3. Turning movement or intersection counts are used in designing channelizations(saluran), planning turn prohibitions(perancangan larangan

Transcription

1 1.0 TRAFFIC FLOW 1.1 Traffc data collecton The type of data collecton n a specfc volume study depends upon the applcaton n whch the nformaton wll used. 1. Street counts (total volume wthout regards to drecton) are used n developng volumes, preparng traffc flow maps, determnng trends, etc.. Drectonal counts are used for capacty analyss, determnng sgnal tmng, justfyng traffc controls, plannng mprovements, obtanng accumulatons of vehcles wthn a cordon(gars kepungan) etc. 3. Turnng movement or ntersecton counts are used n desgnng channelzatons(saluran), plannng turn prohbtons(perancangan larangan membelok), computng capacty, analyzng hgh accdent ntersectons, evaluatng congeston, etc. 4. Classfcaton counts (obtan volumes of the varous types of classes of vehcles n the traffc stream) are used n establshng structural and geometrc desgn crtera, computng expected hghway user revenue, computng capacty (effect of commercal vehcles), determnng correcton factors for machne counts Occupancy counts are made to determne the dstrbuton of passengers per vehcle accumulaton of persons wthn an area, proporton of persons utlzng transt facltes. 6. Pedestran counts are used n evaluatng sdewalk and crosswalk needs, justfyng pedestran sgnals, tmng traffc sgnals, etc. 7. Cordon counts (kraan spadu kepungan) are made at the permeter of an enclosed area (shoppng center, ndustral area, etc.). Vehcles and/or persons enterng and leavng the area durng a specfed tme perod are counted. These data provde nformaton relatve to the accumulaton of vehcles or persons wthn the cordon. 8. Screen lne counts are classfed counts taken at all streets ntersectng an magnary lne (screen lne) bsectng an area. These counts are used to determne trends, expand urban travel data, traffc assgnment etc Countng measurements / technques 1. Machne counts can be used to obtan vehcular counts at md-block locaton. Total volumes, drectonal volumes, or lane volumes can be obtaned, dependng upon equpment avalable.. Manual counts are necessary n certan studes snce the desred data cannot obtaned by mechancal counters. (eg. Turnng movement counts, classfcaton counts, occupancy studes, pedestran counts,

2 freeway counts). Tables 1.0 shows an example of form for ntersecton manual traffc countng. 3 Fakult : Kejuruteraan Muka surat Jabatan : Kejuruteraan Awam Tajuk : ANALISIS KENDERAAN Eds No. semakan Tarkh Efektf DIPERSIMPANGAN Tarkh Pndaan Lokas : Part Raja Lengan : Belok kanan Kelas Masa (mnt) kenderaan Jumlah Kelas 1 motorskal Kelas 3 Van / lor kecl Kelas Kereta / teks Kelas 4 Lor besar / bas Table 1.0 : Form for ntersecton (manual traffc countng) 4

3 3. Movng vehcle method. Ths technque wll estmates the average volume on a test secton as well as the average travel tme. The method nvolve : a. A test vehcle beng drven repeatedly over the test secton at any reasonable speed. b. An observer recordng the followng data for each test secton. Number of oncomng vehcles meetng the test car. Number of vehcles overtakng the test car. Number of vehcle overtaken by the test car v. Travel tme of the test car. Countng perods The tme and length that a specfc locaton should be counted s also dependent upon the data desred and the applcaton n whch the data are to be used. 1. Some of the more commonly used ntervals are: a. Weekend counts coverng the perod from 6 p.m., Frday to 6 a.m., Monday. b. 4-hour counts normally coverng 4-hour perod between noon Monday and noon Frday. c. 16-hour counts from 6 a.m. to 10 p.m. Ths perod contans most of the daly flow ncludng evenng traffc. 5 d. 1-hour counts usually from 7 a.m. to 7 p.m. to cover most daytme traffc movements, especally n commercal or busness areas. Such counts made n shoppng centers or dstrcts where stores are open at nght are usually extended untl after 9 p.m. e. Peak-perod counts whch vary dependng upon sze of metropoltan area, proxmty to major generators (such as the CBD or ndustral areas), and the type of the faclty (gateway, radal arteral, etc.). Commonly used perods are 7 to 9 am and 4 to 6 p.m.. Specal condtons should be avoded unless the purpose of the count s to obtan data concernng these unusual condtons. Examples of such condtons nclude: a. Specal events (holdays, sports, exhbtons, sales., etc). b. Abnormal weather condtons whch do not generally occur. c. Temporary closure of streets effectng the volume pattern. d. Transt or truckng strkes. 3. Adjustment factors must be appled to the data to remove seasonal or other varatons, to provde a realstc estmate of the average volume condton, or to expand a count to a volume estmate of a longer perod. 6

4 Types of counts 1. Control staton counts : made to obtan Average Annual Daly s Traffcs (AADTs) and the seasonal factors whch are used to expand sample counts. Control counts are made by hourly recordng counters by drecton. There are three types of control counts. a. Trend counts are obtaned contnuously at desgnated locatons whch reflect the statewde change n travel. b. Monthly counts are from 7 days to month long. There are lmted number of these locatons. c. Quarterly counts are obtaned for the same week-long perod once each quarter. There are taken along a hghway at the ponts of low, hgh or rapdly changng traffc volume, at or near end of the hghway routes, and at begnnng or endng ponts used.. Sample counts : are obtaned for the short perods of tme, generally one day, usually by nonrecordng counter, at ponts of sgnfcant change n traffc volume, on freeway ramps and connectors, and at other necessary locatons. 3. Vehcle classfcaton counts are made manually on hgh volume hghways (over 5,000 AADTs) and mechancally wth a vehcle classfyng counter on low volume hghways. The manual countng s from 4 to 8 hours at profle ponts. 7 Traffc volume data Purpose of volume studes : To obtan factual data concernng the movement of vehcles and/or person at selected ponts on the street or hghway system. Volume data are expressed n relaton to tme the base beng determned by the type of nformaton desred and the applcaton n whch t s to be used.. Annual traffc n vehcle per year s used for a) Determnng annual travel n geographc area. b) Estmatng expected hghway user revenue. Fgure 1.0 : Bar chart shows the varatons of passenger on hghway c) Computng accdent rates. d) Indcatng trends n volume, especally on toll facltes. 8

5 9 Masa (jam) 00 pg 00 pg 00 pg pg mlm 0 mlm 0 mlm 1.00 mlm 0.00 mlm mlm ptg ptg ptg ptg 10 ptg 10 ptg 10 tgh pg pg pg pg pg pg 0.00 Peratus spadu (%) Peratus spadu lalu lntas a. Determnng length and magntude of peak perods.. Hourly Traffc n vehcles per hour s used for: e. Programmng captal mprovements exstng facltes are needed. d. Locatng areas where new facltes or mprovements to c. Developng the major or arteral street system. system. b. Evaluatng the present traffc flow wth respect to the street hghway. a. Measurng the present demand for servces by the street or (AADT) n vehcle per day s used for:. Average daly traffc (ADT) or Average Annual Daly Traffc pg Fgure : Percentage of hourly traffc volume b. Evaluatng the capactes defcences. c. Establshng traffc controls volume s usually among the warrants for the: 1. Installaton of sgn, sgnals, and markngs. Desgnaton of through streets, one-way streets, unbalanced flow, and traffc routng. 3. Prohbton of parkng, stoppng, and turnng. d. Geometrc desgn or redesgn of streets and ntersectons. v. Short Term Counts (coverng 5, 6, 10, or 15 mn. ntervals) are usually expanded nto hourly flow raters. Such counts are prmarly used to analyze: a. Maxmum flow rates. Tme Car Small van Medum van Bus Lorry Motorcyc le Total Flow rate hourly volume V1 = 63 flow ratemax = 68 V =

6 PHF = 0.91 Table : Contoh Short Term Count Dan Kraan Flow Rate b. Flow varatons wthn peak hours c. Capacty lmtatons on traffc flow. d. Characterstcs of peak volumes. 1. Traffc data analyss Analyss of traffc data often begns wth some type of statstcal analyss, and often nvolves some type of graphcal dsplay. Flows and volumes Analyss of flow begns wth the collecton of volume counts. Traffc flow data are dsplayed n a number of dfferent graphcal formats (maps, graphs, fgures). Traffc flow are subject to three basc types of varatons: Trends whch are nonrepettve changes over extended perods of tme (perhaps several years).. Peak-ng patterns whch are repettve changes over tme ntervals such as a day, week or year. 3. Random varatons. A major objectve of the analyss of flow data s often to separate the effects of these dfferent types of varaton. Average daly traffc volumes n vehcles per day) are often used for plannng purposes or for plottng long-term trends. Extractng repettve patterns or long-run trends from randomly varyng data s a or goal of much of the analyss of traffc flow data. In dong ths, t s mportant to understand the effect of the aggregaton nterval on the varaton. The dsperson of the n a dstrbuton s measured by the varance, whch s gven by, Where, σ = ( x x ) n _ 1

7 x x σ = varance x = an ndvdual measurement x = mean of the dstrbuton n = sample sze Commonly, the measure of dsperson actually cted s the standard devaton, s the square root of the varance. In cases n whch the standard devaton of a populaton s estmated from a relatvely small sample, a more accurate estmate s by 13 ( ) = x x n 1 s Where, s = standard devaton A short-cut calculaton of s s gven by the formula n = x ( x ) n ( n 1) s If means are taken from a number of samples from the same populaton, the resultng dstrbuton of means has a standard devaton gven by, s x s x = n Where, s = standard devaton of the mean s = standard devaton of the populaton, estmated from a sample n = sample sze Speed and travel tme data A second category of traffc data commonly collected and analyzed s that related to speed and travel tme. 14

8 Two basc types of data are nvolved: 1. Spot speed data, whch are taken at a pont or over a very short dstance nterval, and. Travel tme data, whch are taken over more extended sectons of roadway There are a varety of technques for collectng spot speed data, the most common of whch s the use of radar. All these technques lend themselves to collecton of farly large samples, usually from 50 to 00 observatons. Travel tme data, on the other hand, are usually collected by usng test vehcles that traverse the roadway secton n queston; the actual travel tmes are usually recorded by human observers, although recordng tachometers are often used to record the speeds contnuously durng the run Because use of test cars to collect data s expensve, travel tme data samples tend to be small, on the order of 6 to 1 runs. Statstcs normally calculated n the reducton of spot speed data nclude the mean, the standard devaton, and the standard devaton of the mean. 15 For analyss of spot speed data, tme-mean speed s commonly used. For other applcatons, however, space-mean speed s sometmes preferred. In addton, common practce ncludes preparng a plot of the cumulatve dstrbuton and calculatng the 85th percentle speed (that s, the speed such that 95 percent of the vehcles are travelng slower). The 85th percentle speed s roughly the mean plus one standard devaton and s mportant because t s approxmately the break pont between the bulk of the dstrbuton and ts upper tal; as such s often used for purposes such as settng speed lmt Example A Calculate the mean, standard devaton, and standard devaton of the mean for the spot speed dstrbuton below. Plot the cumulatve dstrbuton curve and determne the 85 th percentle speed. Speed groups, km/h Lower lmt mdpont Upper lmt Frequency observed

9 Soluton, Problems, 1. Calculate the mean,. standard devaton, and ( s 3. standard devaton of the mean, n = x ( x ) ) n ( n 1) s s = x x n 4. Plot the cumulatve dstrbuton curve and 5. determne the 85 th percentle speed Lower lmt Speed groups, km/h frequency observed, md upper pont, v lmt f Cumulatve Calculatons frequency, percent f v f ( v ) Totals n = Tme mean speed Standard devaton, f v v km/hr = n = = s n = f ( v ) ( fv ) 10(39650) (6790) = n ( n 1) 10(10 1) = 10.1 km/hr Standard devaton of the mean Parameters connected wth traffc flow space, and s usually expressed n vehcle per hour. Flow s defned as the number of vehcles per unt tme passng a pont n vehcles, s a hghly complex process The nteracton between vehcles and ther drvers, and also among.1 Introducton to traffc flow Traffc flow analyss, Densty and Traffc volume Car movng observer analyss 85 th percentle s approxmately 64 km/hr speed km/hr cumulatve percent th percentle speed 85 percent Cumulatve dstrbuton plot = 0.9 km/hr s v s 10.1 = = n 10

10 There are at least eght basc varables or measures used n descrbng traffc flow, and several other stream characterstcs are derved from these. The three prmary varables are: (a) Speed ) (v, (b) Volume (q ) (c) Densty (k ) Three other varables used n traffc flow are: (a) Headway (h ) (b) Spacng (s ) (c) Occupancy (R ) Also, correspondng to measures of spacng and headway are two parameters: (a) Clearance (c ) (b) Gap (g ). Speed 19 Speed s defned as a rate of moton, as dstance per unt tme, generally n mles per hour (mph) or klometers per hour (km/hr). Several dfferent speed parameters can be appled to a traffc stream. These nclude the followng: ) Average runnng speed o A traffc stream measure based on the observaton of vehcle travel tmes traversng a secton of hghway of known length. o It s the length of the segment dvded by the average runnng tme of vehcles to traverse the segment. o Runnng tme ncludes only tme that vehcles are n moton. ) Average travel speed o A traffc stream measure based on travel tme observed on a known length of hghway. o It s the length of the segment dvded by the average travel tme of vehcles traversng the segment, ncludng all stopped delay tmes. 0

11 s, t o It s also space mean speed. o Thus, f travel tmes t t t..., t, are observed for n vehcles, 3 1 n traversng a segment of length L, the average travel speed s: L nl v = = n n t = 1 n = 1 t (1) Where, v s = average travel speed or space mean speed (mph) L = length of the hghway segment t = travel tme of the th vehcle to traverse the secton (hours) n = number of travel tmes observed ) Space mean speed o A statstcal term denotng an average speed based on the average travel tme of vehcles to traverse a segment of roadway. o It s called a space mean speed because the average travel tme weghts the average to the tme each vehcle spends n the defned roadway segment or space. 1 Example 1 Three vehcles are traversng a 1-km segment of a hghway and the followng observaton s made: Vehcle A: mn hr/km = 50 km/h Vehcle B: mn hr/km = 40 km/h Vehcle C: 1.7 mn hr/km = 35.3 km/h What s the average travel speed of the three vehcles? Soluton Average travel tme = = hr Average travel speed = 1/0.044 = kph. o Another way of defnng the "average speed" of a traffc stream s by fndng the tme mean speed (v,). Ths s the arthmetc mean of the measured speeds of all vehcles passng, say, a fxed roadsde pont durng a gven nterval of tme, n whch case, the ndvdual speeds are known as "spot" speeds. n = v v = 1 () n Where, v = spot speed

12 n = number of vehcle observed. v) Tme mean speed o The arthmetc average of speeds of vehcles observed passng a pont on a hghway; also referred to as the average spot speed. o The ndvdual speeds of vehcles passng a pont are recorded and averaged arthmetcally. Example Three vehcles pass a mle post at 50,40, and 35.3 mph, respectvely. What s the tme mean speed of the three vehcles? Soluton Average tme mean speed = n v = 1 v = = = mph t n 3 o Exhbt 7-1 shows a typcal relatonshp between tme mean and space mean speeds. o Space mean speed s always less than tme mean speed, but the dfference decreases as the absolute value of speed ncreases. 3 o Based on the statstcal analyss of observed data, ths relatonshp s useful because tme mean speeds often are easer to measure n the feld than space mean speeds. v) Free-flow speed o The average speed of vehcles on a gven faclty, measured under low-volume condtons, when drvers tend to drve at ther desred speed and are not constraned by control delay. 4

13 . Densty Densty s the number of vehcles (or pedestrans) occupyng a gven length of a lane or roadway at a partcular nstant. For the computatons n ths manual, densty s averaged over tme and s usually expressed as vehcles per klometer (veh/km) or passenger cars per klometer (pc/km). Drect measurement of densty n the feld s dffcult, requrng a vantage pont for photographng, vdeotapng, or observng sgnfcant lengths of hghway. Densty can be computed, however, from the average travel speed and flow rate, whch are measured more easly. Equaton (3) s used for undersaturated traffc condtons. q k = (3) v Where, q = Rate of flow v = Average travel speed (kph) k = Average densty (veh/km) 5 Example 3 A hghway segment wth a rate of flow of 1,000 veh/h and an average travel speed of 50 km/h would have a densty of Densty s a crtcal parameter for unnterrupted flow facltes because t characterzes the qualty of traffc operatons. It descrbes the proxmty of vehcles to one another and reflects the freedom to maneuver wthn the traffc stream. Roadway occupancy s frequently used as a surrogate for densty n control systems because t s easer to measure. Occupancy n space s the proporton of roadway length covered by vehcles, and occupancy n tme dentfes the proporton of tme a roadway cross secton s occuped by vehcles. If one could measure the lengths of a vehcles on a gven roadway secton and compute the rato, 6

14 R (L = vehcles lengths, D = roadway secton lengths) (4) = L D then R could be dvded by the average length of a vehcle to gve an estmate of the densty k..3 Headway and spacng Spacng s the dstance between successve vehcles n a traffc stream, measured from the same pont on each vehcle (e.g., front bumper, rear axle, etc.). spacng Headway s the tme between successve vehcles as they pass a pont on a lane or roadway, also measured from the same pont on each vehcle. These characterstcs are mcroscopc, snce they relate to ndvdual pars of vehcles wthn the traffc stream. 7 Wthn any traffc stream, both the spacng and the headway of ndvdual vehcles are dstrbuted over a range of values, generally related to the speed of the traffc stream and prevalng condtons. In the aggregate, these mcroscopc parameters relate to the macroscopc flow parameters of densty and flow rate. Spacng s a dstance, measured n meters. It can be determned drectly by measurng the dstance between common ponts on successve vehcles at a partcular nstant. Ths generally requres complex aeral photographc technques, so that spacng usually derves from other drect measurements. Both spacng and headway are related to speed, flow rate and densty. (5) (6) (7) 8

15 Example 4 Four vehcles, 5.5, 6.0, 6.4, and 6.7 meter long, are dstrbuted over a length of a freeway lane 15.4m long. What s the lane occupancy and densty? Soluton Occupancy, R = = = Average length of vehcles = = Densty, = 6.18 veh/km 6.15 Lane occupancy (LO) can also be descrbed as the rato of the tme that vehcles are present at a detecton staton n a traffc lane compared to the tme samplng. Where, LO t 0 = (8) T 0 t = total tme vehcle detector s occuped T = total observaton tme 9 Drecton of travel L L = Length of vehcle C C = Dstance between loops of detector Fgure 1.0 : Loop detector Fgure 1.0 llustrates the use of a detector n traffc engneerng work. Here L + C t 0 = (9) v s Where, L = average length of vehcle C = dstance between the loop of the detector. It s necessary to know the effectve length of a vehcles as measured by the detector n use to calculate lane accupancy. 30

16 .4 Categores of traffc flow Vehcle flow on transportaton facltes may be generally classfed nto two categores as shown n below Table : Types of transportaton facltes Characterstcs Unnterrupted flow Freeways Multlane hghways Two-lane hghways Interrupted flow Sgnalzed streets Unsgnalzed streets wth stop sgn Arterals Transts Pedestran walkways Bcycle paths It should be noted that unnterrupted and nterrupted flow are terms that descrbe the faclty and not the qualty of flow. A congested freeway where traffc s almost comng to a halt s stll classfed as an unnterrupted flow faclty, because the reason for congeston s nternal to the traffc stream Freeways, operate under the purest form of unnterrupted flow. 31 Multlane and two-lane hghways may also operate wth almost unnterrupted flow, partcularly n long segments between ponts of fxed nterruptons, such as segments where sgnal spacng exceed mles. Pedestran, bcycle and transt flow are generally consdered to be nteruppted, although unnterrupted flow condtons can occur for example, n a long busway, wthout stops. 1. Unnterrupted flow Relatonshps among basc parameters Exhbt 7- shows a generalzed representaton of these relatonshps, whch are the bass for the capacty analyss of unnterrupted-flow facltes. 3

17 The curves of Exhbt 7- llustrate several sgnfcant ponts. ) A zero flow rate occurs under two dfferent condtons. One s when there are no vehcles on the faclty densty s zero, and flow rate s zero. Speed s theoretcal for ths condton and would be selected by the frst drver (presumably at a hgh value). Ths speed s represented by Sf n the graphs. ) Densty becomes so hgh that all vehcles must stop. 33 The speed s zero, and the flow rate s zero, because there s no movement and vehcles cannot pass a pont on the roadway. The densty at whch all movement stops s called jam densty, denoted by Dj n the dagrams. Between these two extreme ponts, the dynamcs of traffc flow produce a maxmzng effect. As flow ncreases from zero, densty also ncreases, snce more vehcles are on the roadway. When ths happens, speed declnes because of the nteracton of vehcles. Ths declne s neglgble at low and medum denstes and flow rates. As densty ncreases, these generalzed curves suggest that speed decreases sgnfcantly before capacty s acheved. Capacty s reached when the product of densty and speed results n the maxmum flow rate. Ths condton s shown as optmum speed So (often called crtcal speed), optmum densty Do (sometmes referred to as crtcal densty), and maxmum flow vm. 34

18 The slope of any ray lne drawn from the orgn of the speed-flow curve to any pont on the curve represents densty. Smlarly, a ray lne n the flow densty graph represents speed. Exhbt 7- shows the average free-flow speed and speed at capacty, as well as optmum and jam denstes. 1.3 Road capacty Level of servce Roadway and traffc condtons, rangng from deal to forced flow, have been dvded nto sx level of servce for qualtatve evaluaton. (Fgure ) Fgure : schematc concept of relatonshp of levels of servce to operatng speed and volume/capacty rato. 35 For unnterrupted flow, the levels are defned as follows: LOS Descrpton A Free flow, low volumes and denstes, hgh speed. Drvers can mantan ther desred speeds wth lttle or delay. Stable flow, operatng speeds begnnng to be restrcted somewhat B by traffc condtons. Drvers stll have reasonable freedom to select ther speed. Sutable for rural desgn standards C Stable flow, but speeds and maneuverablty are more closely controlled by hgher volumes. Sutable for urban desgn standards. Approaches unstable flow, tolerable operatng speeds whch are, D however, consderably affected by operatng condtons. Drvers have lttle freedom by maneuver. E Unstable flow, wth yet lower operatng speeds and, perhaps, stoppages of momentary duraton. Volumes at or near capacty. F Forced flow, low volumes. Both speed and volumes can drop to zero. Stoppages may occur for short or long perods. These condtons usually result from queues of vehcles backng up from a restrcton downstream. 36

19 HIGHWAY AND FREEWAY CAPACITY Ths chapter descrbes the basc defntons and concepts relatng to capacty and level of servces. The hghway capacty manual (HCM)(TRB,000) s the standard reference work used n ths area. Several major types of transportaton facltes and road user categores are descrbed n the HCM: (a) Unnterrupted flow facltes. ) Freeways ) Multlane hghways ) Two-lane hghways (b) Interrupted flow facltes ) Sgnalzed ntersectons ) Unsgnalzed ntersectons ) Urban streets (c) Other road user ) Transt ) Pedestrans ) Bcycle The analyses of these facltes vary consderably. 37 However the detals presented n the HCM on transt, bcycles, and pedestrans focus on those aspects that nteract wth traffc usng the street/hghway. In general, the capacty of a faclty s the maxmum hourly rate at whch person/vehcle reasonably can be expected to traverse a pont or unform secton of a lane or roadway durng a gven perod under prevalng roadway, traffc, and control condton. (TRB, 000). Roadway condton refer to: (a) Type of faclty (b) Geometrc characterstc (c) Number of lanes (by drecton) (d) Lane and shoulder wdth, (e) Lateral clearances. (f) Desgn speed, (g) Horzontal and vertcal algnment and (h) Avalablty of queung space at ntersecton. Traffc condton refer to: (a) Dstrbuton of vehcle types usng faclty, (b) Amount and dstrbuton of traffc n avalable lanes of faclty and (c) Drectonal dstrbuton. Control condton refers to: (a) The types specfc desgn of control devces (eg. Traffc sgnal and ther tmng). (b) Traffc regulatons. 38

20 Level of servces (LOS) s qualtatve measure descrbng operatonal condton wthn a stream and ther percepton by motorst and/or passengers. Factor effectng LOS: 1. Speed,. Travel tme, 3. Freedom to maneuver, 4. Traffc nterrupton, comfort and convenent. The parameters that are selected to defne LOS for each faclty type called measures of effectveness (MOE). Basc Freeway capacty study A freeway s a dvded hghway faclty havng two or more lanes n each drecton for the exclusve use of traffc, wth full control of access and egress. In the hghway herarchy, the freeway s the only facltes that provdes completely unnterrupted flow. A freeway s composed of three subcomponents: 1. Basc freeway segment. Weavng areas 3. Ramp juncton. 39 Basc freeway segment (a) Outsde the nfluence of ramp or weavng maneuvers. Weavng area (b) Merge area followed by dverge (c) On ramp followed by off-ramp wth auxloary lane 40

21 Ramp juncton d) Isolated On-ramp e) Isolated Off-ramp USE OF THE HIGHWAY CAPACITY MANUAL FOR FREEWAY CAPACITY CALCULATION The determnaton of LOS for basc freeway secton generally nvolves three components: 1. Flow rates,. Free-flow speed and 3. Level of servces An equvalent passenger-car flow rate s calculated usng equaton (a) to allow for the effect of heavy vehcles and varaton of traffc flow durng the hour n the traffc stream. V v p = (a) PHF N f f HV p 41 Where, vp = 15-mn passenger-car flow rate (pc/h/ln) V = hourly volume (veh/h) PHF = peak hour factor N = number of lanes fhv = heavy-vehcle adjustment factors fp = drver populaton factors Freeway traffc volumes that nclude a mx of vehcle types must be adjusted to an equvalent flow rate expressed n passenger car per hour per lane. Adjustments for the presence of heavy vehcles n the traffc stream apply for trucks, buses, and recreatonal vehcles (RVs). The adjustment factors fhv, for heavy vehcles are computed usng the followng equaton: 1 f = (b) HV 1 + P ( E 1) + P ( E 1) T T R R where, fhv = heavy-vehcle adjustment factor ET,ER = passenger-car equvalent for trucks/buses and recreatonal vehcles n the traffc stream respectvely. PT,PR = proporton of trucks/buses and RVs n the traffc stream, respectvely 4

22 In many cases where recreatonal vehcles represent a small percentage, t may convenent to consder all heavy vehcles as typcal trucks. (1) Determnaton of free flow speed The mean speed of passenger cars measured under low to moderate flows, up to 1300pc/h/ln, s the free-flow speed (FFS) freeway. FFS can be measured drectly n the feld or estmated usng gudelnes provded based on HCM. For a feld measurement, the study should be conducted at representatve locaton wthn the secton beng evaluated. A systematc sample of at least 100 cars across all lanes durng offpeak hours could provde a satsfactory measurement. If feld-measured data are used no subsequent adjustments are to the free-flow speed as t reflects the net effect of all condtons at the study ste. If feld measurement of free flow speed s not possble, t can be estmated usng the followng equaton: 43 FFS = BFFS f f f f (c) LW LC N I D where, FFS = estmated free-flow speed (mph) BFFS = Base free-flow speed, 70 mph (urban) or 75 mph (rural) flw = adjustment for lane wdth from Table 7-6 (mph) flc = adjustment for rght-shoulder lateral clearance from Table 7-7 (mph) fn = adjustment for number of lanes from Table 7 8 (m/h) fid = adjustment for nterchange densty for Table 7-9 (m/h). The base free-flow speed (70 or 75 mph) s reduced by the adjustment factors n Tables 7-6 to 7-9 when roadway condtons dffer from base condtons. No adjustment factors are recommended for left-shoulder clearance of less than ft. The adjustments n Table 7-8 are based exclusvely on data collected on urban and suburban freeways. Rural freeways typcally carry two lanes n each drecton. () Determnaton of Level Of Servces The level of servce on a basc freeway secton can be determned drectly from fgure 7-3 usng free-flow speed and the flow rate. The followng steps llustrate the procedure: 1. Defne and segment the freeway secton as approprate. 44

23 . On the bass of measured or estmated free-flow speed, construct a typcal curve as shown n Fgure 7-3. The curve should ntersect the y-axs at the free-flow speed. 3. Enter the flow rate, vp, on the horzontal axs and read up to the curve dentfed n step and determne the average passengercar and level of servce correspondng to the pont. 4. Determne the densty of flow as follows: v p D = (d) S pc Where, D = densty (pc/m/ln) vp = flow rate (pc/h/ln) Spc = average passenger-car speed (m/h) The level of servce can also be determned usng the densty ranges provded n Table Table 7-1 LOS Crtera For Basc Freeway Segments LOS Crtera A B C D E FFS = 75 m/h Maxmum densty (pc/m/ln) Mnmum speed (m/h) Maxmum v/c Maxmum servce flow rate (pc/h/ln) FFS = 70 m/h Maxmum densty (pc/m/ln) Mnmum speed (m/h) Maxmum v/c Maxmum servce flow rate (pc/h/ln) FFS = 65 m/h Maxmum densty (pc/m/ln) Mnmum speed (m/h) Maxmum v/c Maxmum servce flow rate (pc/h/ln) FFS = 60 m/h Maxmum densty (pc/m/ln) Mnmum speed (m/h) Maxmum v/c Maxmum servce flow rate (pc/h/ln) FFS = 55m/h Maxmum densty (pc/m/ln) Mnmum speed (m/h) Maxmum v/c Maxmum servce flow rate (pc/h/ln) Note : the exact mathematcal relatonshp between densty and v/c has not always been mantaned at LOS boundares because of the use of rounded values. Densty s the prmary determnant of LOS. The speed crteron s the speed at maxmum densty for a gven LOS. sources: TRB,

24 Applcaton Procedures for the applcaton of the Hghway Capacty Manual (TRB,000) to freeway problems fall under three categores: 1. Operatonal analyss,. Desgn, and 3. Plannng. Table 7- Passenger-Car Equvalent on Extended Freeway Segments Factor Type of Terran Level Rollng Mountanous ET (trucks and buses) ER (RVs) Source : TRB, Table 7-3 Passenger-Car Equvalent for Trucks and Buses on Upgrades Upgrade (%) Length (m) ET Percentage of Trucks and Buses < All > > > > > > > > > > > > > > > > > > > > > > > > >

25 Table 7-4 Passenger-Car Equvalent for RVs on Upgrades Upgrade (%) Length (m) ER Percentage of RVs All > >0.50 > > >0.50 > > >0.50 > > > Table 7-5 Passenger-Car Equvalent for Trucks nad Buses on Downgrades Downgrade (%) > > 5 6 > 5 6 > 6 > 6 Length (m) All 4 > 4 4 > 4 4 > 4 ET Percentage of Trucks Table 7 6 Adjustments for Lane Wdth Lane wdth (ft) Reducton n Free-flow Speed, flw (mph) Table 7-7 Adjustments for Rght-Shoulder Lateral Clearance Rght-Shoulder Later Clearance (ft) Reducton n Free-Flow Speed, flc (mph) Lanes n One Drecton Table 7-8 Adjustments for Number of Lanes Number of Lanes (One Drecton) Reducton n Free-Flow Speed, fn (mph) Note: For all rural freeway segments, fn s 0.0 Source : TRB, Table 7-9 Adjustments for Interchange Densty Interchanges per Mle Reducton n Free-Flow Speed, fid (mph)