersections, cited in Holland, Handley & Feetam. The crash risk associated with a stroke can be found here.

Cardiovascular diseases

Cardiovascular diseases include diseases such as angina pectoris (chest pain), cardiac arrythmias, heart failure and hypertension (abnormally high blood pressure). Studies that have distinguished these different conditions have indicated that only cardiac arrythmias and angina pectoris increase crash risk. Actual heart attacks are only responsible for a small part of the total relationship of cardiovascular illness with driving risk. The crash risks associated with cardiovascular diseases can be found here.

Diabetes Mellitus

Diabetes is a disorder that is characterized by high blood sugar levels, especially after eating. The incidence of diabetes becomes much more common with increasing age, with 17-20% of 70-year-olds having difficulty regulating glucose as compared with 1.5% of 20-year-olds. There are two types of diabetes: insulin dependent (type I) and non-insulin dependent (type II). The former are dependent on insulin injections, the latter can control blood sugar levels by diet, weight reduction, exercise and oral medication.

Estimates of the crash risk associated with diabetes used to be as high as twice the rate of average drivers. However, improved medications, better possibilities for diabetic patients to monitor their own blood glucose levels, and improved understanding of diabetic control seem to have reduced the crash risk (Hansotia, 1993, cited in Holland, Handley & Feetam). An important drawback of tighter control of blood glucose levels is that hypoglycaemic episodes are now much more common. During these episodes with low blood sugar levels, cognitive functions are diminished. Even with only modest levels of hypoglycaemia at times when individuals may be totally unaware that they are hypoglycaemic. All in all, serious diabetes (treated with oral drugs or with insulin) is still one of the strongest predictors of crashes, showing a stronger relationship than other illnesses examined. The crash risk associated with diabetes can be found here.

Persons with diabetes are also at higher risk of developing the eye disorders cataract, glaucoma and abnormalities that affect the retinal blood vessels (diabetic retinopathy), all affecting visual acuity and visual field (see eye disorders).


Many older adults suffer from more than one disease. In a study by Holte & Albrecht it was found that two out of three persons aged 60 years and above suffer from at least one illness. Nearly every second person suffers from more than one illness. Suffering from more than one disorder can reduce the driver's possibility to compensate for the effects of these disorders. In addition, suffering from more than one disease often means that multiple medication has to be prescribed (polypharmacy), which increases the likelihood of pharmacokinetic or pharmacological interactions (see medication).



Regardless of the age of the patient, there is an excellent consensus on medications that present a risk to drivers. These are, among others, benzodiazepines, tricyclic and 'second generation' antidepressants, painkillers that act on the peripheral and central nervous system (analgesics), and first generation antihistamines. Older patients, however, are likely to exhibit altered sensitivity to medication, and this should be taken into account when prescribing. This change usually involves an increased effect, including side effects and adverse reactions, and the duration of action of a drug may be significantly prolonged.

Since many older adults suffer from more than one disease (see comorbidity), older patients are also more likely to be prescribed multiple medication (polypharmacy). The more different medications are being taken, the greater the likelihood of pharmacokinetic or pharmacological interactions. Medicines which are not prescribed but that can be obtained over-the-counter can add to this effect.

In evaluating the possible impact of a medication on driving, it is important to remember that medication is prescribed for an illness, and that the illness may itself affect driving-related abilities (see age-related disorders that affect driving). A particular medication could affect driving independently, it could worsen any deterioration in driving ability caused by the illness, or it could even act to reduce the risk to the patient caused by the illness. The vital point is not whether the specific drug has an effect on driving performance, but rather, whether the individual is capable of functioning safely in their environment.