2 Lane 2 way Rural hwys

CTC-340

HMWK

• CH 16 # 1, 3, 6 use HCS+ software

2 lane roads

• 80% of nations 4,000,000 miles of paved roads are rural– 85% of these are 2 lane

• Only roadway link where traffic in 1 direction has a direct impact on traffic in other direction

• Wide set of geometric standards and operating conditions

• They provide:– mobility - county seat to county seat– access - to land

Highway Classes

– Class 1 motorists expect to travel at high speeds – major intercity highways, primary arterials, collectors

• Serve mobility needs

– Class 2 – access routes, scenic & recreational routes, routes through rugged terrain

• Serve access needs

– Class 3 - – F 16.1

Highway Classes

– Class 3 – serve as main streets – reduced speed limit, no passing, more roadside driveways, and unsignalized junctions

– F 16.1

Design Standards

• Set by AASHTO standards

• Most important design factor = design speed

• T16.1 shows recommended design speeds for different facility types

• F 16.2 recommended design criteria for max grades

Passing

• unique feature – using opposing lane to pass vehicles – directional flows interact

• as flow increases in one direction so does the desire to pass

• but flow is also increasing in the opposing direction which reduces the opportunities to pass

– capacity is based on both directions of travel

•

Passing

• Heavy vehicles have a major impact on roadway capacity – Platoon formation behind slow moving

vehicles is common

Passing Sight Distance– Must maintain safe stopping sight distance over entire

highway– Passing is an important part of the capacity of a 2 lane

road– Must know required passing sight distance

– d1 +d2 +d3 +d4

• d1 = Distance traveled during PR time + initial accel to point of encroachment into left lane

• d1 = 1.47t1*(S-m+at1/2)

– t1 = PR Time (sec); S = speed of passing vehicle (mph); m = difference between spd of passing and passed veh (mph); a = accel of passing veh (mph/s)

Passing Sight Distance• d2 = Distance traveled while passing vehicle is in left

lane

• d2 = 1.47St2

– t2 = time passing vehicle occupies left lane

• d3 = Distance between passing vehicle at end of maneuver and opposing vehicle

• d3 = 100 – 300 feet

• d4 = Distance traversed by opposing vehicle for 2/3 time the passing vehicle occupies the left lane or 2/3 d2

• d4 = 2/3 d2

Passing Sight Distance– Assumptions

• Spd of Passing car is 10mph greater than passed car• Acceleration rate = 1.4 – 1.5 mph/s• PR times = 3.6 – 4.5 s• Time in left lane = 9.3 – 11.3 s – based upon spd

parameters• Clearance distance – lower spds use lower range• Minimum values for PSD T 16.2• Warrant PSD T16.3 – used for posting NO PASSING

signs• Want to maximize passing zones

Capacity & LOS

– Models based on simulation & limited field study – hard to find 2 lane roads @ capacity

• Capacity– Max under base conditions = 3200pc/h total– 1700 pc/h in 1 direction– Base conditions

• 12 foot lanes, 6 foot usable shoulder, level terrain, no HV, 100% PSD available, 50/50 traffic split, no traffic interruptions

Capacity Analysis

• density is not meaningful since capacity is measured as the total of both directions of

travel

Capacity & LOS

• LOS– 3 MOE

• Average travel speed (ATS)– Average spd of all vehicles traversing the segment for a

specified time period (peak 15 minutes)– Can be both directions or 1 depending on analysis

• % time spent following (PTSF)– Aggregate time that all drivers spend in queues, unable

to pass, with speed restricted by queue leader– % of vehicles following at headways <= 3.0 sec

Capacity & LOS

• LOS– Percent FFS – comparison of prevailing

speed to FFS– T16.4 - LOS criteria

Capacity & LOS

• Class 1 use ATF & PTSF• Class 2 uses only PTSF – not meant for

mobility• Class 3 use PFFS

– Operational deterioration occurs at a relatively low v/c ratio

• Only roadway where this occurs

– Leads to improper passing and accidents• Safety issues will demand that road be reconfigured

LOS

• LOS deteriorates rapidly at low flows on 2 lane roads– as volumes increase the passing

opportunities decrease– F 16.4

• LOS A - D cover 0 - 1600 pcph

• LOS E covers 1600 - 3200 pcph

Narrow Lanes and shoulders

• look at usable shoulder

• on 2 lane roads shoulder functions as storage area for breakdowns, slow vehicle lane to allow queued vehicles to pass

• small shoulders have a large impact on capacity

Analysis

• 2 types– Single directional analysis of general

extended sections in level or rolling terrain (>= 2 mi)

– Single direction analysis of specific grades

Analysis

• FFS– Should be field info

• Representative sample of 100 or more vehicles• Total 2 way traffic flow >= 200pc/h• All vehicle speeds observed or systematic

sampling• Sample should mirror analysis type• If Total 2 way traffic flow >= 200pc/h then

• FFS = Sm + 0.00776(vf/fHV)

• Sm = mean spd of sample, vf = observed flow rate

Analysis

• FFS = BFFS – fLS – fA

• BFFS can be taken as – (Cl 1 – 55-65mph, Cl 2 – 45 – 50mph, Cl 3 –

40 -50mph)– Design spd

– fLS = lane & shoulder width T 16.5

– fA = Access point density T 16.6

– Can be taken as Spd Lmt + 5-7mph

Analysis

– Find FFS– 2 lane rural hwy, 10.5’ lanes, 4’shoulders, 20

access points/mi, BFFS = 55mph

Demand Flow Rate

• v = V/(PHF*fHV*fG)

– 2 conversions based on 2 different sets of adjustments (ATS & PTSF)

– need to convert both subject & opposing volumes

– Determination of demand flow rates are iterative (only 1 iteration)

Grade adjustment factor

• Depends on type of grade

• General terrain for ATS & PTSF T16.7

• Specific Upgrades for ATS T16.8

• Specific Upgrades for PTSF T16.9

• Specific downgrades for ATS & PTSF T16.10

HV Factor

• fHV = 1/(1+PT(ET-1) + PR(ER-1))

• T16.10 -gen’l terrain & specific downgrades for ATS & PTSF

• T16.11 & 16.12 - specific upgrades for ATS

• T16.13 - specific upgrades for PTSF

• T16.10 – specific downgrades

HV Factor

• Truck crawl speed– May need to go down hill slowly to maintain

control

• fHV = 1/(1+PTC*PT(ETC-1) +(1-PTC)*PT(ET-1)+PR(ER-1))

• T16.14

• PTC = % of trucks at crawl speed

Estimating Average Travel Speed

• ATSd = FFS – 0.00776(vd+vo)-fnpA

• vd= demand flow rate in direction of analysis

• vo= demand flow rate in opposing direction of analysis

• fnpA = adjustment to ATS for no passing zones in study area – T 16.15

Determining PTSF

• PTSFd = BPTSFd +fnpP(vd/(vd+vo))

• BPTSFd = 100(1-exp(avdb))

• fnpP= adjustment to PTSF for no passing zones in the study area T 16.16

• a,b calibration constants T16.17

• LOS T16.4

Impact of Passing lanes

• Passing lanes allow platoon to break up– Can avoid long platoons behind a vehicle– Steps

• 1) assume no passing and find ATSd, PTSFd

• 2) Find the 4 subsegments of the segment– Lu = subsegment upstream of passing lane

– Lpl = subsegment that is the passing lane including tapers

– Lde = effective downstream length of the passing lane T14.23

– Ld = subsegment downstream of the effective downstream length of the passing lane

– Sum is equal to the total length of the directional segment

Impact of Passing lanes

– Lde reflects observations that the passing lane improves the ATS and the PTSF downstream of the passing lane

– Effective distance varies depending on demand flow rate and whether ATS or PTSF is involved

– Effective distance does not include the passing lane

– Ld = L – (Lu +Lpl +Lde)

– Lde , Ld will differ for ATS & PTSF

Impact of Climbing Lanes

– Added to avoid long queues– Warranted when

• Directional flow rate on the upgrade exceeds 200 vph

• Directional flow rate for trucks on the upgrade exceeds 20 vph

• Any of the following conditions apply– A speed reduction of 10mph for a typical heavy truck– LOS E or F exists on upgrade– LOS on upgrade is 2+ LOS below existing LOS on the

approach

Impact of Climbing Lanes

– ATS & PTSF values may be modified to take climbing lane into account except that

• Ld = Lu = Lde

• fpl = are selected from T14.26