The Issue (or Non-Issue)

I have seen several comments about driving concerns by people on The Spectrum. This is mostly a non-issue because driving and distance perception is not a problem with most autistics. Most of us enjoy driving and have no problems. Regardless, there are a few who find driving to be a challenge, which is the reason for this article.

The major concern I've seen is inability to judge distances. There are certainly people on The Spectrum who are superior in this regard, but as we shall see, judging distances isn't a critical aspect of driving.

("The Spectrum" generally means the autism Spectrum, including Asperger's syndrome, HFA, PDD and Kanner's autism. Many of us are comfortable with the generic reference of "autistic".)

Why Judging Distances is Not a Critical Skill

Binocular vision (stereo vision) contributes to depth perception at close distances, but only within 6-7 meters (18-20 feet). Beyond that, the brain relies on less precise cues, such as shadows and the relative motion of objects at different distances to generate depth information.

What this means is that we rely on visual cues to determine distance. Consider that the length of a car is typically between 2.6 meters / 8.6 ft. (2005 Smart Car) and 5.4 meters / 17.7 ft. (2005 Bentley Arnage). So at one car length in front of you (count 1.5 or 2 meters to the front of your car), most peoples' binocular vision doesn't work.

Incidentally, visual acuity is enhanced by synthetic aperture from binocular vision over single eye vision, but that's a different issue.

But as to safety...

The only categories for risk relating to judging distance are:

• alcohol
• drugs
• age
• passing on two-lane roads (opposing traffic)

As to alcohol and drugs, the advise is easy:

1. Don't drink and drive
2. If you drink and drive, don't smoke.
3. If you drive drunk, drive fast so you get home before you pass out.

As to age, seniors need to be aware of reaction times. There are programmes which explain or teach that to seniors. The issues of reaction time and poor vision become the limiting factors for driving at old age. And I guess alcohol. Other than that, "poor distance perception" is not considered a risk factor for accidents.

The list leaves the issue of two-lane roads. One with poor distance perception is limited in ability to pass on two-lane roads (with oncoming traffic); but it's a no-brainer -- if you can't judge the distance, don't bother passing. In some cases, you can rely on the lane striping to indicate safe passing zones, but for the most part it isn't necessary to pass.

Outside Links Related to Depth Perception and Driving Safety

Canadian Ophthalmological Society recommendations Vision standards for driving in Canada

Depth perception

Automobile accidents sometimes occur because of the driver's inability to judge distances accurately. However, judging distance is a skill that can be learned, even by people with monocular vision. Judgements of depth can be made based on monocular clues, such as the relative size or interposition of objects, and clearness of details. A more refined form of distance judgement, called stereopsis, is based on information coming from both eyes.

A driver who has recently lost the sight of an eye or stereopsis may require a few months to recover the ability to judge distance accurately.

So Here's How

Observe spacial relationships while a passenger or while receiving instruction.

There are various relationships between portions of an intersection and vehicles which can give spatial cues. For example, people with superior spatial skills will often use cues to "cheat" when parallel parking.

Tell an instructor or person teaching you that you would like help in judging distances and recognizing spacial clues.

If you tell a driving instructor you are unsure of spacial relationships and have trouble judging distances, the instructor will almost certainly endeavour to help you. The hardest part is convincing the instructor that the difficulty is real.

The second hardest part is explaining to the person what kind of help you are asking for. Most people don't think about the spatial clues they use while driving.

In the case of relatives, expect the opposite. A relative may presume that you are incapable of driving because of autism or other disability. Only learn with a relative if you trust the relative to accept the task of instruction. There are many cases in which a relative or spouse will be an unacceptable instructor.

Observe relative movements

This can be by direct interpretation (the car is getting closer) or by clues (the car is occupying a larger proportion of the windshield). Alternatively, you can go by sound (the loud noise made by the collision).

This is the most important part of spatial judgement, and is part of "defensive driving". If a car is getting closer, then ... well, the car is getting closer. If you can anticipate that, you will have a much easier time following the car.

Do not try to pace a car at close range (unless your car has large numbers painted on the sides).

At slow speeds, such as at a light, you need to have enough space to react and stop should the car in front stop for any reason. This may sound obvious, but this is a common mistake among new drivers.

Note that this is the opposite of pacing a car on the highway, but the two go in-hand. This is explained in the next two items:

The "Three Second Rule" for following.

If you pass the same landmarks as the car in front of you within three seconds, you are following too closely.   Count 3 to 6 seconds from the time a car in front of you passes an object and you pass the same object.

Some places reference a "Two Second Rule", whereas others claim that "two seconds" is insufficient. (My take on it is that the difference of a few seconds is a total of a few seconds from the entire trip, plus the few seconds attributable to people who take advantage of the gap by cutting in front.)

The "Car Length" rule for following

Leave 1 car length for each 10 MPH or 20 Km (or 10 MPH or 15 Km) of car speed. If you have a hard time with car lengths, use the Three Second Rule.

Cheat.

Use things like the need to easily see traffic signals as an excuse to stop well before an intersection.

Expect things to get much easier.

As you can see, most distance perception while driving is learned. After a brief amount of practice, you will be able to estimate distances without thinking about it.

Miscellaneous Cues

Passing

You can safely cut in front of the car you are passing when you can see its headlights

When stopping at an intersection with a crosswalk, you're in about the right place when the far line is barely hidden by the front of your car.

Stop your car so you can see the rear tires of the car in front of you.

This is about a car length.   This always leaves enough room to go around the car if it stalls or just decides not to proceed. Leave more room on uphill stops, because the driver in front may drift back before engaging the clutch.

If you can't see it, don't do it.

My favorite is when a large truck stops to allow me to turn past the truck. The problem is that I can't see the next lane past the truck. I can never tell if that's intentional or misguided courtesy on the part of the truck driver.

"Unable."

When a pilot is unable to comply with a request from a controller, the pilot will respond, "Unable." Don't be afraid to say you are unable or unwilling to do something.

"St. Pancake"

I presume you wouldn't walk in front of a D-9 Cat like Rachel Corrie, but if a vehicle is large, there's a chance the driver will not see you. Typically, if you can see a vehicle's mirrors, the driver can see you in them.

Visual Effects

Most of these are insignificant effects. They are here as a matter of intellectual interest:

Visual angle

Change in size or shape of an object with changing distance;

Linear perspective

This refers to the apparent convergence of parallel lines, e.g., railroad tracks or lane lines that appear to converge to a vanishing point on the horizon;

Texture gradients

Many objects have a surface structure or "grain". The farther away an object, the smaller its details and the more densely packed those details become and this gradient of texture provides information on distance. Also, equal spaces appear increasingly smaller with increasing depth or distance. Very uniform or homogenous road surfaces take away such cues;

Interposition

Near objects partially conceal objects farther away such that the fully exposed object appears nearer;

Elevation

The horizon is higher in the vertical dimension of the optical flow field than is the foreground. Near objects appear below the eye level while distant objects appear at or above eye level.

Motion parallax

Nearer objects are displaced more rapidly than objects farther away as the vehicle changes direction (or the driver's head is moved).

Aerial perspective or Clearness

Closer objects appear sharp and distinct while objects much farther away appear blurry or less distinct due to dust, water vapor, or other particles in the air.

Familiarity

Familiar objects have an expected size and shape based on prior experience.

Relative size

When two similar objects are viewed at the same time, the larger one will appear closer.

Equidistant tendency

In the absence of effective distance information, objects that appear close to each other will tend to be perceived as equally distant from the viewer. Other cues might also be mentioned but they are thought to play a less important role in driving. These cues include accommodation (i.e., change in the shape of the lens within the eye), convergence (i.e., tendency of the eyes to turn toward each other while observing very close objects), stereopsis (i.e., the slightly different images in each eye that results from their different vantage points), and brightness constancy (i.e., dimmer objects appear to be further away than the same object at a closer distance). These cues will not be discussed further because they a) either operate only for distances so close or so far as to be unimportant for driving, or b) because they are relatively weak cues as compared to visual angle or motion-based cues.

Blue Advancing-Red Receding effect

The blue advancing-red receding effect skews the ability to determine the true state of a parked or moving vehicle. Particularly in a dark environment, it could be difficult to determine whether the vehicle is actually parked or moving. At night or under darkened conditions, one's eyes will perceive that a lamp emitting a higher frequency, shorter wavelength of light (violet or blue) will appear to be moving closer, while a lamp with a lower frequency, longer wavelength of light (red) will appear to be moving away.

Black Hole effect

An illuminated object appears to be lower as one approaches it. This generally affects pilots at night. It is the result of a length of an object occupying an ever increasing space in the field of vision as one gets closer. There is a tendency to "fix" the length of the object. As one gets closer, this tendency suggests that one is higher than one actually is. This can lead to controlled flight into terrain.

If this sounds a bit complex, that's because it is. I had been able to perceive this effect intentionally on approach to Napa, California. Of course when an effect is being intentionally observed, one is not at risk.

In the absence of landing aids, the pilot must observe altitude or mentally compensate for the effect.

Misc.

(stuff that doesn't fit elsewhere)

Avoid excitotoxins

Aspertaime and MSG. The effects of aspertaime on autistics is not documented. If it makes you jittery or "spaced out", stay away from it when driving. Mixing excitotoxins with driving or operating heavy machinery is probably risky.

I noticed a large truckstop chain, when test marketing selling Chinese food, prominently displayed "No MSG". Since long haul truckers are typically not more cosmopolitan than urban diners, I was puzzled. Then I realized -- they have to drive long distances, at which time they don't want a "dose" of MSG.

If someone cuts in front of you, that person may be a good person to follow.

Watch brake lights

In addition to alerting you that a car in front is slowing, brake lights can be a good indication of a cop ahead.

Don't let the cellphone distract you.

I hadn't heard of distraction being an issue with autistics. Regardless, ignore the conversation when performing critical operations such as intersections, lane changes, etc. This is more of a problem with hands-free telephone sets than with the kind the user must hold.

I use "standby". I expect the other party to understand the driving takes priority. Or as pilots say, "aviate, navigate, communicate" (meaning operate the vehicle first).

Driving on Ice and Snow

First, it is generally not necessary to learn to drive on poor traction surfaces, provided of course you can avoid such conditions while driving. But if you wish to be able to handle such conditions:

Learn how to skid a car.

At the beginning of each season, take the car to a clear parking lot or the like. While driving slowly and away from objects like trees, other cars, large children,

Turn the wheel and accelerate.

That should put the car into a skid. Recover by straghtening the wheels.

Drive so that someone else is the first person to slide off the road.

But not by cutting someone else off. The idea is that if you're significantly more cautious than the most aggressive drivers, you will leave yourself sufficient margin of error. After all, the faster drivers are driving in generally the same conditions that you are.

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This is linked to pages on relationships and autistics, and relates to the ability of autistics to establish intimate relationships.

The myth of lack of empathy has been used as some sort of justification for abuse of people on the Spectrum

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back to Relationships index

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First posted 5-Apr-03. Last revised 1-Jun-08.

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