Visual Task Analysis for Driver

Question 1

Adaptation Effects for a driver?

Answer 1

• People drive in all different types of weather conditions and at all times of day
• Speedy visual adaptation is important for drivers
• Illumination levels constantly changing while driving – e.g. enter tunnel
• Poor adaptation is one of factors that particularly affects older drivers

Question 2

Time taken to respond for a driver?

Answer 2

• Any task which involves moving observer must consider effect of space scotoma
  o Size of scotoma will increase as driving speed increases
• Driving induces a motion-induced blindness which manifests as a scotoma
  o In motion-induced blindness, salient objects in full view can repeatedly fluctuate into & out of conscious awareness when superimposed onto certain global moving patterns
• If small object in VF remains unaltered when everything around it is changing – visual system discounts that stimulus as if it is a scotoma -> removing object from consciousness

Question 3

Flicker for driver?

Answer 3

• In driving, not really a major consideration
• Can be experienced while driving, usually due to sunlight incident through a regular array of trees
  o May be problem for those who are visually sensitive

Question 4

Position in VF/VF size for drivers?

Answer 4

• Driving uses most of VF
• Detection & identification of objects with peripheral vision is particularly important aspect of task
• Steering is guided by monitoring distance centrally (~1 sec ahead) to estimate road curvature, & by monitoring near (~0.5sec ahead) peripherally to judge position in lane

Question 5

Viewing distance for drivers?

Answer 5

• Driving makes strong demands on convergence & accommodation as driver must be able to see instruments on dashboard as well as objects in distance

Question 6

Visual subtense of task detail (size/acuity) for drivers?

Answer 6

• Good central VA and consequently a clear retinal image are necessary for early recognition & reading of road signs
• Aids in early detection of small & hazardous objects – e.g. pedestrians

Question 7

Contrast for drivers?

Answer 7

• Important as have to have well-lit road in v dark circumstances
• Light should be uniformly illuminating road ahead

Question 8

Clarity for drivers?

Answer 8

• V important
• Impacted when there are adverse weather conditions

Question 9

Colour of task for drivers?

Answer 9

• Colour vision important for recognition of traffic lights & road signs
• Problems might be expected where a colour-deficient driver confuses red, amber & green of traffic lights – not a key factor as the lights are always in the same order vertically
• Colour vision defects have not been conclusively found to be associated with higher accident rates
• Protanopic and deuteranomolour drivers – have poor sensitivity to reed light – at disadvantage when comes to responding to tail and brake lights

Question 10

Motion for drivers?

Answer 10

• Crucial attributes of driver’s visual system: dynamic VA (DVA) and ability to perceive lateral motion & motion in depth
• Weak correlation found between reduced DVA & accident rate
• Awareness of reduced DVA cited as one of reasons elderly people give up driving earlier than others

Question 11

Stereopsis requirements for drivers?

Answer 11

• Under conditions of poor visibility (e.g. night) – majority of monocular cues to depth are missing & stereopsis becomes major cue in depth perception
• Stereopsis is inoperative beyond ~500m (Panum’s visual area) so is of little benefit in high-speed driving – it is valuable for nearer tasks, e.g. parking or child location
• No correlation has been found between defective stereopsis & increased accident frequency
  o BUT where a binoc visual anomaly results in diplopia or a large (>4 prismD) vertical phoria -> there is evidence of an association w/ poor accident records

Question 12

VF Requirements for drivers?

Answer 12

• Good peripheral vision is essential for driving
• Restriction of VF can never be fully overcome – increasing head & eye movements & adding extra mirrors to car can help
• Full VF important for maintaining driver’s orientation & in establishing relationships between the many objects in field of view
• VFs can be artificially reduced by e.g. thick spec frames or car design or cap/hat with visor
• Various pathological disorders also cause field defects – e.g. glaucoma, retinitis pigmentosa, cataracts

Question 13

Training Requirements for drivers?

Answer 13

Driver must be trained - to get license

Question 14

Describe other visual factors relevant to driving?

Answer 14

• Vehicle lighting
• Glare
• Vehicles provide lighting to see and be seen
  o In order for driver of vehicle to see road ahead, intensity of headlamps can be high enough to act as a significant glare source to other road users
   Particularly case for more elderly drivers who suffer more from glare, probably due to changes in ocular media & retinal adaptation abilities as age

Question 15

How can drivers reduce glare?

Answer 15

o Dipped headlights: use in built up areas w/ street lighting. Use also when going uphill
Main beam: use when driving unlit roads – dip when oncoming vehicles & coming up behind vehicles going in same direction
Fog lamps: significant glare source to vehicles behind – only use when necessary
Headlamps are checked during MOT to prevent glare from mis-aligned headlamps
o Vehicle loading can cause front of vehicle to point up when rear is more heavily loaded – can misdirect headlamp beam & cause glare  self-levelling suspension & self-levelling headlamps can be used – expensive
o Polarised headlamps could be used, w/ cross-polarised windscreens  couldn’t use completely crossed polarised lamps otherwise vehicles would be invisible to other vehicles until car body was in range of headlamps.
 Relative velocity of 2 vehicles travelling along dark single carriageway road can legally be up to 120mph
 Would be v expensive
 Luminance of lamp would have to be more than doubled to give same lighting on road – polaroid affect diminishes light on road to <40% of light from lamp

Question 16

How can weather affect drivers?

Answer 16

• Rain:
  o Way light is reflected back from road surface changes in wet conditions
  o Less light is reflected back to driver of vehicle from their headlamps, whilst more light is specularly reflected to drivers of oncoming vehicles – producing glare
   Good reason for reducing speed when driving in rain at night
• Fog:
  o Fog acts to scatter light
   This reduces amount of light being directed in front of vehicle & increases amount of back scatter from fog
• Both of these factors reduce contrast of any obstructions, pedestrians or vehicles that may be in road
• Slow down & be v careful

Question 17

Describe 3 aids to drivers?

Answer 17

• Yellow filters:
  o On car lamps – less scatter of a monochromatic light e.g. yellow & would be near peak of voltage & current curve, giving maximal efficiency
   No evidence to show works – using filter over conventional lamp reduces total quantity of light available
• Signalling:
  o Brake lights & indicators not used to illuminate road but act as warning
  o Intensity of indicators is lower than headlamps or rear lamps
  o Flashing of indicators (6/120 per minute) brings them to attention of other drivers
  o Intensity of brake lamps is higher than that of rear lamps – lamps are on for short period of time so does not create glare & helps to bring light to attention of other drivers (combined with on-off nature of the light)
• Other Driving Aids:
  o Retroreflectors e.g. cat’s eyes & road signs make use of light from vehicles to guide drivers
  o Anti-reflection coatings can be helpful in reducing light scatter through spec lenses – reducing glare -> must be kept clean to do this

Question 18

Describe driving & ageing?

Answer 18

• More complain of night driving difficulty
  o Night myopia reduces with age (as amplitude of accommodation reduces) so other factors must be causing problem
• Problems include reduced retinal adaptation ability, reduced dynamic VA & increased scatter from ocular media

Question 19

Describe night driving & myopia?

Answer 19

• When no stimulus for accomm, eye attains resting-state of accomm with eye focussed at 50-100m (1-2D)
  o Such a lack of stimulus can occur either in bright empty field, such as looking at sky or dark field as may be experienced by drivers at night
  o Even with dim street lighting – where level of light adaptation is 0.5 cd/m2 will be some “night myopia”  only to extent of around -0.3DS on average
  o At one extreme, 17% of young people may experience myopic shift in their focus of greater than or equal to 0.75DS in mesopic (middle light) conditions
• Night myopia should not be prescribed for – evidence shows makes no difference
  o When moving from mesopic to photopic conditions – extra lens power could be hazard as acuity reduction under night-time driving is largely due to neural factors rather than refractive error
  o Clip-ons etc that can be put on glasses to help with night myopia

Question 20

Describe vision, driving & accidents?

Answer 20

• Driving is v complicated process & involves range of visual & physical tasks as well as external factors e.g. road lighting, driving conditions, alcoholic intoxication, road type, traffic speed, amount of traffic & complexity of road layout
• Accident statistics show accidents are more common at night & youngest & oldest drivers have most accidents  Younger drivers: don’t have experience  Older drivers: drop in visual performance with age & may explain why these drivers have more accidents
• Davison (1985) shows accident variables & visual abilities were correlated for hyperphoria, R, L & binoc VA
• Many of these variables only became correlated for older drivers, aged 45 & above, & hyperphoria only for drivers aged >55
• Older drivers >55 show correlation between dynamic VA & accident rates
  Types of accidents older drivers are involved in also tend to reflect their failing visual performance
• Older drivers suffer from accidents where they have missed a sign, have failed to notice traffic whilst giving way at junction & failing to notice traffic whilst turning
• This ties in with correlations between accidents & RE acuity in UK (driving on left) & LE acuity in US (driving on right)