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10 of lane and shoulder width to crashes on two-lane rural roads An additional concept that merits attention is that of the and quantified these by developing models later included in presence of a tipping point--the principle that small changes the Interactive Highway Safety Design Model (IHSDM). A have little or no effect on a system until a crucial point is significant and potentially useful conclusion from the literature reached (19). This concept, which has been extensively used is that the important element in crash reduction is the total in epidemiological research, could also be used in roadway available roadway width. The studies on converting two-lane design because of the available flexibility in the values of design roads to four-lane roads show that, in general, safety gains elements. It could be hypothesized that safety and operational are achieved with such conversions (14, 15). The findings of consequences from altering the values of design elements while NCHRP Report 330: Effective Utilization of Street Width indicate remaining within the suggested Green Book values are minimal that there are certain designs for urban arterials where the and, thus, do not create significant problems. Moreover, small implementation of strategies that involve the use of narrower departures from these values may have no significant impact, lanes has an effect on safety (16). Such strategies include the use and thus the safety consequence tipping point for any single of center two-way left-turn lanes or removal of curb parking, design value may not be detectable. Highway design typically and most of these strategies involved projects with restricted requires a multi-level assurance by professional engineers right of way and arterials with speeds of 45 mph or less. The that the approved design will not result in unacceptable levels study also concluded that even though the use of narrower of safety consequence. Projects requiring a design exception lanes, when considered alone, may increase specific crash types, could be considered as those that are the farthest from the the presence of other design features, such as the addition of most desirable design value. The recently completed NCHRP two-way left-turn lanes, may offset these increases. This study Project 15-22, "Safety Consequences of Flexibility in Highway also underscores the potential of interactive effects between Design," found that the small deviations noted in the case various design elements and suggests careful evaluation of the studies analyzed indicate that a generally conservative approach use of narrower-than-typical lanes. is taken when considering values that vary from traditional A more recent review of safety in geometric design standards design (20). by Hauer (6) critically examined the belief that adherence to design standards is directly linked to safe roadways. This review Cross-Section Elements indicated that design guidelines have an inherent safety level, but that little is known about the impacts of their flexibility The literature review conducted for this research focused application in roadway design. Another issue identified by on three cross-section elements: lane width, shoulder type Hauer was the notion that there are two different kinds of and width, and median type and width. This section discusses safety. One could be called nominal safety and is measured the findings for these design elements. Several of the find- "in reference to compliance with standards, warrants, guide- ings have been cross-referenced with the interim report from lines, and sanctioned design procedures" (6). Substantive NCHRP Project 17-27, "Parts I and II of the Highway Safety safety, by comparison, is based on the roadway's actual safety Manual," (21) and NCHRP Web-Only Document 126: Method- performance--that is, crash frequency and severity. Designing ology to Predict the Safety Performance of Rural Multilane nominally safe roads does not ensure substantive safe roadways Highways (22). since adherence to values of each guideline does not inherently produce a safe design. Several of the studies examined focused Lanes on developing models that investigate and quantify the sub- stantive safety changes from altering design dimensions (17). Wider lanes are traditionally associated with higher oper- Another aspect of safety noted by Fambro et al. (18) is the ating speeds and increased safety. The Highway Capacity concept that safety is a continuum and not a single yes/no Manual (HCM) (23) documents that wider lanes for multi- decision. This implies that a change in the value chosen for lane highways result in higher free-flow speeds. On the other a particular design element "can be expected to produce an hand, very little has been found on the safety implications incremental, not absolute change in crash frequency and of wider lanes. It is reasonable to assume that wider lanes severity" (17). However, there is a need to better understand may provide additional space to the driver to correct poten- the effect on the level of safety from these incremental changes, tial mistakes and thus avoid crashes. However, a driver could and such efforts are essential in understanding and quantify- be expected to adapt to the available space, and the positive ing the substantive safety of a roadway. This is critical for safety effects from the wider lanes may be offset by the higher projects where design flexibility is considered. Stakeholders speeds. do not easily accept designs that are considered nominally Most completed research on this topic has focused on the safe, but require the evaluation of design alternatives that may lane width of two-lane, two-way roads, and very little is known deviate from the nominal designs. of the effect of lane width of multilane rural highways (24).