STATE HIGHWAY GEOMETRIC DESIGN MANUALSECTION 7:ROADSIDE FEATURESMay 20057 - 1Revised Draft:12 May 2005 7 Roadsi de Features 7.1 GeneralThe roadside should be made as forgiving as possible, as amatter of good road design practice.An adequate clear zoneshouldbeprovidedtogivedriversoferrantvehiclesanopportunitytorecoverandstopsafelyorreturntotheroadway, and where this is not possible, to reduce the severityof the resulting crash as much as possible.Recommendedroadside design procedures to provide an adequate clear zoneinclude: Remove obstacles or redesign themso the roadside canbe traversed safely. Relocate obstacles to a point where they are less likelyto be struck. Reduce impact severity by using appropriate breakawaydesigns. Redirectvehiclesbyshieldingobstacleswithlongitudinal traffic barriers and/or crash cushions. Delineate obstacles where the above alternatives are notappropriate.The application of consistent geometric design standards forroads and streets provides motorists with high a degree ofsafety.Designfeaturessuchashorizontalandverticalcurvature,laneandshoulderwidths,signingandroadpavement markings each play an important role in keeping amotoristontheroad.Roadsidesafetyfeatures,suchasbreakaway supports, bridge safety barriers and crash cushionsprovide an extra margin of safety when vehicles inadvertentlyleave the road.Most of these devices are installed on thebasis of a subjective analysis of their safety benefits.Eachprojectisuniqueandoffersanindividualopportunitytoenhancethatparticularroadsideenvironmentfromsafetyperspective.The benefits derived froma road safetytreatment can becalculated by first estimating the number of vehicles that arelikely to run off the road at a particular location.A run offtheroadincidentisdefinedasanencroachmentandthenumber of encroachments likely to occur at a given locationis mainly related to the traffic volume, road alignment andlane width.All encroachments do not, however, result in crashes becausethe end result depends upon the physical characteristics of theroadside environmentFlat, traversable slopes will minimiseoverturning type crashes while the elimination of roadsidehazards or their relocation to less vulnerable areas and the useof breakaway type devices will also improve roadside safety.Hazards that cannot be otherwise treated should be shieldedby properly designed safety barriers or crash cushions.When the number of crashes likely at a given location hasbeen estimated it must be translated into crash risk which isdirectly related to accident severity.7.2 Ditches and Back Slopes7.2.1 GeneralThe majority of rural roads will have ditches in one formorother.Their primary function is to collect and carry away thesurfacewaterawayfrom thecarriageway.Ditchesaredesigned to accommodate the runoff fromheavy rain withminimalfloodingordamagetotheroad.Deepditchesconstructed close to the carriageway are the most efficient inremoving and retaining the water fromthe road surface butthey are, however, a hazard for errant vehicles.Good ditchdesign requires a consideration of roadside safety as well ashydraulic efficiency.Ditches are classified by their slope change, ie. an abrupt orgradual slope change.The abrupt slope change type includes'V' ditches, rounded ditches with a bottomwidth