Sight Distance Analysis

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sightline analysisAs part of any development project where a roadway or driveway is proposed, it is imperative to ensure that there is proper sight distance to allow for safe traffic operation along a proposed roadway and at intersections where a proposed roadway or driveway meets an existing roadway. Sight distance is defined as the length of the roadway ahead that is visible to the driver. The available sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path. The two primary types of sight distance considered as part of most site development projects are Stopping Sight Distance (SSD) and Intersection Sight Distance (ISD).

Intersection Sight Distance

Intersection Sight Distance (ISD) is the corner sight distance available for a vehicle approaching an intersection to see oncoming vehicles approaching from crossing legs (the left and right). The height of eye for passenger cars is assumed to be 3.5 ft above the surface of the minor road and is typically measured 15′ off the edge of major road travelway. The height of object (approaching vehicle on the major road) is also assumed to be 3.5 ft. An object height of 3.5 ft assumes that a sufficient portion of the oncoming vehicle must be visible to identify it as an object of concern by the minor road driver. Intersection Sight Distance (ISD) standards are set by local municipalities, as well as, state and federal highway agencies. Intersection Sight Distance (ISD) requirements are a function of the design speed of the major roadway, the type of vehicle making the turn, the type of turn, and intersection geometry.

Stopping Sight Distance

Stopping Sight Distance (SSD) is the sum of the distance traveled during a driver’s brake reaction time (i.e., perception/reaction time) and the braking distance (i.e., distance traveled while decelerating to a stop). In basic terms, Stopping Sight Distance (SSD)Stopping sight distance is defined as the distance needed for drivers to see an object on the roadway ahead and bring their vehicle to a safe stop before colliding with the object. Stopping sight distance needs to be provided along new roadways to ensure that a vehicle traveling along a roadway can come to a complete stop when an object enters its travel lane. Stopping Sight Distance (SSD) is a function of the design speed of a roadway, the reaction time for an average person to apply the brakes, and the deceleration rate of a vehicle when the brakes are applied under wet pavement conditions. In measuring Stopping Sight Distance (SSD), the height of the driver’s eye is estimated to be 3.5 ft and the height of the object to be seen by the driver is 2.0′ which is equivalent to the taillight height of a passenger car. Stopping sight distance is influenced by both the vertical and horizontal alignment of a roadway. Stopping Sight Distance is factored into the minimum horizontal and vertical curve criteria as set by the American Association of State Highway and Transportation Officials (AASHTO).

A common scenario where a Stopping Sight Distance (SSD) situation may arise on a site development project is when vehicles must cross a lane of oncoming traffic to make a left turn into a facility. If there is oncoming traffic at the point when the left turning vehicle is trying to make the turn, the left turning vehicle must wait until there is a gap in oncoming traffic. When this occurs, vehicles approaching in the left turning vehicle’s lane must be able to see the left turning vehicle and be able to come to a complete stop so as to avoid hitting the stationary left turning vehicle. In this type of situation, it is imperative that there is sufficient Stopping Sight Distance to prevent a rear end type accident.

Sight Distance Specialists

The engineering staff of Arthur H. Howland & Associates, P.C. has a great deal of experience in performing Stopping Sight Distance and Intersection Sight Distance Analyses. If you are involved in a project that may require a sight distance analysis, it is important to have qualified, experienced professionals working on your team. Contact us at (860) 354-9346 to discuss how Arthur H. Howland & Associates can help you with your sight distance needs.

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