FORMULATION OFTHEANALYTICALWETWEATHER SAFETYINDEXAnalyticalThe analytical formulation of a Wet Weather Safety Index (wwSla) isdefined by the flow diagram (Figure 17),the definition of terms (Table2)and the equation 1isting (Table 3 ).It is a systematic organization oftheoretical, analytical and empirical work that has been accomplished inthe areas of vehicle handling and stability, tire-pavement interaction anddriver performance over the past 20 years.References are given by eachequation in Table 3.Basic to the formulation of a wwsi is the need to have an indicator ofaccident probability during dry weather and accident probability in wetweather.The indicators selected and formulated are discussed on pages 27through 37.They are summarized here as (1)TF,the traffic conf1ict factorwhich is present during either wetand dry weather, and is based on thespectrum of vehicle speeds and confiicts between all these vehicles withothers in the traffic pattern as weli as with vehicles seeking access oregress from the traffic stream; and(2)wF,thewet weather conf1icts factor,a factor which is present only when rain is falling or the pavement is stillwet.The value of wF calculated is dependent on the differences between thecomputed maximum safe speed for traffic and the frequencies of vehiclesmoving at higher speeds.Although the logic behind the development of these two factors wasexplained previously,the way in which they are put together to form theRHI (Road Hazard Index) and the WWSI (Wet Weather Safety Index) wii1 betreated here.TTI Res Rpt 22/-1FNovember 1977
CalculateChoose LargerCatculate spCalculateCaleulatowpENPUT,BATAXSPRAYt0Sru11ergfsDam1gP;10:T80DorNOEeuatlon9Equat1on17tquetton 10- MDS: L: MO,: 50,R;eS,iV,tM,ENSaitSHAo*PA,-VA,PA,"VA,TheSoullerCalculate FM,of't, Pr..'.qustlon 1Yandy,Pv,. "'v,.-Py,. 'y,and.SoeuaCAN; CTM; CH;VCalcutate rs,CalculatoTCalculateEquatlon12Equation2Calculate FiSTARTwerageFN,FHFMPROGRANTesandFRCalculateFn,TestCalculateEquatten 131svO0.017CALCULATEO FACTORSEeuat1otp, s, fr."tye.Vv, wCalculate FH,Fm,.Fn,.e.fpEeuatfons14,15,$16F,.TF, ME.CalcwlateRHRHF,WSTteuatfon 21CalculateTnEquattons5,6&Calculate WSI,Equation 22Tigure 17,Plow ChartferOeteminutton orws1
TABLE2.DEFINITIONOFTERMS.INPUTDATA1. Ig, Design rainfall intensity (in./hr)2.P,Design tire pressure (1b/in.2)3.TD, Design tread depth (32nds of an in.)4.TxD,Surface texture(in.)5. s, Surface slope (in./in.)6.L,Drainage path length (ft)7.WDa,Puddling water depth (in.)8.SDg,Geometric sight distance (ft)9.VT50 Percentile truck speed (mph)5010.fr, Average truck spacing (ft)11.R, Road curve radius (ft)12.e, Curve superelevation (in./in.)13.Sr, Reverse crown slope (in./in.)14.W,,Road lanewidth (ft)15.W,,Paved shoulder width (ft)16.Wt,Track width (ft)17.SN40, Skid number at 40 mph (dimensionless)(i=1+n), Matrix of normalized vehicle frequency and18.Pv.,VvVicorresponding velocities (dimensionless and miles/hr)19.PA,VA, (i-1+k), Matrix of normalized automobile frequency andcorresponding velocities (dimensioniess and miles/hr)20.Pr,,Vt.(j=1+m),Matrix of normalized truck frequency and1corresponding velocities (dimensioniess and miles/hr)21.CH, Critical vehicle frequency (vehicles/hr)(DHv may be used to approximate CV)22.CAH, Critical automobile frequency (vehicles/hr)23.CTH, Critical truck frequency (vehicles/hr)24. V, First assumption of critical vehicle speed (miles/hr)25. T(w), Decimal portion of time road is wet (dimensionless)
TABLE 2.DEFINITION OF TERMS.(continued)CALCULATED FACTORS1.SSD,Sight distance(ft)2.SDw, Climatic sight distance (ft)3.WD,water depth (in.)4.V, Final estimate of critical speed (miles/hr)5.Yy，Hydroplaning speed (miles/hr)6.V.,Critical wet weather speed (miles/hr)7.W,Availablesidespace(ft)8.FN,Available friction number (dimensionless)9.FNs,Stopping distance friction number (dimensionless)10.FNc,Cornering friction number (dimensionless)11.FNp, Passing friction number (dimensionless)12.FN,Emergency correction friction number (dimensionless)13.KSPRAY, Reduction of sight distance due to truck spray (dimensionless)14.TF,Traffic confiict factor15.WF,Wet surface speed conflict factor16.RHF,RoadHazardFactor17.wwSI,Wet Weather Safety Index19.FNvThe average friction number of FNs,FNc, FNp,and FNeavg
TABLE3.EQUATIONS(continued)ReferenceNumber10,11LOFN,= 100(a,SN40 + by)a, = 1.14 × 10-6 (v2 - 10v + 3530)106by = -7.44 × 10~5(v2 - 10V - 3000)107Definitionft = VT-。 × 5280/CTH8'50V2WD/2 cVH150New Dev.9KSPRAY =14.5×106(4500-15001g)KSPRAY12*,13*10SD..68rNew Dev.+0.8]IdL27v?14,1511FN, " 0.3(SD - 3.67V)v214,16120. 08R + 40 - 100 eFNc"v214,1713FN.220 - 100 S,SP100v2(1- cos e)14,1514FNe " T5(00V(.47 0S10) + 2New Dev.15W = 0.8 W.+ 51516e = 0.2 W.+38rTxD0.11_0.43,0.5913WD = 0.00338 [17- TXDs0.42* Equation 5 was developed from information presented in these studies