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TAC recommends sight distance standards

Joe Naiman

Village News Reporter

The county’s Traffic Advisory Committee has recommended the adoption of sight distance standards.

A unanimous TAC vote Sept. 9 forwards that recommendation to the San Diego County Board of Supervisors, whose Jan. 11, 2023, meeting is expected to include adoption of the sight distance standards. The sight distance standards are based on the driver speed of oncoming traffic along with whether or not the through traffic is on a level road or one with a grade.

“It’s a useful tool for county engineers,” said TAC Secretary Kenton Jones.

Normally the TAC addresses regulatory issues such as stop or yield controls, speed limits, and parking prohibitions. The sight distance standards will be used for assessing the adequacy of a development or for improving operational safety on existing roads. “It really has nothing to do with the setting of the speed limit,” Jones said.

Speed limits normally address a segment of a road; the sight distance standards will apply to individual driveways or intersections. “The data through the whole segment is a little bit different throughout,” Jones said.

The county’s public road standards require that all intersections involving a public road conform to intersectional sight distance based on roadway design speed and that any deviations from the county public road standards follow the American Association of State Highway Transportation Officials manual which provides in-depth analysis of stopping sight distance, the sum of driver perception and reaction distance, and deceleration distance.

The manual provides for adjustment factors for uphill and downhill grades and has formulas used for calculations including the determination of driver reaction times. The sight distance standards including minimum standards for existing intersections were based on those formulas.

The standards can, however, be used by and in response to developers. Adopting a standard is one factor for a public agency’s design immunity in the event of a collision or other accident and subsequent lawsuit. A developer who meets the sight distance standards will have a strong chance of the county’s Department of Public Works providing approval for that portion of the project.

The county public road standards themselves are intended for roadway and intersection design rather than for existing roads and intersections although DPW may address an existing road segment or intersection with a high accident rate by recommending design improvements.

Corner sight distance is measured from a point on the minor road at least 10 feet from the major road and from an eye height of 3 feet 6 inches on the minor road to an object height of 4 feet 3 inches on the major road. The minimum sight distance is the greater of the design speed for the road classification or the prevailing speed if known.

Additional corner intersection sight distance may be required for left turns at divided highways or onto two-lane highways with more than two lanes (for example, a turn lane). The minimum corner sight distance is 200 feet for 20 mph, 300 feet for 30 mph, 400 feet for 40 mph, 500 feet for 50 mph, and 600 feet for 60 mph.

Design stopping sight distance is for new construction. The calculations allow 2 1/2 seconds for perception and reaction while also incorporating braking capability. The total stopping sight distance is 77 feet at 15 mph, 112 feet at 20 mph, 152 feet at 25 mph, 197 feet at 30 mph, 246 feet at 35 mph, 301 feet at 40 mph, 360 feet at 45 mph, 424 feet at 50 mph, and 492 feet at 55 mph.

Operational stopping sight distance is measured from the minor road 8 feet from the pavement and from an eye height of 3 feet 6 inches on the minor road to an object height of 3 feet 6 inches on the major road. The calculations assume a perception and reaction time of 1 1/2 seconds.

The actual deceleration varies with grade, but for a level road the stopping sight distance is 49 feet at 15 mph, 73 feet at 20 mph, 101 feet at 25 mph, 132 feet at 30 mph, 166 feet at 35 mph, 204 feet at 40 mph, 246 feet at 45 mph, 292 feet at 50 mph, and 341 feet at 55 mph.

“We will take the tool to the Board of Supervisors,” Jones said.

 

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