Rule of thumb?

[ACtrafficengr]

I'm looking for documentation for a "rule of thumb" about sight distance obstructions that goes something like this:

From the travel lane of the approach, hold your hand out at arms length, thumb pointed upwards. If the apparent width of the object is more than twice the width of your thumb, it is a visibility obstruction.

Of course, it depends somewhat on the width of your thumb and the length of your arm.

Is there an accepted width of an object for it to be considered a sight distance obstruction? If so, it is probably an angular measurement rather than an abolute width, since a nearby narrow object can be as obstructive as a wider one farther away.

 

[KRSServices]

I'd be careful with that little jem. I would use the general rules in the RTAC manual. Crouch or squat to an approximate height of 1.05 m. Using 300 m as a guideline (speeds of 60 mph or greater), look 300 m in the direction of the desired sight line relative to the centreline of road approach. If you have a clear line of sight, you should be good to design the approach properly and calculate the actual sight line. If it is obstructed, the design in that location likely will not satisfy the sight line test or stopping sight distance test. A sight line should always be unobstructed.

KRS Services

http://www.krs-services.com

 

[ACtrafficengr]

I'm assuming you are saying that vehicles should be visible for your equivalent of AASHTO's Intersection Sight Distance. I can't argue with that. My question was, when is something wide enough to be considered an obstruction?

 

[sc]

Something is wide enough when it exceeds the apparent width of a bike rider or pedestrian at a distance required for braking. For example if a car is travelling at 60km/h it may require 40m to stop, therefore at 40m from where the pedestrian is the driver should be able to clearly see the pedestrian.

Then do as KRS suggests and squat and check. No rule of thumb exists because most people cannot judge distances vs speed vs stopping, etc effectively. In my role as a road safety auditor, I quite often pace out distances, squat on the road, think about what problems may occur etc,and then make a subjective assessment.

Remember at speed, many drivers have very different reaction times, hence the difficulty in assessing what will or will not be an obstruction.

regards
sc