Capital Hill PUD Traffic Safety Analysis (14-020) <br />March 3, 2017 <br />intersection sight distance, but is not within view of the departing vehicle's line of sight. At both <br />existing driveways where additional traffic will be added after construction, measurements were <br />made on the Capital Drive approaches that indicate approximately a six percent grade at the <br />existing driveway near Cresta De Rute Street and approximately a nine percent grade at the <br />driveway near the end of Capital Drive. AASHTO recommends a stopping sight distance of 143 <br />feet on up gradients of six percent and 165 feet for down gradient approaches based on a 25 <br />MPH design speed. The AASHTO stopping sight distance for up gradients of 9 percent with a 25 <br />MPH design speed is 140 feet and on down gradients is 173 feet. Trimming vegetation inside the <br />curve approaching the northernmost driveway will enhance the available stopping sight distance <br />to meet the standard for a 9 percent downgradient on the approach. <br />One new access connection is proposed on Capital Drive within the apex area of the curve <br />southeast of the intersection of Cresta De Ruta Street that will serve two residences at the <br />proposed new parcels 19 and 20. This driveway location was selected because it is the safest <br />location for access within this curve (safer than existing adjacent driveways). At the new <br />proposed driveway location, the approach gradient on Capital Drive is approximately 16 percent <br />up gradient to the right (north) and 16 percent downgradient to the southwest on the horizontal <br />curve. The available intersection sight distances and location are shown on Figures 4 and 4A. The <br />location within the curve should consider the 15-mile per hour curve advisory speed at this <br />location when calculating safe intersection sight distance. The AASHTO recommended stopping <br />sight distance for a downgradient of 16 percent is 203 feet and for an up gradient is 133 feet. <br />In considering intersection sight distance, volume of approaching traffic should also be <br />considered. <br />2.1.4 Existing Traffic Speeds and Volumes <br />A tube counter was placed on Capital Drive east uphill and toward the site from the existing <br />intersection at Alta Vista Court to determine the approximate 24-hour traffic volume currently <br />serving that section of the existing roadway and to collect vehicle speed data. The tube counter <br />collected traffic data for the 24-hour period from 5:00 PM on November 2, 2016 to 5:00 PM on <br />November 3, 2016. The count data indicated that there were approximately 208 to 209 ADT <br />vehicle trips traveling on the roadway during that time. <br />A tube counter was placed on Capital Drive north of Cresta De Ruta Street to collect traffic data <br />between 4:00 PM on November 911 and 8:30 AM on November 11". The collected count data <br />indicated that this section of Capital Drive serves between 54 and 64 ADT two-way vehicle trips <br />per day, or an average of 59 vehicle trips/day. <br />The collected traffic data simultaneously counted speed data while compiling traffic volume <br />data. The raw collected traffic data is included as Appendix C. The results of the speed data are <br />included in the following table. <br />© Branch Engineering, Inc. <br />Page 16 <br />