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HomeMy WebLinkAbout011.03 - Appendix J.4 - Access Spacing Deviation Letter P.O. Box 249 Bozeman, MT 59771 406.922.7300 www.406traffic.com M E M O R A N D U M To: City of Bozeman Engineering staff From: Nate Larson, PE, PTOE, RSP1 Date: 1/31/2025 Subject: Stockyard; Access spacing comment/deviation This memo addresses Engineering’s comment on the spacing of the project’s proposed property access to Story Mill Road, a Collector with a posted speed limit of 25 mph, south of the East Griffin Drive intersection. The comment reads as follows: BMC 38.400.090(D)(2) Access - The minimum separation between the right-of-way boundary and the inside edge of a full access on a collector street is 330 feet. The applicant has proposed a separation of 180 feet. The applicant must revise the design to provide the minimum spacing, revise the access to be a partial access, or request a modification per Paragraph H of this section. The information in this memo is intended to support a request to modify/relax the minimum separation requirement in this individual case based on the projected traffic operating conditions at the East Griffin Drive intersection located 180 feet to the north, as indicated in the comment, in accordance with the conditions indicated within BMC 38.400.090(H)(3), which are numbered a through f. Conditions a through c: Traffic Operations These three items, which are “traffic volumes”, “turning movements”, and “traffic controls”, are provided in the TIS report (completed in September 2024), but some additional explanation is helpful. The primary concern for this access’ proximity to the Griffin intersection is that vehicles turning left out of the site could be blocked by northbound traffic queued at the intersection. Because no stop sign controls, or is proposed to control, northbound traffic at this location, control delay for northbound traffic is low and queues are predicted to be negligible. The traffic analysis model employed for TIS analysis indicates that in the higher-volume peak hour (PM), with all site-generated traffic and future growth considered the northbound queue extent is projected to be 0.0 vehicles in the 95th-percentile case. Peak volume will be too low at this location to register the potential for an average queue of even one vehicle, while it would take an 8th vehicle in queue (at standard average 25-foot spacing) to extend beyond 180 feet and effectively block exiting left-turn movements. Both the Griffin/Story Mill intersection and the project site access in question are “T” intersections rather than 4-legged ones, PTOE #1185, Certified 2003 406 Traffic and Transportation Consulting, PLLC Stockyard: Story Mill Road Access Spacing, 1/31/2025 2 and are expected to retain this configuration for the long-term future. This means that future potential development in the area is highly unlikely to result in prominent additional operational interference with the proposed Story Mill Road access. Condition d: Site Design While the internal arrangement of the buildings and parking areas proposed on these two tracts south of Griffin has not been finalized, the proposed location of the Story Mill Road access was selected with care to serve both tracts and is intended to optimize efficient site circulation and preserve flexibility in the final site design, which must account for multiple non-traffic-related spatial constraints. Condition e: Sight Distances The MDT Traffic Engineering Manual addresses intersection sight distance in Chapter 28, Section 9, Part 2. ISD guidance therein requires a design speed assumption. For this situation, 25 mph (the current posted speed limit) was assumed because it is potentially consistent with Story Mill Road’s alignment and character at this access location. This is due in part to its alignment being set a century or so ago, long before modern design speed principles were established. Because the access is not a public street and only serves parking areas for non-truck-oriented land uses, the MDT sight distance requirements for a Single-Unit Truck were used. These are in the Manual’s Figure 28.9E for left turns and 28.9H for right turns. No adjustments were appropriate for approach grade (only applicable where such grade exceeds 3%), as the site is relatively flat and the final finished approach grade for this access is not expected to be substantially lower than Story Mill Road. The minimum sight distances indicated by MDT are 350 feet for left turns and 315 feet for right turns. Because the proposed access is on the outside part of a slight curve in Story Mill Road’s alignment, the available sight distance will exceed these requirements, and will include full visibility of vehicles entering Story Mill Road from the Griffin intersection. The rough estimates of available sight distance, to the nearest 25 feet, are 525 feet to the north past a building on the historic Story Mill property and 475 feet to the south past a stand of dense vegetation. Note that the sight distance looking south only applies to left turn movements out of the proposed access. The most important finding regarding sight distance is that relocating the Story Mill Road access point to the south to meet the relevant requirement of 330 feet would preclude meeting the MDT minimum sight distance requirement by a considerable margin. Condition f: Location and Alignment of Other Access Points The project’s TIS included the assumption that the full movement access proposed for Story Mill Road to this part of the property (Tracts 5 and 6) would work in concert with its two other full movement access points, which are on the north side, accessing Griffin. As with the Story Mill Road access, those on Griffin have been located to optimize circulation and site design. A benefit to the proposed full-movement access is to minimize the need for unnecessary trips through the Griffin intersection. Such unnecessary intersection traffic would result if the proposed Story Mill Road access does not permit all four possible in/out turning movements. This potential effect has not been quantified. The examination of the conditions in BMC 38.400.090(H)(3) documented in this memo indicates strong support for approval of the applicant’s request for an access spacing deviation in this specific case. If you have any questions about this memorandum, please feel free to reach out to me directly at 406.922.7300 or nate@406traffic.com, or to inquire through the appropriate applicant team contact.