July 19


3 Tips for Evaluating Intersection Sight Distance

By Mike Spack

July 19, 2016

A Policy on Geometric Design of Highways and Streets, access point, American Association of State Highway and Transportation Officials, corridor, intersection sight distance, traffic safety

Guest post by Jonah Finkelstein, EIT Spack Consulting

Intersection Sight Distance is an important measurement in traffic engineering and comes heavily into play when analyzing access options and modifications for existing and proposed developments. The American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets says, “Specified areas along intersection approach legs and across their included corners should be clear of obstructions that might block a driver’s view of potentially conflicting vehicles.” These sight triangles and lines at intersections help give drivers enough sight distance to check for conflicting vehicles before make safe movements from a cross street onto or through crossing roadway. Recently, the determination of intersection sight distance and their corresponding time gaps were required to assess the potential impacts and safety concerns of adding access points along a corridor. The following are some important tips we discovered during the field review of these accesses.

  1. Determine the operational stop bar location, elevation of the roadway at the access.

For existing access this is an easy task, however for proposed access this is more difficult. It is important to take all measurements from the location that an average driver will be stopped (4.4 meters from the edge of the major roadway according to AASHTO). Excavation and roadway elevation are also important to note, as all measurements should be taken from roughly 1.08 meters above the finished roadway elevation, to replicate the height of the average driver’s eye when driving a passenger car (2.33 meters should be used for trucks and heavy vehicles). This means that if the finished road will be lower or higher than the existing ground elevation the measurement elevation needs to be adjusted to match the final elevation.

  1. Determine the slope of the access approach.

It is important to note the approach grade of the proposed access, as for every additional 1 percent slope over the included 3 percent slope, an additional time requirement is added to the necessary sight distance time gap. This information can usually be found in the construction plans, however if not supplied an estimated field review can be used until more exact data can be retrieved. This value is even more important in Minnesota due to our freezing temperatures in winter, which magnify the effects of the approach slope. Icy conditions slow down the ability of a vehicle to quickly accelerate from a stop and increase the time needed for vehicles to make a safe movement from a stop.

  1. Record sight distance with respect to time and not distance.

By measuring sight distance with respect to time, and not distance, an accurate representation of the actual supplied gaps in traffic can be determined. If only measuring the sight distance in feet, an access may meet the required intersection sight distance for all movements in supplied feet, however, if vehicles along the roadway are traveling faster than the posted speed limit the required time gaps for safe movements will not be met. The resulting time gaps for cross street traffic to pass through or onto the mainline will not be supplied. This can result in longer delays or increased accidents due to the non-sufficient gaps in traffic.

Have you discovered any helpful tips, tricks or processes during your intersection sight distance field review? Let us know in the comment section below.

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  • I enjoyed your post. I also had a few notes and questions:

    1) Although measuring gaps using time rather than distance may be preferable in that it accounts for varying speeds, doesn’t this also mean that your results are based on whatever random behavior is observed at the time you happened to be measuring in the field?

    2) The latter part of the article talks about “time gaps for safe movements”. This seems a bit misleading to me, since time gaps are not directly related to safety. The notion of intersection sight distance is based on the idea that through traffic should not need to slow by more than 15% to accommodate maneuvers by side-street vehicles. Providing a specific time gap is intended to accomplish only this goal. Any safety-based analysis would need to account for the physics of a stopping vehicle (including perception-reaction and deceleration) rather than just one that slows by 15%. Thankfully, stopping sight distances are almost always less than the sight distance required to meet time gap parameters, so providing adequate intersection sight distance typically has the side-benefit of ensuring the intersection can operate safely.

    The AASHTO Green Book states, “Intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to allow the intersection to operate smoothly.” (p. 9-36) Also, “If the available sight distance for an entering or crossing vehicle is at least equal to the appropriate stopping sight distance for the major road, then drivers have sufficient sight distance to anticipate and avoid collisions. However, in some cases, a major-road vehicle may need to stop or slow to accommodate the maneuver by a minor-road vehicle. To enhance traffic operations, intersection sight distances that exceed stopping sight distances are desirable along the major road.” (p. 9-29)

    There are other weaknesses associated with the way we currently evaluate intersection sight distance using time gaps. For instance, that Highway Capacity Manual’s gap acceptance model indicates that most vehicles actually accept gaps that are shorter than those desired for intersection sight distance. Based on my observations, heavy vehicles in particular often will not wait for a gap of 11.5 seconds or more before turning onto a major street. This means that not only is intersection sight distance not a safety parameter, but it isn’t a very effective operational parameter either.

    3) A bit of a philosophical conundrum arises as a result of the above-noted concerns. Specifically, if we evaluate intersection sight distance based on the time gaps for vehicles that are traveling at speeds in excess of the posted speed limit, we may be ensuring that speeding vehicles don’t need to slow down to the speed limit rather than promoting safe operation of the intersection. If we really want vehicles to travel no faster than 25 mph on a particular roadway and post that speed limit, is it really appropriate to require sight distances that allow vehicles to travel at 35 mph without being impeded by side-street vehicles? Especially when providing lesser sight distance could still accommodate safety by allowing those vehicles sufficient space to react and come to a complete stop if needed to avoid a collision?

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    Mike Spack

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