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.
- 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.
- 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.
- 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.