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E-2008-0321 TRAFFIC ENGINEERING SAFETY AND SPEED STUDY Rattlesnake Valley Greenough Drive & Duncan Drive & Rattlesnake Drive City of Missoula, Montana Public Works Department August 2008 Background: In April 2008, citizens from the Rattlesnake Valley living along Greenough Drive, Duncan Drive, and Rattlesnake Drive made a request to the Missoula City Council and the City Public Works Department (Department) to reduce the posted speed limits on these collector roadways. The Department agreed to perform a traffic engineering safety and speed study to determine if such a change was feasible and justifiable. In accordance with Missoula Municipal Code, Chapter 10.18 SPEED LIMITS, the City Engineer may alter posted speed limits as a result of an engineering and traffic investigation, with the concurrence of the City Council. The following report summarized the results of the study for portions of Greenough Drive, Duncan Drive, and Rattlesnake Drive. Study Area Description: Figure 1 shows the study area. The study segment on Greenough Drive and Duncan Drive are within the Rattlesnake Valley starting from where Greenough Drive crosses under Interstate 90 to approximately 1.9 miles north where Teddy Turn intersects Duncan Drive. This segment is functionally classified as a collector road. Existing Greenough/Duncan along the study segment is a two lane suburban collector road with 12-foot wide travel lanes and 2 to 4-foot wide paved shoulders. The posted speed varies from 25 to 35 miles per hour (mph) from I-90 to Lolo Street, and from 25 to 35 mph north of Lolo Street to Teddy Turn. The study segment on Rattlesnake Drive is from the intersection with Missoula Avenue in the central Rattlesnake Valley approximately 1.7 miles north to the intersection with Creek Crossing at the entry to the Lincolnwood Neighborhood. This segment is functionally classified as a collector road. Existing Rattlesnake Drive along the study segment is a two lane collector road with 11-foot wide travel lanes and 4 to 6-foot wide pave shoulders. Several short sections of Rattlesnake Drive have curbs and sidewalks installed, but the majority of the road exists as a rural section without those structures. The posted speed is 35 mph. Traffic: Figure 2 shows the Annual Average Daily Traffic (AADT) taken from the Transportation Planning section of the Office of Planning and Grant data from 2007. This is the most current standardized AADT information available. Traffic volumes generally increase from north to south. ---PAGE BREAK--- Figure 3 provides the average annual growth rate expressed in percentage per year for traffic on these roadways from 2003 to 2007. The smaller traffic volumes in the northern portion of the Rattlesnake Valley show more variability than the larger traffic volumes in the south. Overall traffic volumes are documented to be relatively flat or falling on Greenough Drive and Duncan Drive. Traffic volumes on Rattlesnake Drive are documented to be increasing over the study period. Crash Summary: Five and one-half years of crash data were retrieved for this study. Figure 4 is a spot map showing approximate locations of the crashes. The exact locations and descriptions require the review of the actual crash reports. The Department used a crash rate to assess the relative significance of the number of crashes experienced on a given intersection compared to other similar locations within the City of Missoula. The crash rate is given as follows: Crash rate = of crashes/year) * 1,000,000) ( AADT * 365) Crash rate expressed as crashes per year per 1 million vehicles through the intersection. A comparison between the crash rate on these roads and the citywide crash rate for similar road types indicated that the crash rate is low on these particular segments. That doesn’t mean there aren’t specific issues or problems that could be addressed. It merely means that there are other local roads with higher crash rate. Figure 5 summarizes the crash rates at several intersections. The intersection Greenough Drive at Vine Street experienced the highest number of crashes during the study period. A complete list of all reported crashes within the study area is attached. Typically, rates above 1.5 crashes per million entering vehicles indicate there may be an unusual problem. Intersections with crash rates above 2.0 are considered to have poor safety performance. The highest crash rate intersection within the study area at Greenough Drive at Vine Street experienced a crash rate of 1.01. Again, the rate doesn’t mean there aren’t problems that could be addressed; it is merely a means of making a quantitative assessment of the intersection’s safety performance as compared to other similar intersections. The overall results indicate a relatively low crash rate as compared to other similar intersections in Missoula. Speed Profile: The Department collected speed data within the study area. The Department utilized an automated recording system at four locations. Speed monitors were placed at the following locations: ● Rattlesnake Drive between Murray Street and Powell Street; ● Rattlesnake Drive adjacent to Rattlesnake School north of Pineview Drive; ● Rattlesnake Drive north of Sunflower Drive; ● Greenough Drive north of Waterworks Hill by Cherry Gulch. ---PAGE BREAK--- These sites were selected because they are located where the appropriateness of the posted speed was in question, and because these sites encountered the least interference from intersections. FIGURE 6 Figure 6 above shows the Posted Speed, Average Speed and 85th Percentile Speed of traffic at the four locations within the study area. The Posted Speed is typically recommended to be between the two values of Average Speed and 85th Percentile Speed. The existing posted speeds within the study area show close correlation between the average speeds and 85th percentile speeds. Another type of engineering speed study would only measure the speed of free flow vehicles. Free flow vehicles are those that are not under any influence other than the roadway. A free flow vehicle will not be following other vehicles, slowing to turn, delivering mail, etc. The data collected for this study measured the speed of all vehicles. If only free flow speeds had been collected, the average and 85th percentile speeds would be somewhat higher. Other Speed Considerations: There are other factors that are considered when establishing a speed limit in addition to the speed profile. Accident frequency and type are important factors when reviewing speed limits. The documented accident frequency in the study area is relatively low. Wildlife occurrence, school zones or parks areas, and road character are also relevant factors. These factors are known and accounted for by most local drivers that use these road sections regularly. Most drivers travel at the speed they perceive to be safe and reasonable for the observed conditions. A local driver will incorporate these known factors in their driving behavior. Although an origin/destination investigation was not conducted as part of this study, it is assumed that most drivers in the study area are local residents that have knowledge of the road conditions. The adjacent development along the roadway and road features play a large role in creating the environment that affects the driver’s behavior relating to speed. These environmental factors are taken into account by the vast majority of drivers using the roadways. Curb and gutter defines and confines the road section, a boulevard tree canopy narrows the perception of road width, and the occurrence of frequent driveways are among the factors that have a dramatic natural effect on 0 5 10 15 20 25 30 35 40 45 Posted Speed mph Average Speed mph 85th% Speed mph Rattlesnake Southbound between Murray/Powell Rattlesnake Northbound at Rattlesnake School Rattlesnake Northbound north of Sunflower Greenough Northbound near Cherry Gulch ---PAGE BREAK--- reducing traffic speed. Sidewalks also have the benefit of separating pedestrians from adjacent traffic hazards. In turn the activity on sidewalks affects driver behavior. These study segments contains only some of these characteristics. Adjacent land use is primarily low density residential with occasional undeveloped and open space parcels. The majority of these road sections do not have curbs, gutters, or sidewalks that have been demonstrated to reduce traffic speeds. Another factor taken under consideration is crash history. However, the mere presence of crashes does not in itself constitute a need for a lower posted speed. Reducing the speed limit for the sake of crash reduction has been demonstrated to have the opposite effect in that it actually may decrease the safety of the corridor. Certain crash types indicate the need for other roadway improvements. Improvements in the pavement condition, intersection visibility, shoulder widths, and the installation of curbs and sidewalks all have been demonstrated to improve road safety. Conclusion: As a result of this traffic engineering safety and speed study of Greenough Drive, Duncan Drive, and Rattlesnake Drive the Department does not recommend any reduction to the posted speeds. The posted speeds show close correlation to the observed average and 85th percentile operating speeds on these roads. Most collector roadways within the City of Missoula have a posted speed limit of at least 30 mph. The speed limit could be set to a uniform rate on Greenough Drive within the limits of this study to potentially reduce driver confusion of the posted speed limit and rear end accidents that stem from speed limit changes. A uniform speed limit may even out operating speeds. These roads remain important neighborhood through corridors providing mobility between the upper and lower sections of the Rattlesnake Valley. The Department is working with the City Council to take several steps to ensure these corridors are incrementally improved in their role as neighborhood transportation corridors. The Department is planning several transportation system improvements in the Rattlesnake Valley over the next several years. Some of the improvements are listed in the City's Capital Improvement Program and others are associated with neighborhood subdivision projects. The intersection of Greenough Drive at Vine Street may warrant additional consideration due to the reported accident history at that location. The completion of curbs and sidewalks on Greenough Drive, Duncan Drive, and Rattlesnake Drive is recommended to assist in reducing traffic operating speeds and improving corridor safety. Missoula Municipal Code: Chapter 10.18 SPEED LIMITS 10.18.020 Established. Where no special hazard exists that requires lower speed for compliance with motor vehicle traffic laws, the speed of any vehicle not in excess of the limits specified in this section or established as authorized by this chapter shall be lawful, but any speed in excess of the limits specified in this section or established as authorized by this chapter shall be prima facie evidence that the speed is not reasonable or prudent and that it is unlawful: A. Twenty-five miles per hour on all streets within the city limits unless posted otherwise; ---PAGE BREAK--- B. The prima facie speed limits set forth in subsection A of this section may be altered as follows: Whenever the city engineer determines upon the basis of an engineering and traffic investigation that any prima facie speed set forth in this section is greater or less than is reasonable or safe under the conditions found to exist at any intersection or other place or upon any part of a street or highway, the city engineer may, with the concurrence of the city council, determine and declare a reasonable and safe prima facie speed limit thereat which shall be effective when appropriate signs giving notice thereof are erected at such intersections or other place or part of the street or highway. (Ord. 2867 1993; prior code §20-64). 10.18.030 Alleys. The maximum speed limit for all alleys within the city shall be five miles per hour. (Ord. 2084 1979). 10.18.070 Specification of rate of speed in violation complaint. In every charge of violation of any speed regulation in this chapter, the complaint, also the summons, or notice to appear, shall specify the speed at which the defendant is alleged to have driven, also the speed applicable within the district or at the location. (Prior code §20-68) . ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK---