Freeways and Multilane Highways

CTC-340

HMWK

• Ch 13 Read Section 13.7

• Ch 13 # 2, 3

• Ch 14# 1, 4, 5, 6, 8

Capacity

• HCM2000 definition– The capacity of a facility is the maximum

hourly rate at which persons or vehicles can reasonably be expected to traverse a point on a uniform section of a lane or roadway during a given period of time under prevailing roadway, traffic and control conditions

LOS

• LOS– Table 13-1 MOEs

• Service Flow– Max flow for a given LOS

• v/c ratio – ratio of flow to capacity of facility– Should be <1– If v/c > 1 => congestion

Freeways

• Pure uninterrupted flow– Cars enter and exit via ramps– No at grade intersections, driveways etc

• Based on number of lanes

Multilane Highways

• Only uninterrupted flow if signals at least 2 miles apart

• Classified by lanes and median type

• F 14.1

LOS

• MOE is density – pc/ln/mi

• Capacity = 45pc/ln/mi– Freeways – multilane hwys => 40 - 45pc/ln/mi– F 14.2, 14.3, T 14.1

• LOS pg 286 - 290

Freeway Analysis

• F14.2 & 14.3

• Operational Analysis• How is the existing facility operating

• vp = V/(PHF*N* fHV * fp ) = pc/hr/ln

• vp is the demand flow rate under ideal conditions

• With vp can find FFS from curves F 14.2

• fp= user population

• If V = 4600vph, PHF = 0.90, N =3, fHV =0.87, fp = 1

• What is LOS• v= 1958pc/h/ln => LOS =? F 14.2 &3

• Service Flow Rate & Service Vol Analysis

• SFi = MSFi*N*fHV*fp

– Can find service flow for each LOS i• Use Tables 14.3 or 14.4 to get MSF

• Design Analysis– Used to determine # of lanes needed for

freeway – To determine the number of lanes

– N = DDHV/(PHF*MSF*fHV*fp)

– Iterative process– Number of lanes depends on design speed– Design speed depends on # of lanes

Free Flow Speed - Freeways

• Free Flow Speed– Found when volume < 1000vph/l– FFS = BFFS-fLW - fLC – 3.22TRD^0.84– fLW – lane width adjustment T14.5– fLC – lateral clearance T14.6

• 6’ on right side 2’ on left• Factor only takes right side into account

– TRD – total ramp density T14.8• Total ramps within 3 miles of midpoint of study area/6miles

Multilane Highways

– FFS = BFFS-fLW - fLC - fM – fA

– fLW – lane width adjustment T14.5– fLC – lateral clearance T14.7– Greatest LC = 6 feet on both sides

– Undivided Hwys – no median side clearance – taken into account with fM – assume 6’ clearance

– TWLTL – assume 6’ clearance– Divided hwy – based on median, dist to opposing traffic,

periodic objects(light poles) never more than 6’

– fM –median type T 14.8• Undivided, TWLTL, divided

– fID – access points T14.9• Driveways or roadways per mile on the right hand side - only

include active intersections

Multilane Highways

– BFFS – use 60mph if no data available for rural and suburban multilane sites

• Can estimate speed limit with BFFS• 7mph higher than spd limits 40 – 45mph• 5mph higher than spd limits 50 – 55mph

Example

• Old 6 lane freeway – 11’ lanes, barriers on right side 2’ from pavement edge, TRD = 3ramps/mile– FFS = 75.4- fLW - fLC – 3.22TRD^0.84– fLW – lane width adjustment T14.5– fLC – lateral clearance T14.6

• 2’ on right side 6’ on left• Factor only takes right side into account

Example

• Old 6 lane freeway – 11’ lanes, barriers on right side 2’ from pavement edge, TRD = 3 ramps/mile– FFS = 75.4- fLW - fLC – 3.22TRD^0.84– fLW – 1.9 mph T14.5– fLC – 1.6 mph T14.6– TRD = 3

• FFS = 75.4 – 1.9 – 1.6 – 3.22*3^.84= 64.1

Example

• 4 lane suburban multilane highway – 12’ lanes, barriers on right side 2’ from pavement edge, TWLTL, 30 access points/mile posted spd lmt = 50 mph– FFS = BFFS-fLW - fLC - fM – fA

– fLW – 0.0 mph T14.5– fLC – 0.9 mph T14.7 total LC = 6+2 = 8’– fM –0.0 mph T 14.8– fID – 7.5 mph T14.9

• FFS = 55 – 0 – 0.9 – 0 – 7.5 = 46.6• 55 mph 50 + 5 mph for BFFS

Heavy Vehicle Factor

• Heavy Vehicle = any vehicle with more than 4 tires– Trucks and buses

• Have similar characteristics• Based on a typical mix of trucks• 150#/hp

– RVs• Privately owned, self contained or pulled• 75 -100 #/hp

Passenger Car Equivalents

• # of cars displaced by the heavy vehicle

• ET = PCE for trucks and buses

• ER = PCE for RVs

• Increases volume to account for HVs

• EX: 1000 vehicles 10% trucks, 2% RV– ET = 2.5 ER = 2.0

• 1000 * 0.1*2.5 = 250pce/hr

• 1000*0.02*2 = 40 pce/hr

• 1000*0.88 = 880 pce/hr

• Total pce = 1170 pce/hr– Traffic stream acts like it has 1170 vph on it

– fHV = vph/(pce/hr) always <= 1.0

– fHV = 1000/1170 = 0.85

• fHV = 1/(1+PT(ET – 1) +PR(ER – 1))

• Extended Freeway & Multilane Sections– Long section = single section where no grade

of 3% or greater is longer than 0.25 miles and if no grade of less than 3% is longer than 0.5 miles

Terrain

• Level terrain – short grades less than 2%– HVs can maintain same speed as cars

• Rolling terrain - HV speed substantially below pc speeds but do not get to crawl speed for extended periods

• Mountainous terrain – HV operate at crawl speed for extended periods or frequently– Rare– T14.11

Specific Grades

• Any grade <3% longer than 0.5 miles or >3% and longer than 0.25 mile is a specific grade

• T14.12, T14.13

• Trucks on Downgrades T14.14– RVs same as level sections

Composite Grades

• Series of grades – Want to get the equivalent uniform grade for

pce– Average grade technique

• Find total rise divide by total run• Good when all grades <4% and < 4000ft length

– Composite grade technique• Use truck performance curves to determine

equivalent length of grade

Driver Population factor

• Based on the number of drivers not familiar with the road.– Range from 1 – 0.85– Usually use 1 unless a field study has been

done or it is a highly recreational area