Click for next page ( 55


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 54
55 provided, consideration should be given to provid- operating speeds, higher access classifications for the ing a loon. public roadway, and higher driveway volumes. For suburban unsignalized intersections: A driveway should not be located within the functional Median widths at suburban unsignalized inter- area of an intersection or in the influence area of the sections generally should be as narrow as possible upstream and downstream driveways. while providing sufficient space in the median for Left-turn lane storage requirements should be consid- the appropriate left-turn treatment. ered when determining the driveway influence area and Median widths between 4.2 and 7.2 m (14 and 24 ft) can limit how closely driveways can be spaced. will accommodate left-turn lanes, but are not wide On roadways that are undivided or have TWLTLs, the enough to store a crossing or turning vehicle in the alignment of driveways on opposite sides of the road median. needs to be considered. Driveways on opposite sides of Medians wider than 7.6 m (25 ft) may be used, but cross- a lower-volume roadway may be aligned across from road vehicles making turning and crossing maneuvers each other. Alternatively, they should be spaced so that may stop on the median roadway. those drivers desiring to travel between the driveways Median widths of more than 15 m (50 ft) gener- on opposing sides of the roadway need to make a dis- ally should be avoided at suburban, unsignalized tinct right turn followed by a left turn (or a left followed intersections. by a right). A much longer separation is needed on a Median opening lengths at rural divided highway inter- higher-speed, higher-volume roadway. sections generally should be kept to the minimum pos- On roadways with restrictive medians, the spacing sible. Increases in median opening length were found between right-turn access points on opposite sides of the to be correlated with higher rates of undesirable driving road can be treated separately. behavior. In contrast, researchers found no reason that Ideally, driveway access for a major development involv- the median opening in urban and suburban areas should ing left-turn egress movements should be located where not be as long as necessary. effective coordination of traffic signals would be achiev- Median opening spacing for rural areas typically ranged able if there is a need to signalize the driveway. from 150 to 805 m (500 to 2,640 ft); a minimum median Driveway connections to public roadways are subject opening spacing of 150 m (500 ft) was recommended to the same intersection control device analyses as are in rural areas. Typically, median opening spacing sub- street intersections. If existing or future volumes war- stantially longer than 150 m (500 ft) was considered to rant installing a traffic signal, and signalized spacing be appropriate, unless two public road intersections or requirements cannot be met, left-turn access should be major driveways are located relatively close together. subject to closure in one or both directions. Median opening spacing for urban areas typically ranged from 90 to 805 m (300 to 2,640 ft); a minimum median For driveways near signalized intersections, Gattis stated opening spacing of 90 m (300 ft) was recommended in that the needed minimum separation distance (i.e., corner urban areas. Researchers stated that, whenever practical, clearance) will depend on the function, operation, and median opening spacing greater than 90 m (300 ft) design features of the roadway and the characteristics of the should be used in urban areas. access connection, considering the basic principle of locat- U-turn maneuvers should not be encouraged at loca- ing one connection outside of the functional area of another tions with limited sight distance. Furthermore, sight connection. For a driveway upstream of or approaching a distance is an important issue in determining locations signalized location on a major road, the functional area where U-turns by larger vehicles should be permitted or was defined to include the PRT, maneuver distance, and encouraged. ISD based on the criteria in the AASHTO storage length of the traffic on that approach. The recom- Green Book for Cases B1, B2, and F should be available mended spacing would provide separation between the to accommodate U-turns and left turns at unsignalized conflicting movements occurring at the signal and the median openings. conflicting movements occurring at the driveway. In addi- tion, this spacing would enable the driveway to operate Gattis et al. (2010) presented guidelines for driveway spac- without being obstructed by the traffic backing up from ing near intersections, both signalized and unsignalized. For the signal. unsignalized intersections, they stated that spacing should not interfere with safe and relatively unimpeded movement on the through roadway, and driveway spacing practices should Summary of Key Findings provide reasonable access to abutting private property. Other general guidelines included: This section summarizes key findings from the research noted in this chapter. This is an annotated summary; conclu- The needed distance between successive connections sions and recommendations are those of the authors of the (both driveways and side streets) increases with higher references cited.

OCR for page 54
56 Intersection Alignment Provide smooth channelization that is intuitive to drivers and results in vehicles naturally using the Avoid approach grades to an intersection of greater intended lanes. than 6%. On higher design speed facilities (50 mph and Provide adequate accommodation for the design greater) a maximum grade of 3% should be considered vehicles. (Rodegerdts et al. 2004). Design to meet the needs of pedestrians and cyclists. Avoid locating intersections along a horizontal curve of Provide appropriate sight distance and visibility the intersecting road (Rodegerdts et al. 2004). for driver recognition of the intersection and con- Strive for an intersection platform (including sidewalks) flicting users." with a cross slope not exceeding 2%, as needed for acces- Maximum entering design speeds are based on a the- sibility (Rodegerdts et al. 2004). oretical fastest path of 20 to 25 mph for single-lane Approach curvature can be used as a treatment to force roundabouts and 25 to 30 mph for multilane round- a reduction in vehicle speed through the introduction abouts (Rodegerdts et al. 2010). of horizontal deflection at high-speed intersection Roundabout alignment is described as "optimally located approaches, but it is discouraged at downhill approaches when the centerlines of all approach legs pass through the (Ray et al. 2008). center of the inscribed circle" (Robinson et al. 2000). A skew angle greater than 20 degrees should not be Common inscribed circle diameters for single-lane used in design when the design vehicle is a large vehicle roundabouts vary from 90 to 180 ft, depending on design or semitrailer (Son et al. 2002). vehicle (Rodegerdts et al. 2010). A minimum skew angle of 15 degrees should be used Designers should provide no more than the minimum to accommodate age-related performance deficits at required ISD on each approach, [because] excessive intersections where right-of-way is restricted (Staplin ISD can lead to higher vehicle speeds that reduce the et al. 2002). safety of the intersection for all road users (Robinson et al. 2000). Crash experience at selected intersections in the United Auxiliary Lanes States indicates an overall reduction in crash frequency at intersections converted to roundabouts (Rodegerdts Adding left-turn lanes is effective in improving safety et al. 2007). at signalized and unsignalized intersections, reducing Pedestrian refuge should be a minimum width of 6 ft crashes between 10% and 44% (Harwood et al. 2002). to adequately provide shelter for persons pushing a Positive results can also be expected for right-turn lanes, stroller or walking a bicycle (Robinson et al. 2000). with reductions in total intersection crashes between 4% "At single-lane roundabouts, the pedestrian crossing and 14% (Harwood et al. 2002). should be located one vehicle-length (25 ft) away from A method was developed to identify where installation the yield line. At double-lane roundabouts, the pedes- of right-turn lanes at unsignalized intersections and trian crossing should be located one, two, or three car major driveways would be cost-effective, indicating lengths (approximately 25 ft, 50 ft, or 75 ft) away from combinations of through-traffic volumes and right-turn the yield line" (Robinson et al. 2000). volumes for which provision of a right-turn lane would "The pedestrian refuge should be designed at street be recommended. The economic analysis procedure can level, rather than elevated to the height of the split- be applied by highway agencies using site-specific values ter island. This eliminates the need for ramps within for ADTs, turning volumes, crash frequency, and con- the refuge area, which can be cumbersome for wheel- struction cost for any specific location (or group of chairs" (Robinson et al. 2000). similar locations) of interest (Potts et al. 2007a). Ramps should be provided on each end of crosswalks to connect the crosswalk to other crosswalks around the roundabout and to the sidewalk network (Robinson Modern Roundabouts et al. 2000). A detectable warning surface, as recommended in the A series of projects during the decade (20002010) led to ADAAG, should be applied to the surface of the refuge the publication of two FHWA Informational Guides con- within the splitter island (Robinson et al. 2000). taining recommendations and guidelines for all aspects of Use of standard AASHTO island design for key dimen- roundabout design. sions, such as offset and nose radii, is encouraged. For General overarching principles of geometric design of sidewalks, a setback distance of 5 ft, with a minimum roundabouts (Rodegerdts et al. 2010) included: of 2 ft is advised (Robinson et al. 2000). "Provide slow entry speeds and consistent speeds For nonmotorized users such as bicyclists, one important through the roundabout by using deflection. consideration during the initial design stage is to main- Provide the appropriate number of lanes and lane tain or obtain adequate right-of-way outside the circula- assignment to achieve adequate capacity, lane volume tory roadway for the sidewalks. All nonmotorized users balance, and lane continuity. who are likely to use the sidewalk regularly, including

OCR for page 54
57 bicyclists in situations where roundabouts are designed Reducing the crossing distance for bicyclists, to provide bicycle access to sidewalks, should be con- Realigning intersection approaches to reduce or sidered in the design of the sidewalk width. Recom- eliminate intersection skew, mended designs for single-lane roundabouts encourage Modifying the geometry to facilitate bicycle move- bicycle users to merge into the general travel lanes and ment at interchange on-ramps and off-ramps, navigate the roundabout as a vehicle, explaining that Providing refuge islands and raised medians, and the typical vehicle operating speed within the circula- Grade-separated crossings. tory roadway is in the range of 15 to 25 mph, which is "Pedestrian facilities should be provided at all inter- similar to that of a bicycle (Rodegerdts et al. 2010). sections in urban and suburban areas. In general, design of pedestrian facilities with the most challenged users in mind--pedestrians with mobility or visual Innovative Intersection Designs impairments--should be done, and the resulting design will serve all pedestrians well. ADA requires that new A number of new or innovative intersection designs were and altered facilities constructed by, on behalf of, or considered during the decade; each of the following was for the use of State and local government entities be described in one or more studies. designed and constructed to be readily accessible to and usable by individuals with disabilities" (Rodegerdts Displaced Left Turns showed considerable savings in et al. 2004). average control delay and average queue length, as well Practitioners should incorporate key elements that as an increase in intersection capacity, in one series of affect a pedestrian facility into their design (Rodegerdts microsimulation analyses (Hughes et al. 2010). et al. 2004): Median U-turns are typically a corridor treatment applied "Keep corners free of obstructions to provide enough at signalized intersections, but are also used at isolated room for pedestrians waiting to cross. intersections to alleviate specific traffic operational and Maintain adequate lines of sight between drivers safety problems (Hughes et al. 2010). and pedestrians on the intersection corner and in the Median width of Restricted Crossing U-Turns is a cru- crosswalk. cial design element to accommodate large trucks with- Ensure curb ramps, transit stops (where applicable), out allowing vehicles to encroach on curbs or shoulders pushbuttons, etc., are easily accessible and meet (Hughes et al. 2010). ADAAG design standards. Quadrant Roadways should be designed so that the left Clearly indicate the actions pedestrians are expected turn with the highest demand is the one that receives the to take at crossing locations. most direct path (Hughes et al. 2010). Design corner radii to ensure vehicles do not drive Double Crossover Intersections are found to have greater over the pedestrian area yet are able to maintain throughput than a conventional intersection, along with appropriate turning speeds. lower values for number of stops, average stop time per Ensure crosswalks clearly indicate where crossings vehicle, average queue, and maximum queue length should occur and are in desirable locations. (Bared et al. 2005). Provide appropriate intervals for crossings and mini- Arterial Interchanges have an overall capacity near 75% mize wait time. of a four-lane freeway (Eyler 2005). Limit exposure to conflicting traffic, and provide ref- J-Turn and Offset-T designs had reductions in crashes uges where necessary. between 40% and 92% (Maze et al. 2010). Ensure the crosswalk is a direct continuation of the Two-Level Signalized Intersections produced mod- pedestrian's travel path. eled results with the shortest delay times in most Ensure the crossing is free of barriers, obstacles, and evaluation scenarios as well as the least sensitivity to hazards." variations in traffic volume compared with other inno- vative intersection types; however, delay increased when flow was unbalanced between the two crossing Transit Considerations roads (Shin et al. 2008). The additional right-of-way needed to construct each of General intersection design principles and guidelines these innovative designs was mentioned as a potential for transit issues (Eccles et al. 2007) include: drawback by every report and author that addressed the "Provide simple intersection designs. issue of the intersection's footprint. Provide clear visual cues to make busway inter- sections conspicuous. Pedestrian and Bicycle Facilities at Intersections Maximize driver and pedestrian expectancy. Separate conflicting movements. Suggested strategies (Raborn et al. 2008) for modifying Minimize street crossings. intersections to accommodate bicycles and pedestrians Incorporate design features that improve safety for included: vulnerable users.

OCR for page 54
58 Coordinate geometric design features and traffic vehicles on the crossroad cannot see both roadways of control devices." the divided highway. There are four types of busways found at intersections: Where the AASHTO passenger car is used as the median busways, side-aligned busways, separated right- design vehicle, a minimum median width of 25 ft is of-way busways, and bus-only ramps. Each busway recommended. type has unique characteristics that are considerations Where a large truck is used as the design vehicle, for guidance on safety issues, basic geometry (includ- a median width of 70 to 100 ft generally should be ing placement of bus stops), and traffic control, along selected. If such a median width cannot be provided, with examples of appropriate intersections for each consideration should be given to providing a loon. type of busway (Eccles et al. 2007). Recommended practices (Potts et al. 2004) for suburban unsignalized intersections include: Median widths at suburban unsignalized inter- Access Management at Intersections sections generally should be as narrow as possible while providing sufficient space in the median for Right-turn-plus-U-turn could have better operational the appropriate left-turn treatment. performance than direct left turns under certain traffic Median widths between 14 and 24 ft will accommo- conditions, implying that directional median opening date left-turn lanes, but are not wide enough to store designs could provide more efficient traffic flow than a crossing or turning vehicle in the median. full median openings (Zhou et al. 2002). Medians wider than 25 ft may be used, but crossroad U-turns at signalized intersections resulted in a 1.8% vehicles making turning and crossing maneuvers saturation flow rate loss in the left-turn lane for every may stop on the median roadway. 10% increase in U-turn percentage and an additional Median widths of more than 50 ft generally should 1.5% loss for every 10% U-turns if the U-turning move- be avoided at suburban, unsignalized intersections. ment was opposed by protected right-turn overlap from Median opening lengths at rural divided highway inter- the cross street (Carter et al. 2005a). sections generally should be kept to the minimum pos- Recommended practices (Potts et al. 2004) for rural sible. Increases in median opening length are correlated unsignalized intersections include: with higher rates of undesirable driving behavior. In They should have medians that are as wide as practical, contrast, the median opening in urban and suburban as long as the median is not so wide that approaching areas can be as long as necessary (Potts et al. 2004).