Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021) / Chapter Skim
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46 CHAPTER 4. ANALYSIS OF EXISTING CRASH DATA INTRODUCTION Run-off-road crashes and associated roadway data from several existing databases were evaluated and analyzed to achieve two objectives.
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47 efforts undertaken by NCHRP 17-22, NCHRP 17-11, and FHWA Rollover Causation Study projects. The NCHRP 17-22 database also provides estimates of vehicle roadside encroachment and impact characteristics data -- for example, encroachment speed and angle distributions at the POD -- using crash reconstruction techniques.
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48 Table 4.1. Relative strengths and limitations of databases to address research objectives.
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49 Table 4.1. Relative strengths and limitations of databases to address research objectives (continued)
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50 The remainder of this chapter is organized as follows: FARS.
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51 Driver characteristics, including driver alcohol use, occupant seat belt usage, and injury severity. Distribution of vehicle travel speeds.
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52 Figure 4.1. Number of fatal crashes involving ditches and other roadside features as FHE from 1991 to 2009.
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53 Table 4.2. Top five or six MHEs (FARS: 2004 to 2008)
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54 Posted Speed Limit Table 4.5 presents the frequency and relative percentage of fatal crashes by FHE and PSL. The majority of the crashes involving ditches, culverts, and utility poles as the FHE occurred on highways with a PSL of 55 and 45 mph (89 and 72 km/h)
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55 Table 4.3. Frequency and relative percentage of fatal crashes by highway functional class (FARS: 2004 to 2008)
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56 Table 4.4. Frequency and relative percentage of fatal crashes by FHE and highway type (FARS: 2004 to 2008)
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57 Table 4.5. Frequency and relative percentage of fatal crashes by FHE and posted speed limit (FARS: 2004 to 2008)
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58 Table 4.8. Frequency and relative percentage of fatal crashes by FHE and lighting condition (FARS: 2004–2008)
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59 Table 4.10. Frequency and relative percentage of fatal crashes by FHE and the deployment of driver side air bags (FARS: 2004–2008)
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60 Table 4.12. Frequency and relative percentage of fatal crashes by FHE and seat belt use.
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61 Table 4.13. Some characteristics of travel speed distribution and fitted probability density function by posted speed limit (FARS: 2004–2009; only SVROR crashes on the roadside area)
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62 NASS GES NASS GES data are obtained from a probability sample of police-reported traffic crashes covering all vehicle types and injury severity levels. About 55,000 crash cases per year have been sampled in recent years (from an estimated crash population of about 5.8 million per year)
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63 Because the estimates produced from the GES and CDS are based on a probability sample of crashes, not a census of all crashes (as is the case with FARS) , the estimates are subject to sampling errors and may differ from the true values.
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64 is in consideration, only one vehicle is damaged. As shown in Table 4.14, on average, a fatal ditch/culvert-initiated crash involved 1.056 fatalities, 0.192 incapacitating injuries, 0.216 nonincapacitating injuries, 0.095 possible injuries, and one damaged vehicle.
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65 Table 4.15. Estimated numbers and distribution of crashes by injury severity (FARS and GES databases, 2004–2009)
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66 presented. These fitted travel speed distributions are used in conjunction with other data to estimate encroachment speed distributions.
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67 from 2002 to 2007. The final stage is a sampling of crashes from 10 crash strata that are determined by severity of the injured, type and model year of the vehicle involved, disposition and hospitalization of the injured, and tow status of the vehicles involved.
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68 Like GES, when coding fixed objects struck by the involved vehicles, CDS uses the same code for ditch and culvert. It is therefore not possible to tell from the coded data whether a vehicle involved in a collision with "ditches/culverts" actually struck a ditch or a culvert.
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69 sample size and large sampling weights discussed earlier. Nonetheless, close to 69% of the rollovers were estimated as being initiated by soft soil and 15% by ditch slope, indicating that soft soil and slope are the main initiation objects in ditch rollover crashes.
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70 discussed. In summary, it is a biased sample by design in which crashes are selected with unequal probabilities.
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71 higher were selected for studying crash severity. The maximum PSL where crashes occurred was only 60 mph (97 km/h)
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72 Focusing on crashes involving ditches as the FHE (i.e., the first property-damaging or injury-producing event in the crash) ensured that the injury to occupants and damage to vehicles did not occur before the vehicle struck the ditch.
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73 Those involving roadside ditches/culverts as the FHE, and the MHE was either the ditch/culvert or a rollover. This study limited its scope to roadside ditches/culverts.
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74 be multiplied by an average number of fatalities and injuries involved in a crash and summed to obtain the total per-crash cost. At the nationwide level, data from FARS and GES are usually used for this purpose (41)
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75 Costs of Rollover and Non-rollover Crashes As presented earlier for fatal crashes, after striking a ditch as the FHE, the MHE includes rollover (54%) , standing tree (16%)
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76 Table 4.24. Estimated per-crash cost for rollover, non-rollover, and all crashes.
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77 cost is attributable to the occurrence of fatal (K) and incapacitating injury (A)
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