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32 Other Considerations and Resources Countermeasures ISD is just one consideration for intersection enhancementâin fact, there are many other effective countermeasures available to practitioners. The countermeasures for ISD can be catego- rized with other measures, discussed in the following, that improve intersection safety and may be considered in a comprehensive approach to intersection safety. Clear sight triangles as much as possible without geometric improvements. Sometimes the sight distance restriction is caused or exacerbated by the presence of overgrown foliage or other natural occurrences. Common examples are high corn crops close to the intersection and tree limbs limiting the corner sight triangle. Also, allowing parking too close to the intersection will limit ISD. Geometric improvements to improve ISD. Often, changes at the major road may address ISD issues. For example, right-turning vehicles on the major road may block the line of sight for turning vehicles from the minor road. In this instance, the use of an offset right turn lane instead of a conventional right turn lane would address this ISD issue. Flattening or realigning a curve is another example. Increase awareness and visibility of the intersection. Giving motorists a good view of the intersection ahead can help put them on the alert and enable them to respond to conditions more quickly. Measures under this category include tree trimming and removing unnecessary signs on the approach to the intersection, as well as: ⢠Enhanced delineation leading up to the intersection, ⢠Larger regulatory and warning signs at the intersection, ⢠Splitter islands on the minor-road approaches, ⢠Intersection lighting, and ⢠Overhead flashing beacons. Warn drivers of presence of conflicting vehicles. For some particularly hazardous inter- sections, it might be beneficial to employ advanced technology devices to warn motorists on the minor road of the suitability of available gaps. Similar technology could be employed to warn motorists on the major road of the presence of vehicles approaching from the minor road. Choose appropriate traffic control. In some cases, it may be appropriate to change from yield to stop control, from stop control to a multi-way stop, or even to signal control if other measures do not solve the crash problem. Better accommodations for pedestrians and bicyclists. In some cases, the limited sight dis- tance may be a problem to pedestrians or bicyclists. In this case, warnings to approaching motor- ists through signing or pavement markings may be effective. C h a p t e r 5
Other Considerations and resources 33 Resources AASHTO has produced several resources that provide practitioners the basic technical information needed when examining ISD-related issues. The Highway Safety Manual and A Policy on Geometric Design of Highways and Streets, 6th Edition (AASHTO 2011, often referred to as the Green Book) may be reviewed by practitioners prior to using this guidance. HSM tools and resources are available online (http://www.highwaysafetymanual.org/Pages/default. aspx), and the Green Book is available for purchase through the AASHTO Bookstore (https:// bookstore.transportation.org/collection_detail.aspx?id=110). The Unsignalized Intersection Improvement Guide (UIIG) was developed to assist practitionersâ specifically local transportation agenciesâin selecting countermeasures at unsignalized inter- sections to reduce the number and severity of crashes. The UIIG provides useful information on 75 countermeasures that focus on the safety, operation, and accessibility of unsignalized inter- sections. More information is available at: http://www.ite.org/uiig/default.asp. The Crash Modification Factors Clearinghouse is a web-based database to help practitioners identify the most appropriate countermeasure for their safety needs using CMFs. More informa- tion is available on the website http://www.cmfclearinghouse.org/index.cfm. NCHRP Report 600: Human Factors Guidelines for Road Systems, Second Edition (Campbell, Lichty, et al. 2012), outlines other behavioral considerations that may affect crashes at inter- sections. Issues such as reaction times and gap acceptance are discussed in this resource. The report can be accessed at http://www.trb.org/Main/Blurbs/167909.aspx.
