2-Traffic Accident Studies and Analysis

1、Chapter 2Traffic Accident Studies and AnalysisCrash Data SelectionCrash Rates and Their UseA. Crash Data AnalysisA. Crash Data Selection -Information on crashes related to the study being conducted should be separated from the large database or from hard copies. -One of the first considerations in s

2、eparating related crashes for conducting accident studies is to decide a time frame. Data outside the selected time period will not be considered.-There are two factors to be compromised: (1) Desire for larger sample sized, and (2) Desire for time frames within which conditions have not changed too

3、much. -Once the study time frame is decided, crash data should be separated out corresponding to that time period.-There are two types of study locations: spots and sections. -Spots are short segments of the roadway that help identify the problem point location, such as intersections, curves, and sh

4、ort bridges. Geometric and other features at a spot should be noticeably different from surrounding spots. Recommended spot lengths are from 300 to 450 m. -Sections are longer, rather homogeneous sections of the roadway. Recommended section lengths are from 1500 to 3000 m.-Precise definitions for th

5、e limits are required for analyzing intersection crashes. A fixed distance of 30-60 m along each of the approach leading to the intersection is used in most of the cases. （Functional Zone）-Reducing crash data based on other variables may be required based on the purposed of the study. Interest may b

6、e on the crash type, time of the day, months of the year, weather, types of vehicles, driver conditions, age of the drivers, etc. B. Crash Rates and Their Use Basis for Using Crash Rates -Absolute number of crashes may not give the true representation of the safety situation. Many statistics are pre

7、sented in the form of rates. -Two based categories in crash rates: (1) population based and (2) exposure（使用） based -Common bases for population based rates: -Area population-Number of driver licensed population-Number of motor vehicle registered-Highway (roadway) mileage -Common bases for exposure b

8、ased rates: -Vehicles miles of travel (VMT)-Vehicles hours of travel (VHT)2.Basic Crash Rates a. Crash rate per mile R = crash rate per mileA = total number of crashes on the considered section (usually for 1 year)L = length of the section b. Crash involvement rate: R = crash rate pr 100 million veh

9、icle miles traveled（億車英里事故率） N = total number of drivers of vehicles involved in crashes (usually in 1 year) V = total vehicle miles traveledLAR V0100,000,00NR c. Death/fatality rate based on population R = death/fatality Rate per 100,000 people（十萬人死亡率）B = total number of traffic fatalitiesP = popul

10、ationd. Death/Fatality Rate Based on Driver-Licensed Population R = death/fatality rate per 10,000 licensed drivers（萬駕駛員死亡率）B = total number of fatalitiesD = driver licensed population P100,000BRD10,000BR e. Death/fatality rate based on registered-vehicle R = death/fatality rate per 10,000 registere

11、d-vehicles（萬車死亡率）B = total number of fatalitiesM = total number of registered motor vehicles f. Death/fatality rate based on VMT R = death/fatality rate per 100 million VMT （億車英里死亡率）B = total number of fatalitiesV = total number of vehicles miles traveled M10,000BRV0100,000,00BRExampleCity XFataliti

12、es 75Fatal Crashes 60Injury Crashes 300PDO crashes 2000Total involvements 4100Vehicles-miles 1,500,000,000Registered Vehicles 100,000Licensed Drivers 150,000Area population 300,000 R= 25 deaths/fatalities per 100,000 peopleR= 5 deaths/fatalities per 10,000 licensed driversR= 7.5 deaths/fatalities pe

13、r 10,000 registered-vehiclesR= 5 deaths/fatalities per 100 million VMT 3.Crash Indices a. Severity index - defined as number of fatalities per crash Example:City ACity B70 fatalities95 fatalities2000 crashes3500 crashesSI= 0.035SI= 0.027 b. Risk index: Example:Driver groupPopulationCrashRIOld driver

14、25%30%1.20Middle age driver45%45%1.00Young driver30%25%0.83 crashes of number Totalfatalities of number TotalSI group in population of %group in tinvolvemen crash of %RI Four Major Types of Crash Analyses -Summarizing number of crashes and trends-Identifying crash prone locations-Selecting crash cou

15、ntermeasures -Evaluating existing countermeasures C. Crash Data Analysis 1、Summarizing number of crashes and trends1）同向車輛碰撞（C1）：發(fā)生在同方向行駛的前后車輛間，前車減速或停車時，尾隨其后仍在行駛中的車輛反應不及時存在追尾的可能；或后車企圖超越前車存在追尾或側(cè)擊的可能。2）換車道碰撞（C2）：車輛因變換車道而轉(zhuǎn)換方向，與在被穿越車道上行駛或排隊的車輛發(fā)生碰撞，有導致側(cè)擊或追尾事故的可能。3）超車碰撞（C3）：后面的車輛試圖超越前車過程中，這將導致其與前車或與相鄰車道的排隊

16、車輛發(fā)生側(cè)擊。4）并行碰撞（C4）：發(fā)生在來自不同方向、選擇了同一或相鄰車道的并行車輛之間，有引起側(cè)擊的可能。5）正向碰撞（C5）：發(fā)生在兩相向行駛的直行車流之間，碰撞車輛以相反的方向相互逼近，是車頭與車頭之間的碰撞。6）側(cè)向碰撞（C6）：發(fā)生在直行車輛與相鄰右引道的左轉(zhuǎn)車輛之間，是車頭與車輛中部之間的碰撞。7）合流碰撞（C7）：發(fā)生在直行車輛與相鄰左引道的左轉(zhuǎn)車輛之間、直行車輛與相鄰右引道的右轉(zhuǎn)車輛之間，是車頭與車輛中部之間的碰撞。8）行人過街碰撞（C8）：發(fā)生在交叉口連接公路上，行人或非機動車橫穿公路時，與機動車發(fā)生的正面碰撞。9）交叉口行人碰撞（C9）：發(fā)生在交叉口，行人或非機動車穿越交

17、叉口，與機動車發(fā)生的碰撞。前四類為機動車之間的碰撞，后兩類為機動車與行人、非機動車之間的碰撞。-By crash injury type -Total number of crashes -Fatal crashes -Injury crashes -Property damage only crashes (PDO) -By crash involvement -Number of vehicles involved in crashes -Number of persons involved in crashes -Number of drivers involved in crashes

18、 -By personal injury severity -Number of fatalities/deaths -Number of persons injured -By driver injury severity-Number of driver fatalities/deaths -Number of drivers injured 2、Identifying crash prone locations -Spots maps: created by making the location of each relevant crash on a map. Example: map

19、 hung on a wall, computer graphics, etc. could be color-coded. Very useful for specialized situations such as pedestrian crashes. -Crash frequency: crash-prone locations are identified through lists of locations ranked by number of reported crashes (frequency). -Crash rates: list of locations are id

20、entified through lists ranked according to the accident rate. -After ranking locations of interest either by crash frequency or rate, it is required to select top n locations for further investigation. -One common approach to determining which locations require immediate attention is to identify tho

21、se with crash rates that are significantly higher than the average for the jurisdiction under study. Example: The locations with crash rates in the highest 5% of the normal distribution. In a one-tailed test, the value of z (on the standard normal distribution) for Prob (z) 0.95 is 1.645.xi = crash

22、rate at the location under consideration = average crash rate for locations within the jurisdiction under study s = standard deviation of crash rates for locations within the jurisdiction under studyxs.xsxxzii6451x3、Crash Countermeasures Once a location is identified as “crash-prone” or otherwise wo

23、rthy of improvement, it is necessary to search for affordable and effective countermeasures. First stage: look for over-represented clusters of particular kinds of crashes. Main tool used by many agencies to identify the clusters: Collision diagram a schematic, not to scale graphical representation

24、of the crash pattern at a particular location. Each accident is plotted separately on the approach and near the place where the first harmful event is said to have occurred. Night-to-Day crash ration is often used to decide whether the location or cluster of accidents is over represented at night. -

25、After prominent cluster of accidents at a location is identified, the next stage to generate list of possible countermeasures through the following: -Detailed investigations of crashes (especially crash causes)-Review of site plans and condition diagrams.-Site visits-Other transportation engineering

26、 studies. Ex. Spot speed studies-The practices and previous experiences of the agency. -Final stage of countermeasures selection is to narrow the range of possibilities to one or more measures to be implemented. One popular way to analyze countermeasure alternatives is thorough an economic analysis.

27、 路側(cè)險要交通安全診斷與改善序號序號問題問題對策對策技術技術基本要求基本要求推薦對策推薦對策簡易簡易標準標準1路側(cè)無防護 （ ）設置防護設施和設置警示標線按照規(guī)范標準設置防護設施（1）設置水泥柱簡易護欄（2）設置沙包、石塊等簡易護欄（3）設置警示樁、示警墩（4）種植灌木（5）桶裝集料護欄根據(jù)路側(cè)危險等級設置相應等級標準護欄2橋頭防護不足 （ ）改善橋頭防護設施改造為符合技術標準的防護設施（1）護欄端頭刷油漆（2）護欄端頭設置警示樁混凝土護欄延伸，符合相關技術規(guī)范 3寬路窄橋 （ ）提示寬路窄橋信息控制速度，改造過渡段（1）護欄端頭刷油漆（2）護欄端頭設置警示樁設置寬路窄橋標志，設計良好的過渡段

28、，橋梁護欄端頭刷油漆4路基變窄路段 （ ）提示路幅變窄信息增加過渡段設置路面變窄警示標志（1）設計良好的過渡段（2）設置限速和禁止超車標志5路幅寬度小于4.5m （ ）提供錯車設施增加路幅寬度，禁止大車駛?cè)朊块g隔300500m設置錯車道增加路肩寬度6路側(cè)安全凈區(qū)內(nèi)有障礙物（ ）提供有障礙物信息移除障礙物在標志上刷油漆移走障礙物7下穿高等級公路 （ ）提供限制信息定位有效控制設置立面標線設置限高、禁止超車等標志，添加立面標線8缺乏減速裝置 （ ）設置道路減速裝置設置減速標志設置減速臺等物理減速設施9缺乏交通標志及位置設置不合理 （ ）合理設置交通標志設置警告作用的閃爍燈按照標準合理設置交通標志按

29、照標準合理設置交通標志10無任何標線 （ ）完善標線設施施劃標線注根據(jù)現(xiàn)場調(diào)查，在存在問題的后面括號內(nèi)打（）平面交叉交通安全診斷與改善序序號號問題問題對策對策技術技術基本要求基本要求推薦對策推薦對策簡易簡易標準標準1路權(quán)不清晰 （ ）分配路權(quán)設置渠化依據(jù)相關標準設置停讓控制（1）依據(jù)相關標準設置信號控制或停讓控制（2）根據(jù)車流行駛特征設置導流島或是標線渠化1 2小角度交叉 （ ）設置渠化渠化；調(diào)整線形（1）設置渠化島1，如三角島、環(huán)島、不規(guī)則島等（2）設置警告、限速、指路等交通標志（1）調(diào)整線形：小角度Y型交叉-T型交叉 小角度X型交叉-十字型交叉（2）渠化設置3視距不良 （ ）修剪或清除非建

30、筑障礙移除所有障礙設置相應的交通標志、標線（1）修剪樹木（2）拆遷障礙設施或建筑14轉(zhuǎn)彎半徑過小 （ ）路口拓寬硬化處理可在拓寬區(qū)域平鋪砂石、泥土等材料2 3（1）適當拓寬路口，增大轉(zhuǎn)彎半徑（2）若有條件可硬化處理5車輛速度過快 （ ）控制速度設置減速設施設置減速標志、標線（1）設置相應的標志、標線（2）設置對應道路等級的減速設施，如震蕩標線、減速帶等6缺乏弱勢 群體保護 （ ）交通管理交通管理設置相關標志、標線（1）設置人行橫道、中央分隔帶等弱勢群體保護設施1 2（2）設置相應標志、標線7信號相位或標志標線設置不合理 （ ）合理設置-按照相關標準合理設置標志8功能區(qū)有障礙物 （ ）交通管理移

31、除障礙物設置相應警告設施，如反光膜、警告標志等拆除或?qū)⒄系K設施或建筑移出交叉口功能區(qū)注：（1）根據(jù)現(xiàn)場調(diào)查，在存在問題的后面括號內(nèi)打（）；（2）標有上標的技術表示該技術只適用于上標數(shù)字所代表的平面交叉類型，未標示的可通用。其中，1為一、二級縣、鄉(xiāng)道之間交叉；2為一、二級縣、鄉(xiāng)道與三、四級農(nóng)村公路交叉；3為三、四級農(nóng)村公路之間交叉。4、 Evaluation of the Effect of Countermeasures -Crash data are frequently used to evaluate the highway safety improvements.-Countermea

32、sures are often evaluated by using the experimental technique by using crashes as the measure of effectiveness. Commonly know as Before and After studies. -Before and after studies of improvements at a set of locations are often conducted by comparing the crash history of these locations before impr

33、ovement to the crash history of the same locations after the improvements. -If “Before and After” study is not feasible, “Cross Section Comparison” study could be performed.-For “Cross Section” comparison, two sets of sites should be identified with similar control factors except the improvement cou

34、ntermeasures. Example: The normal test is used to test whether the safety improvement is significant. fA = number of crashes in the “after” periodfB = number of crashes in the “before” periodz1 = test statistic representing the reduction in crashes on the standard normal distributionGiven 5% signifi

35、cance level, the test is: - Because of the number of crashes may be small, the sample sizes may not be sufficient to justify use of the normal distribution. It is more accurate to use the Poisson distribution and a modified binomial test.BAABiffffz950.zzobPriEndHome WorkHomework No. 1Due: next class1. Considering highway fatalities over the past 20 years (you may use the data give