Roundabout Fundamentals and Intersection Control Evaluation Session 3: Multilane Roundabout Design FDOT ICE December 16, 2020 SESSION 3: Multilane Roundabout Design/FDOT ICE Agenda • Multilane Design • Pedestrian and Bicycle Accommodations • Introduction to FDOT ICE 2 SESSION 3: Multilane Roundabout Design/FDOT ICE Multilane Roundabouts • Principles and design process described previously apply to multilane roundabouts but in a more complex way • Multiple traffic streams may enter, circulate through, and exit the roundabout side-by-side • Designer needs to consider how these traffic streams interact with each other 3 SESSION 3: Multilane Roundabout Design/FDOT ICE Multilane Roundabouts • Geometry should provide adequate alignment and establish appropriate lane configurations for vehicles in adjacent entry lanes to be able to negotiate the roundabout geometry without competing for the same space 9 If not, operational and/or safety deficiencies may occur Google Earth 4 SESSION 3: Multilane Roundabout Design/FDOT ICE Multilane Roundabouts • Balancing capacity, safety, property impacts, and costs become more difficult with multilane roundabout designs • Geometry, pavement markings, and signing must be designed together to create a comprehensive system to guide and regulate road users who are traversing roundabouts Google Earth 5 SESSION 3: Multilane Roundabout Design/FDOT ICE Multilane Roundabouts • Additional key considerations for all multilane roundabouts: ¾Lane arrangements to allow drivers to select the appropriate lane on entry and navigate through the roundabout without changing lanes ¾Alignment of vehicles at the entrance line into the correct lane within the circulatory roadway ¾Accommodation of side-by-side vehicles through the roundabout ¾Alignment of legs to prevent exiting-circulating conflicts Google Earth ¾Accommodations for all travel modes 6 SESSION 3: Multilane Roundabout Design/FDOT ICE Lane Arrangement • Ensure that the design provides the appropriate number of lanes within the circulatory roadway and on each exit to ensure lane continuity • Movements assigned to each entering lane are key to the overall design • Pavement markings are integral to the preliminary design process 7 SESSION 3: Multilane Roundabout Design/FDOT ICE Inscribed Circle Diameter (ICD) • Single-lane roundabouts • Largely dependent upon the turning requirements of the design vehicle • Multilane roundabouts • Deflection/speed control • Adequate alignment of the natural vehicle paths • Accommodating design vehicle NCHRP 672 EXHIBIT 6-2 8 SESSION 3: Multilane Roundabout Design/FDOT ICE Inscribed Circle Diameter (ICD) For initial section of ICD, the design vehicle and context of location should be taken into consideration • Urban location • Typically lower speeds with right-of-way constraints • Rural location • Typically higher speeds with larger vehicles Roundabout Type Typical Inscribed Circle Diameter Single-lane 120 – 160 ft Multilane (2-lane entry) 160 – 200 ft 9 SESSION 3: Multilane Roundabout Design/FDOT ICE Entry Width • Required width is dependent upon the number of lanes and the design vehicle • Typical width for a two-lane entry ranges from 24 to 30 ft • Typical widths for individual lanes at entry range from 12 to 15 ft Google Earth • Typical widths with painted gores: ¾ Entry lanes = 11 to 12 ft ¾ Gore = 4 to 6 ft Use painted gores when providing in-lane truck accommodations! Google Earth 10 SESSION 3: Multilane Roundabout Design/FDOT ICE Entry Design • The entry curvature should balance the competing objectives of: 9 Speed control 9 Design vehicle accommodations 9 Adequate alignment of natural paths 9 Need for appropriate sight lines • Requires several design iterations to identify the appropriate roundabout size, location, and approach of alignments Google Earth • Multilane entry radii commonly 75 to 110 feet 11 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap • Designing multilane roundabouts is significantly more complex than single-lane roundabouts due to the additional conflicts present with multiple traffic streams in adjacent lanes: 9 Entering 9 Circulating 9 Exiting • The natural path of a vehicle is the path it will take based on the speed and orientation imposed by the roundabout geometry 12 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap NCHRP 672 • Path overlap occurs when the natural EXHIBIT 6-28 paths of vehicles in adjacent lanes overlap or cross one another • The entry design should align vehicles into the appropriate lane within the circulatory roadway • A good design balances entry speed and path alignment • Common on entries, but also can occur on exits 13 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap NCHRP 672 14 EXHIBIT 6-29 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap NYSDOT 15 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap NYSDOT 16 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap 17 Google Earth SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap Not Just on Entries 18 SESSION 3: Multilane Roundabout Design/FDOT ICE Path Overlap 19 Google Earth SESSION 3: Multilane Roundabout Design/FDOT ICE Checking Path Overlap WisDOT FDM Figure 30.17 Method for checking path overlap 20 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations • Considerations should be made for both tracking on the entry/exit and within the circulatory roadway • Percentage of trucks and lane utilization is an important consideration • Frequency of a specific design vehicle is also an important consideration • Determine whether the design will allow trucks to use two lanes or accommodate them to stay within their own lane 21 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Case 1 Design • Single lane line dividing the entry lanes • Trucks encroach on adjacent lanes on entry and when circulating and exiting WisDOT FDM 11-26 22 Figure 30.4 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Case 2 Design • Wider entry with gore marking • Trucks stay in lane on entry • Trucks encroach on adjacent lanes when circulating and exiting WisDOT FDM 11-26 Figure 30.5 23 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Case 3 Design • Wider entry with gore marking • Typically larger circle with wider circulatory roadway • Trucks stay in lane on entry, circulating, and exiting WisDOT FDM 11-26, Figure 30.6 24 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Case 2 and 3 typically have a painted gore area provided between lanes on the approach 25 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Outside lane is typically wider than the inside lane 12’-15’ 14’-18’+ Google Earth 26 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations (FDOT policy) • Provide adequate pavement area for the simultaneous passage of the design vehicle and a passenger vehicle through the roundabout and for turning movements • Design vehicle swept paths must stay within the travel lanes without encroaching on the inside and outside gutters • Provide a minimum of 18-inches of clearance between curb faces and the outside edge of the design vehicles tire track 27 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations (FDOT policy) • Truck trailer is permitted to mount the RA curb and use the truck apron. 28 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations (FDOT policy) • Develop swept path diagrams for all turning movements in the following combinations: ¾Design vehicle in the outside lane and passenger vehicle in the inside lane ¾Design vehicle in the inside lane and passenger vehicle in the outside lane • When truck volume is very low, consider allowing the design vehicle to command both lanes (Case 1 design) 29 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations (FDOT policy) • Acceptable for the design vehicle path to encroach into the adjacent travel lane within the circulatory roadway • Need to provide sufficient space for the passenger vehicle plus 2 feet of clearance between the two vehicles 30 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Width of gore marking is determined by the entry lane widths and swept paths of design vehicle Google Earth 31 SESSION 3: Multilane Roundabout Design/FDOT ICE Design Vehicle Accommodations Google Earth Only provide extra width where needed! 32 SESSION 3: Multilane Roundabout Design/FDOT ICE Circulatory Roadway Width • Usually governed by the type of vehicles that need to be accommodated adjacent to one another • Outside lane typically larger ¾ They don’t have to be 15’/15’ ¾ Provides additional space for larger vehicles ¾ Improves entry and exit path tangents 14’-18’ 12’-15’ Google Maps Google Maps 33 SESSION 3: Multilane Roundabout Design/FDOT ICE Circulatory Roadway Width • Does not need to remain constant! • Provide only the minimum width necessary to serve the required lane configuration ¾ Major movement may have 2 lanes circulating 2 ¾ Minor movement may only have 1 11 lane circulating 2 NCHRP 672 34 EXHIBIT 6-26 SESSION 3: Multilane Roundabout Design/FDOT ICE Spirals • Spiral transitions lead drivers into the appropriate lane for their desired exit • Enable vehicles to reach their intended exit without being trapped or needing to change lanes • More effective on larger circles where spiral curves are longer • Avoid the use of spiraled designs