Roundabouts: An Informational Guide – Second Edition (2010) / Chapter Skim
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Pages 109-143
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From page 109... ...
Chapter 5/Safety Page 5-1 Roundabouts: An Informational Guide CHAPTER 5 SAFETY CONTENTS 5.1 INTRODUCTION .
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Roundabouts: An Informational Guide Page 5-2 Chapter 5/Safety LIST OF EXHIBITS Exhibit 5-1 Vehicle Conflict Points for T-Intersections with Single-Lane Approaches .
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Chapter 5/Safety Page 5-3 Roundabouts: An Informational Guide Exhibit 5-26 Approach Models .
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Roundabouts: An Informational Guide Page 5-4 Chapter 5/Safety 5.1 INTRODUCTION The use of roundabouts is a proven safety strategy for improving intersection safety by eliminating or altering conflict types, reducing crash severity, and causing drivers to reduce speeds as they proceed into and through intersections. Decreased vehicle speeds will also decrease the speed differentials with other road users.
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Chapter 5/Safety Page 5-5 Roundabouts: An Informational Guide helps to reduce the likelihood of collisions. As with other crossings requiring acceptance of gaps, roundabouts present visually impaired pedestrians with unique challenges, as described in Chapter 6.
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Roundabouts: An Informational Guide Page 5-6 Chapter 5/Safety cannot eliminate them entirely due to violations of those devices. Many of the most serious crashes are caused by such violations.
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Chapter 5/Safety Page 5-7 Roundabouts: An Informational Guide are queued waiting for gaps. These conflicts are typically the least severe of all conflicts because the collisions involve the most protected parts of the vehicle and the relative speed difference between vehicles is usually less than other conflicts.
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Roundabouts: An Informational Guide Page 5-8 Chapter 5/Safety 5.2.2 VEHICULAR CONFLICTS AT MULTILANE ROUNDABOUTS Multilane roundabouts have some of the same safety performance characteristics as their simpler single-lane counterparts. However, due to the presence of additional entry lanes and the accompanying need to provide wider circulatory and exit roadways, multilane roundabouts introduce additional conflicts not present in single-lane roundabouts.
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Chapter 5/Safety Page 5-9 Roundabouts: An Informational Guide As with single-lane roundabouts, the most severe vehicular crossing conflicts are eliminated and replaced by less severe merging conflicts. The additional conflicts unique to multilane roundabouts are generally low-speed side-swipe conflicts that typically have low severity.
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Roundabouts: An Informational Guide Page 5-10 Chapter 5/Safety roundabouts, the overall severity (and often number) of conflicts is typically less than other intersection alternatives.
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Chapter 5/Safety Page 5-11 Roundabouts: An Informational Guide street generally face less severe conflicts, but conflicts still occur. These include those caused by vehicle drivers making left turns from the major street.
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Roundabouts: An Informational Guide Page 5-12 Chapter 5/Safety – Right turns on green (legal)
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Chapter 5/Safety Page 5-13 Roundabouts: An Informational Guide 5.2.4 BICYCLE CONFLICTS Bicyclists face similar conflicts as motor vehicles at both signalized intersections and roundabouts. However, because bicyclists typically ride on the right side of the road between intersections, they face additional conflicts when they need to merge into the flow of motor vehicle traffic or where motor vehicles cross their path.
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From page 122... ...
Roundabouts: An Informational Guide Page 5-14 Chapter 5/Safety • At multilane roundabouts, it is recommended that cyclists travel through the roundabout in the same manner as other vehicles. Therefore, cyclists making left turns may encounter multiple merging conflicts as they change into a lane designated for left-turn movements.
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Chapter 5/Safety Page 5-15 Roundabouts: An Informational Guide 5.3.1 COMPARISONS TO PREVIOUS INTERSECTION TREATMENT The most up-to-date knowledge on the safety effects of roundabout conversions in the United States is summarized in NCHRP Report 572 (2)
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Roundabouts: An Informational Guide Page 5-16 Chapter 5/Safety roundabouts. The benefits are greater for injury crashes than for all crash types combined.
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Chapter 5/Safety Page 5-17 Roundabouts: An Informational Guide Exhibit 5-12 Comparison of Crash Types at Roundabouts Crash type France Queensland, Australia United Kingdom1 United States SingleLane Double Lane 1. Failure to yield at entry (entering-circulating)
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Roundabouts: An Informational Guide Page 5-18 Chapter 5/Safety Exhibit 5-13 Graphical Depiction of Crash Types at Roundabouts (Numbers correspond to those in Exhibit 5-12.) Source: (12)
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Chapter 5/Safety Page 5-19 Roundabouts: An Informational Guide 5.3.3 PEDESTRIANS As described previously, vehicular injury crashes normally decrease when roundabouts are installed at an existing intersection. The safety benefits of roundabouts have been found to carry over to pedestrians as well, as shown in the British statistics of Exhibit 5-14.
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Roundabouts: An Informational Guide Page 5-20 Chapter 5/Safety from vehicle streams that are available at a signal-controlled intersection. For example, at roundabout exits it may be difficult to discern the sound of vehicles that will continue to circulate from those exiting the roundabout.
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Chapter 5/Safety Page 5-21 Roundabouts: An Informational Guide All European countries report that a more careful design is necessary to enhance bicyclist safety. The type of bicycle crashes depends on the bicycle facilities provided at the roundabout.
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Roundabouts: An Informational Guide Page 5-22 Chapter 5/Safety urban compact design. In addition, multilane roundabouts should not be used where they are not needed for capacity purposes in the short term, as single-lane roundabouts are much safer for bicyclists.
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Chapter 5/Safety Page 5-23 Roundabouts: An Informational Guide Exhibit 5-19 Intersection-Level Safety Performance Models and Validity Ranges -- Total Crashes to data acquired in Step 1) from a sample of roundabouts representative of ones in that jurisdiction.
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Roundabouts: An Informational Guide Page 5-24 Chapter 5/Safety where m = expected annual crash frequency; x = total crashes observed; P = predicted annual number of crashes; n = years of observed data; and k = dispersion parameter for a given model (given in Exhibit 5-19 or Exhibit 5-20)
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Exhibit 5-21 Calculation of Total Crashes Chapter 5/Safety Page 5-25 Roundabouts: An Informational Guide These measures can be calculated for total crashes and for injury crashes and, from the differences between the two estimates, for non-injury crashes. A severityweighted ranking measure can be derived by applying weights to injury and non-injury crashes that reflect their relative severity.
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Roundabouts: An Informational Guide Page 5-26 Chapter 5/Safety A safety performance model representative of the existing intersection is required. This, again, will require that one exist for the jurisdiction or that data are available to enable a recalibration of a model calibrated for another jurisdiction using the procedure outlined earlier.
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Chapter 5/Safety Page 5-27 Roundabouts: An Informational Guide tion, an alternate approach can be used. In this, the results of the before–after study presented in Exhibit 5-9 can be applied as accident modification factors (AMFs)
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From page 136... ...
Roundabouts: An Informational Guide Page 5-28 Chapter 5/Safety Step 6: Compare estimated net safety benefit from Step 5 against the annualized roundabout conversion costs, considering other impacts if desired, and using conventional economic analysis tools. How and whether this is done is very jurisdiction-specific.
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Exhibit 5-23 Calculation of Expected Change in Crashes Converting an Intersection to a Roundabout Chapter 5/Safety Page 5-29 Roundabouts: An Informational Guide
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Roundabouts: An Informational Guide Page 5-30 Chapter 5/Safety Exhibit 5-23 (cont.) Calculation of Expected Change in Crashes Converting an Intersection to a Roundabout
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Chapter 5/Safety Page 5-31 Roundabouts: An Informational Guide evaluate the safety performance of an existing roundabout with respect to the three crash types.
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Roundabouts: An Informational Guide Page 5-32 Chapter 5/Safety from a safety perspective, diagnostic procedures can then be used to isolate any problems and to develop corrective measures. The application of the EB method at the approach level would be identical to the procedure presented and illustrated earlier for the intersection level in Section 5.4.
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Chapter 5/Safety Page 5-33 Roundabouts: An Informational Guide Exhibit 5-27 Base Conditions for Design Variables and AMFs Implied for Unit Change in Variables Variable Base Condition Value Entering– Circulating AMF Exiting– Circulating AMF Approach AMF Entry Radius 76 ft 1.010 Entry Width 20 ft 1.052 Approach Half Width 18 ft 1.031 Inscribed Circle Diameter 134 ft 1.022 Central Island Diameter 69 ft 0.992 1.014 Circulating Width 23 ft 1.117 Angle To Next Leg 93 deg 0.973 reality. The correlation matrix provided as Table 3-14 of the NCHRP Report 572 (2)
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Roundabouts: An Informational Guide Page 5-34 Chapter 5/Safety 5.6 REFERENCES 1. Maycock, G
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Chapter 5/Safety Page 5-35 Roundabouts: An Informational Guide 14. Crown, B
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