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40 for different radii of horizontal curve and major-road design Provide appropriate sight distance and visibility for driver speeds. They noted that their model considers the vertical recognition of the intersection and conflicting users." obstruction caused by the road surface on crest vertical curves, and recommended further research to explore the case of a sag vertical curve, where the sightline may be Design Speed obstructed by an overpass. The authors of the first edition of FHWA's Roundabouts: An Informational Guide (Robinson et al. 2000) stated that Modern Roundabouts design speed of a roundabout is determined from the small- est radius along the fastest allowable path. In their observa- The increase in the use of modern roundabouts in the United tions, the smallest radius usually occurred on the path of the States continued at a high pace during the decade from 2000 circulatory roadway as the vehicle curved to the left around to 2010. The need to know more about design, operations, the central island. However, they stated it was "important safety, and other aspects of roundabouts in this country when designing the roundabout geometry that the radius of prompted a number of research projects. Findings from those the entry path (i.e., as the vehicle curves to the right through projects will be summarized in this section of the report. This entry geometry) not be significantly larger than the circu- section contains subtopics that overlap with headings found latory path radius." Recommended maximum entry design elsewhere in this chapter (e.g., design speed and alignment), speeds from the Guide are shown in Table 17. but the roundabout-specific nature of the information made it appropriate to include here, rather than be distributed through- The design process described in the Guide states that out other parts of the report. a vehicle is assumed to be 2 m (6 ft) wide and to maintain a NCHRP Synthesis 299 had little content on roundabouts, minimum clearance of 0.5 m (2 ft) from a roadway centerline or because, to that point, relatively little research had been concrete curb and flush with a painted edge line. Thus the cen- conducted on them in the United States. A series of projects terline of the vehicle path is drawn with the following distances to the particular geometric features: during the decade led to the publication of two FHWA Informational Guides containing recommendations and 1.5 m (5 ft) from a concrete curb, guidelines for all aspects of roundabout design. As such, a 1.5 m (5 ft) from a roadway centerline, and great deal of content was generated on the subject, a sample 1.0 m (3 ft) from a painted edge line. of which is presented in this section. Many projects were regional or local in nature, however, and specific research Their desirable radius relationship was that the entry path reports on many of those projects were not published in a radius was less than the circulatory path radius, which was less forum that was readily available for this synthesis. How- than the exit path radius, ensuring that speeds will be reduced ever, the FHWA Informational Guides summarized much to their lowest level at the roundabout entry. The design speed of the existing information and compiled them into the review process also included the evaluation of the left-turn form of nationally distributed research reports as well as path radius and the right-turn path radius for speeds consistent guidelines suitable for practitioners. Given the importance with the other three radii. Selection of an appropriate design of those two Guides, a sizeable portion of the research vehicle, as defined in the Green Book, would help to define the highlighted here is either primarily or secondarily sourced necessary radii for a given design speed. to those two documents. The second edition of Roundabouts: An Informational Guide (Rodegerdts et al. 2010) recommends maximum enter- General Principles ing design speeds based on a theoretical fastest path of 20 The authors of NCHRP 672: Roundabouts: An Informational to 25 mph for single-lane roundabouts; 25 to 30 mph is rec- Guide, Second Edition (Rodegerdts et al. 2010) offered a ommended for multilane roundabouts, based on a theoretical set of overarching principles to guide the development of fastest path assuming vehicles ignore all lane lines. designs for all roundabouts. They stated that achieving these principles be the goal of any roundabout design: Table 17 "Provide slow entry speeds and consistent speeds Recommended Maximum Entry Design Speeds through the roundabout by using deflection. Recommended Maximum Entry Design Provide the appropriate number of lanes and lane assign- Site Category Speed, km/h (mph) Mini-roundabout 25 (15) ment to achieve adequate capacity, lane volume balance, Urban Compact 25 (15) and lane continuity. Urban Single Lane 35 (20) Provide smooth channelization that is intuitive to drivers Urban Double Lane 40 (25) and results in vehicles naturally using the intended lanes. Rural Single Lane 40 (25) Provide adequate accommodation for the design vehicles. Rural Double Lane 50 (30) Design to meet the needs of pedestrians and cyclists. Source: Robinson et al. 2000.

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41 FIGURE 10 Radial alignment of roundabout entries (Robinson et al. 2000). Alignment culatory roadway, and the design of the exits also provides appropriate alignment to allow drivers to intuitively main- With regard to the alignment of roundabout approaches, the tain the appropriate lane. The report's authors add that these first FHWA Guide (Robinson et al. 2000) states that, alignment considerations often compete with the fastest path speed objectives. in general, the roundabout is optimally located when the center- lines of all approach legs pass through the center of the inscribed circle. This location usually allows the geometry to be adequately Inscribed Circle Diameter designed so that vehicles will maintain slow speeds through both the entries and the exits. The radial alignment also makes the cen- tral island more conspicuous to approaching drivers. If it is not In the first FHWA Roundabouts Guide (Robinson et al. possible to align the legs through the center point, a slight offset 2000), the authors state that the inscribed circle diameter to the left (i.e., the centerline passes to the left of the roundabout's (ICD) in a single-lane roundabout should be a minimum of center point) is acceptable. It is almost never acceptable for an approach alignment to be offset to the right of the roundabout's 30 m (100 ft) to accommodate a WB-15 (WB-50) design center point. This alignment brings the approach in at a more vehicle. "Smaller roundabouts can be used for some local tangential angle and reduces the opportunity to provide sufficient street or collector street intersections, where the design vehi- entry curvature. cle may be a bus or single-unit truck. At double-lane round- abouts, accommodating the design vehicle is usually not a Examples of all three alignments are shown in Figure 10. constraint. The size of the roundabout is usually determined either by the need to achieve deflection or by the need to fit the entries and exits around the circumference with reason- Lane Arrangement able entry and exit radii between them." Thus, the authors recommended that the ICD of a double-lane roundabout gen- NCHRP Report 672 (Rodegerdts et al. 2010) provides a meth- erally be a minimum of 45 m (150 ft). The second edition of odology for conducting an operational analysis of a round- the FHWA Guide modified the ICD recommendations from about, one outcome of which is to determine the required the first edition; the second edition's typical ICD ranges are number of entry lanes to serve each approach. The report's shown in Table 18. authors advise that, for multilane roundabouts, care must be taken to ensure that the design also provides the appropri- ate number of lanes within the circulatory roadway and on Entry Width each exit to ensure lane continuity. The primary caution with multilane roundabouts is path overlap, which occurs when According to the first FHWA Roundabouts Guide (Robinson the natural path through the roundabout of one traffic stream et al. 2000), overlaps the path of another. If the natural path of one lane interferes or overlaps with the natural path of the adjacent determining the entry width and circulatory roadway width involves a trade-off between capacity and safety. The design lane, the roundabout is not as likely to operate as safely or should provide the minimum width necessary for capacity and efficiently as possible. The report advises that a good entry accommodation of the design vehicle in order to maintain the design aligns vehicles into the appropriate lane within the cir- highest level of safety. Typical entry widths for single-lane

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42 Table 18 Typical Inscribed Circle Diameter Ranges Typical Design Common Inscribed Circle Roundabout Configuration Vehicle Diameter Range* Mini-Roundabout SU-30 4590 ft Single-Lane Roundabout B-40 90150 ft WB-50 105150 ft WB-67 130180 ft Multilane Roundabout (2 lanes) WB-50 150220 ft WB-67 165220 ft Multilane Roundabout (3 lanes) WB-50 200250 ft WB-67 220300 ft Source: Rodegerdts et al. (2010). *Assumes 90-degree angles between entries and no more than four legs. List of possible design vehicles is not all-inclusive. entrances range from 4.3 to 4.9 m (14 to 16 ft); however, values Superelevation higher or lower than this range may be required for site-specific design vehicle and speed requirements for critical vehicle paths. Guidelines in the first FHWA Roundabouts Guide state that, in general, "a cross-slope of 2% away from the central island Where wider entries are required, this can be done in two ways: should be used for the circulatory roadway. This technique of by adding a full lane upstream of the entrance and maintain- sloping outward [was] recommended for four main reasons: ing parallel lanes through the entry, or by gradually widening the approach through flaring. When used, the Guide states that It promotes safety by raising the elevation of the central flare lengths should generally be a minimum of 25 m (80 ft) in island and improving its visibility. urban areas and 40 m (130 ft) in rural areas. It promotes lower circulating speeds. It minimizes breaks in the cross slopes of the entrance The second edition of the Guide (Rodegerdts et al. 2010) and exit lanes. revised the previous guidelines to state that typical entry widths It helps drain surface water to the outside of the for single-lane entrances range from 14 to 18 ft, which are roundabout." often flared from upstream approach widths. However, values higher or lower than this range may be appropriate for site- The outward cross-slope design means vehicles making specific design vehicle and speed requirements for critical through and left-turn movements must negotiate the round- vehicle paths. A 15-ft entry width is a common starting value about at negative superelevation; however, the slow speeds for a single-lane roundabout. NCHRP Report 672 also states through the circulatory roadway were generally expected to that care be taken with entry widths greater than 18 ft or for negate the effects of the slope on drivers. those that exceed the width of the circulatory roadway, as drivers may mistakenly interpret the wide entry to be two lanes when there is only one receiving circulatory lane. Safety Researchers on NCHRP 3-65 (Rodegerdts et al. 2007) Intersection Sight Distance found that crash experience at selected intersections in the United States that had been converted to roundabouts Concerning ISD at roundabout approaches, the first edition showed an overall reduction in crash frequency; there were of the FHWA Guide (Robinson et al. 2000) recommended selected intersections at which this was not the case (e.g., the use of a critical headway of 6.5 s to determine the appro- either no change or a small increase in crash frequency), priate length of the conflicting leg of the sight triangle. It but in most cases, the crash counts at those locations were further recommended that designers "provide no more than too small for increases to be statistically significant. In com- the minimum required intersection sight distance on each paring crash frequency to geometry, researchers listed these approach. Excessive intersection sight distance can lead to findings: higher vehicle speeds that reduce the safety of the intersec- tion for all road users (e.g., vehicles, bicycles, pedestrians)." Eight of the ten sites with the lowest crash frequencies The authors also stated that landscaping can be effective in were single-lane roundabouts. restricting sight distance to the minimum requirements. Twenty-six of the 30 sites with the lowest crash fre- quencies were single-lane roundabouts. NCHRP Report 672 (Rodegerdts et al. 2010) advised the Two of the ten sites with the highest crash frequencies use of a critical headway of 5.0 s, based on the critical headway were single-lane roundabouts. required for passenger cars. The authors added that this value Nine of the 30 sites with the highest crash frequencies represented an interim methodology pending further research. were single-lane roundabouts.

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43 Crash frequency increased as the inscribed circle diam- and/or percentage of exiting vehicles could become more eter increased, and as the number of vehicles entering noticeable and recommended further study of the subject in the roundabout increased. the future. Crash frequency increased slightly as the number of legs to the roundabout increased. Researchers suggested that the critical headway estimate of 6.5 s in the first edition of the FHWA Roundabout Guide A review of multilane roundabout characteristics led the appeared to be somewhat conservative for design purposes research team to believe that most sites were not designed for both single-lane and multilane entries. They recom- using the natural vehicle path concept, which was likely mended a lower value of 6.2 s for design purposes, which because the design was completed before the introduction of was approximately one standard deviation above the mean the concept in the first edition of the FHWA Roundabout observed critical headway (Rodegerdts et al. 2007). Guide. Lane widths also appeared to have an effect on crash frequency, particularly lanes that were narrower than those Isebrands (2009) conducted a review of crashes at 17 inter- recommended by FHWA. sections on rural high-speed roadways that were converted to roundabouts between 1993 and 2006; "high-speed" was Analysis of roundabouts in the United States led research- defined as having a posted speed limit of 40 mph or greater. ers to conclude that the general principle was true that as The number of years of before data averaged 4.6, with a the width of an entry increases, the capacity of the entry minimum of 2.5 years and a maximum of 6.6 years. The after increases, while the safety of the entry decreases. However, data showed greater variation, with an average of 5.5 years, extending the principle beyond the number of lanes to the a minimum of 1.8 years, and a maximum of 12.7 years of actual entry width did not appear to have as strong a relation- data. The results of her analysis showed 52% and 67% ship in the United States as in other countries. Researchers reductions in total crashes and crash rate, respectively. suggested that, although the overall relationship between Moreover, the findings showed an 84% reduction in injury capacity and entry width appeared to hold true in terms of crashes and an 89% reduction in the injury crash rate. No the aggregate number of lanes on the approach, changes in fatal crashes occurred since the roundabouts were con- entry width within a single-lane entry has a much lower-order structed, compared with 11 fatal crashes that were reported effect on capacity. in the before period. The number of angle crashes was also reduced by 86%. The NCHRP 3-65 research team also reported that the angle between legs of a roundabout appeared to have a direct influence on entering-circulating crashes. As the angle to the Pedestrian Considerations next leg decreased, the number of entering-circulating crashes increased, suggesting that roundabouts with more than four The first FHWA Roundabout Guide (Robinson et al. 2000) legs or with skewed approaches tended to have more entering- discussed considerations for nonmotorized users. The authors circulating crashes. Analysis of U.S. data did not find a provided design dimensions from Pein (1996) for key round- significant relationship between the capacity of the entry about design features, which are largely repeated in the second and the width of the splitter island, nor with the percent- Roundabout Guide and are included in Table 19. The Guide age of exiting vehicles. The research team believed that as added that "pedestrian crossing locations must balance pedes- drivers became more comfortable and efficient in driving trian convenience, pedestrian safety, and roundabout opera- roundabouts the effect of the width of the splitter island tions." With those issues in mind, the Guide recommended Table 19 Key Dimensions of Nonmotorized Roundabout Design Users User Dimension (ft) Affected Roundabout Features Bicycle Length 5.9 Splitter island width at crosswalk Minimum operating width 4.0 Bicycle lane width Lateral clearance on each side 2.0 Shared bicyclepedestrian path width 3.3 to obstructions Pedestrian (walking) Width 1.6 Sidewalk width, crosswalk width Wheelchair Minimum width 2.5 Sidewalk width, crosswalk width Operating width 3.0 Sidewalk width, crosswalk width Person pushing stroller Length 5.6 Splitter island width at crosswalk Skater Typical operating width 6.0 Sidewalk width Source: Pein (1996).

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44 that pedestrian crossings be designed with the following involved no interaction with a motor vehicle, where inter- characteristics: action is defined as the pedestrian either accepting or reject- ing a gap when a vehicle was present. For those pedestrians "The pedestrian refuge should be a minimum width of who did interact with vehicles and ultimately crossed the leg, 1.8 m (6 ft) to adequately provide shelter for persons researchers categorized their behaviors as Normal, Hesitates, pushing a stroller or walking a bicycle. or Runs. Three categories of motorist yielding behavior were At single-lane roundabouts, the pedestrian crossing identified as well: should be located one vehicle-length (7.5 m [25 ft]) away from the yield line. At double-lane roundabouts, Active yield: The motorist slowed or stopped for a cross- the pedestrian crossing should be located one, two, or ing pedestrian or a pedestrian waiting on the curb or split- three car lengths (approximately 7.5 m, 15 m, or 22.5 m ter island to cross. The pedestrian was the only reason the [25 ft, 50 ft, or 75 ft]) away from the yield line. motorist stopped or slowed. The pedestrian refuge should be designed at street level, Passive yield: The motorist yielded to the pedestrian but rather than elevated to the height of the splitter island. was already stopped for another reason. This situation This eliminates the need for ramps within the refuge occurred most often when there was a queue of vehicles area, which can be cumbersome for wheelchairs. waiting to enter the roundabout or when the vehicle was Ramps should be provided on each end of the crosswalk already stopped for a prior pedestrian crossing event. to connect the crosswalk to other crosswalks around the Did not yield: The motorist did not yield to a crossing roundabout and to the sidewalk network. pedestrian or a pedestrian waiting on the curb or splitter It is recommended that a detectable warning surface, as island to cross. recommended in the Americans with Disabilities Act Accessibility Guidelines (ADAAG), be applied to the Researchers determined that for pedestrians initiating a surface of the refuge within the splitter island." crossing on the entry side of one-lane sites, 15% of motorists did not yield to the pedestrian on either the entry or exit side. The second edition of the Guide recommended minimum The remainder of the exit-side vehicles actively yielded. The splitter island dimensions, as shown in Figure 11, and the remainder of the entry-side vehicles included 20% that were authors encouraged use of standard AASHTO island design classified as passively yielding. For two-lane sites, the per- for key dimensions, such as offset and nose radii. For side- centage of nonyielding vehicles increased to 33% on the walks, authors advised a setback distance of 1.5 m (5 ft), with entry side and 45% on the exit side. For those vehicles that a minimum of 0.6 m (2 ft). did yield, 9% and 2% were classified as passive yield for the entry and exit sides, respectively. When crossing began Research on NCHRP Project 3-65 included the review of on the exit side, yielding improved for entry-side drivers pedestrian crossing activity at 10 legs on seven roundabout but declined for exit-side drivers. In all categories, yield- study sites (Rodegerdts et al. 2007). The researchers deter- ing at two-lane sites was lower than at one-lane sites. On mined that the majority of the 769 observed crossing events average across all sites, approximately 30% of the motorists FIGURE 11 Minimum splitter island dimensions (Rodegerdts et al. 2010).