E 361 Safety Features and Collision Data (Accident Records)

Many comprehensive studies have been made of (highway accidents)¹ traffic collisions, their causes, and the safety measures to be taken for their prevention.  Some of these safety measures are under the control of the highway designer and should be considered when designing a proposed highway or correcting an existing street.  Therefore, the results of one of these studies that was made and tabulated by AASHTO are shown on Figure E 261. The Figure shows that there is a definite correlation between the lowering of design standards and the increase in the (accident)¹ collision rate.  There is also an indication that as the volume increases to an ADT of 9000 vehicles, the (accident)¹ collision rate decreases sharply.  This reduction is attributed to the heavy congestion, which causes a severe reduction of speed, prevents passing maneuvers, and results in an unsatisfactory type of operation.

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Although it may be concluded from the figure that producing heavy congestion is a method of reducing (accident)¹ collision, a more desirable, more efficient, and safer method is meeting the geometric design standards as covered in this Street Design Manual.  In this regard, some of the factors that should be considered in (highway)¹ street design are submitted for the designer’s consideration:

1. Volume: Provide adequate facilities in order to increase the vehicular capacity and reduce the vehicular density.

2. Pavement and Lane Widths: Provide lane widths of at least 10 feet and pavement widths of at least 20 feet.

3. Lateral Clearances: Provide adequate lateral clearances to structures and other roadside features.

4. Sight Distances: Provide adequate vertical and horizontal sight distances for stopping and passing maneuvers.

5. Alignment: Provide good alignment by not incorporating constrictions, jogs, and excessive curvature in the design of proposed streets, and by eliminating them in existing streets.

6. Type of Street (Highway)¹ Street: Site type of (highway)¹ street is sometimes classified by the number of lanes or the degree of access control. Three-lane highways appear to have the highest (accident)¹ collision rate. The rate may be reduced by adding another lane or increasing the degree of access control.

7. Access Control: (See Section E 262.) Statistics accumulated by AASHTO indicate that an excellent degree of safety is obtained with controlled access design. This is where all types of hazardous (highway)¹ street access, such as cross traffic, traffic along the roadsides from parking, parking lots, driveways, etc., and pedestrian traffic are materially reduced or eliminated. Where feasible, some of the following methods should be used:

   1. Cross Traffic: Efforts should be made to either:

      1. Completely eliminate cross traffic by dead-ending intersecting streets or providing a grade separation OR

      2. Minimize conflict areas by means of channelization and right-angle crossings.

   2. Roadside Interference: This type of interference may be reduced or eliminated by providing frontage or service roads or access to off-street parking by means of alleys or side streets.

   3. Pedestrian Traffic: Providing on overhead or underpass structure is one solution. Curbed islands or safety zones reduce the area and time of pedestrian exposure to vehicular traffic on wide streets.

There are other means of introducing safety factors in a (highway)¹street, such as prohibiting parking; providing signalization and traffic signs; etc. These efforts, coordinated with the above-mentioned design features, should result in safer, more efficient, and more satisfactory (highway)¹street operations.

Footnotes

1. The text in parenthesis is from the legacy Street Design Manual text and has been superseded by the text that follows.