to determine when it is "clear to cross"
Look for vehicles by SCANNING or GLANCING left / right
Challenges and strategies for scanning / glancing side to side
Pedestrians need to look both left and right to be sure it is clear to cross in both directions, but this can be difficult for some people with visual impairments.
If they glance or scan incorrectly, they may miss seeing some of the vehicles.
The next page has suggestions for training students to scan or glance effectively, but first I'll explain the scanning challenges and suggested strategies for two common visual impairments:
* Severely restricted visual field, such as with retinitis pigmentosa (RP) (using a telescope may also restrict visual fields)
* Central scotoma with loss of visual acuity, such as with macular degeneration
Severely restricted visual field
CHALLENGE WHEN LOOKING FROM SIDE TO SIDE FOR VEHICLES:
May miss seeing vehicles that are nearby (especially vehicles in the "blind spot")
Try looking for something through a long tube like a paper towel roll.
If you move the tube quickly from side to side and back again, you may see what's at your sides each time you stop moving the tube long enough to move it back to the other side, but you can't see anything in between.
That is what happens when people with less than about 5-10 degrees of central vision try to look quickly from left to right and back -- they see what's on the street far to their left and their right, but nothing in between, including vehicles that are near to them.
The same thing can sometimes happen when using a telescope, too.
In addition to the problem with not being able to see details when scanning quickly with a small field of view,
vehicles that are close are difficult for people with this visual condition to see.
This is because the further that they are from an object, the more of it they can see
-- illustrations of this principle are in Considerations of restricted visual fields.
The videos to the right simulate restricted visual fields.
The first video shows that when glancing quickly with about 5 degrees of visual field, you can see the vehicles that are in the distance more easily than you can see those that are near to you.
The second video compares glancing with normal vision and glancing with simulated visual field restriction.
Both videos then show how much more effective it is to scan slowly.
NOTE: People with restricted visual fields also have problems with scanning when they cross at signalized intersections.
When looking quickly to their left, they sometimes miss seeing a vehicle that is turning into their crosswalk.
Although people with severely restricted visual fields might miss seeing approaching vehicles that are close, it is usually a problem only when the vehicle is in the nearest lane.
Vehicles in further lanes are usually not a problem because if they are approaching closely and are undetected when a person starts her crossing, those vehicles are likely to have passed by the time she reaches their lanes.
So there is only one place where it is extremely important that they not overlook vehicles -- a place which I call "the blind spot" for people with restricted visual fields.
That place is in the nearest lane where the vehicle is so close that if they miss seeing it and they start to cross, the driver might be unable to stop -- yikes!
The photo to the right shows a vehicle in the blind spot, about two car-lengths to the left of the person with the cane wearing a vision simulator.
(Click here if you want to read the scary story of how I discovered this.)
The videos above demonstrate that with a restricted visual field, when you scan slowly you can see vehicles that you missed when you scanned more quickly.
Students need to learn how slowly they must scan to ensure that they don't miss seeing any vehicles in the "blind spot," as explained on the next page.
The most difficult vehicles for people with restricted visual fields to see are
in the nearest lane, about two car lengths to their left (in the "blind spot" as shown in the photo above).
The strategy to help see the vehicles better: scan slowly.
Central scotoma with loss of visual acuity:
CHALLENGE WHEN LOOKING FROM SIDE TO SIDE FOR VEHICLES:
May miss seeing vehicles in the distance.
Students with macular degeneration and other central scotomas have a loss of acuity, which can make it difficult to see details such as objects that are small or far away, as demonstrated in the video to the right below.
There are two strategies that can help them be more likely to see vehicles approaching from a distance: eccentric viewing, and holding their glance long enough to notice if there is any movement.
With training, people with central scotoma can learn to find and use the best part of their peripheral vision, which is called eccentric viewing.
This training should be done before they learn to analyze and cross streets -- the skill can then be used to look for vehicles.
HOLDING THE GLANCE LONG ENOUGH TO SEE MOVEMENT:
Even with skillful use of eccentric viewing, the peripheral vision has poor acuity.
However, the peripheral vision excels at detecting movement.
That means that people with central vision loss may not be able to see vehicles that are parked in the distance, but they can detect them if they are moving.
So when these students look for vehicles, they need to hold their gaze long enough to notice if there is any movement, as illustrated in the video to the right.
The most difficult vehicles for people with central scotomas to see are
approaching from a distance (as shown in the photo to the right).
The strategy to help see the vehicles better: hold the glance long enough to notice if there is any movement.
The next page has suggestions for training people to scan or glance effectively.