SUPPLEMENTAL MATERIALS (6-19-17)

Capital Hill PUD Traffic Safety Analysis (Revised) May 17, 2017 horizontal and vertical alignment features calm traffic speeds on Capital Drive. Most importantly, approaches to the site driveways have adequate line of sight of the roadway to meet the AASHTO recommended criteria for stopping sight distance on the major street approaches to each of the existing and proposed driveways, should a driver at the observation point depart the driveway during the time which an approaching vehicle surpasses the recommended intersection sight distance, but is not within view of the departing vehicle's line of sight. At both existing driveways where additional traffic will be added after construction, measurements were made on the Capital Drive approaches that indicate approximately a six percent grade at the existing driveway near Cresta De Ruta Street and approximately a nine percent grade at the driveway near the end of Capital Drive. AASHTO recommends a stopping sight distance of 143 feet on up gradients of six percent and 165 feet for down gradient approaches based on a 25 MPH design speed. The AASHTO stopping sight distance for up gradients of 9 percent with a 25 MPH design speed is 140 feet and on down gradients is 173 feet. Trimming vegetation inside the curve approaching the northernmost driveway will enhance the available stopping sight distance to meet the standard for a 9 percent downgradient on the approach. One new access connection is proposed on Capital Drive within the apex area of the curve southeast of the intersection of Cresta De Ruta Street that will serve two residences at the proposed new parcels 19 and 20. This driveway location was selected because it is the safest location for access within this curve (safer than existing adjacent driveways). At the new proposed driveway location, the approach gradient on Capital Drive is approximately 16 percent up gradient to the right (north) and 16 percent downgradient to the southwest on the horizontal curve. The available intersection sight distances and location are shown on Figures 4 and 4A. The location within the curve should consider the 15-mile per hour curve advisory speed at this location when calculating safe intersection sight distance. The AASHTO recommended stopping sight distance for a downgradient of 16 percent and a design speed of 25 mph is 203 feet and for an up gradient is 133 feet. In considering intersection sight distance, volume of approaching traffic should also be considered. 2.1.4 Existing Traffic Speeds and Volumes A tube counter was placed on Capital Drive north of Cresta De Ruta Street to collect traffic data between 4:00 PM on November 91h and 8:30 AM on November I Ilh, 2016. The collected count data indicated that this section of Capital Drive serves between 54 and 64 total vehicle trips per day (two-way ADT), or an average of 59 vehicle trips/day. A tube counter was placed on Capital Drive east and uphill toward the site from the existing intersection at Alta Vista Court to determine the approximate 24-hour traffic volume serving that section of the existing roadway and to collect vehicle speed data. The tube counter collected traffic data for the 24-hour period from 5:00 PM on November 2, 2016 to 5:00 PM on November 3, 2016. The count data indicated that there were approximately 210 ADT vehicle trips traveling on that section of the roadway during that time. A tube counter was also placed on Capital Drive south and uphill toward the site from the existing intersection at Spring Boulevard for the 24-hour period from 5:00 PM on December 2, 2014 to 5:00 PM on December 3, 2014 to determine the approximate 24-hour traffic volume serving that section of the existing roadway. The count data indicated that there were approximately 258 total vehicle trips counted during that time. It is unlikely the traffic volume at © Branch Engineering, Inc. Page 1 7