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2 Lane 2 way Rural hwys

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

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2 Lane 2 way Rural hwys

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  1. 2 Lane 2 way Rural hwys CTC-340

  2. HMWK • CH 16 # 1, 3, 6 use HCS+ software

  3. 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

  4. 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

  5. Highway Classes • Class 3 – serve as main streets – reduced speed limit, no passing, more roadside driveways, and unsignalized junctions • F 16.1

  6. 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

  7. 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

  8. Passing • Heavy vehicles have a major impact on roadway capacity • Platoon formation behind slow moving vehicles is common

  9. 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)

  10. 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

  11. 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

  12. 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

  13. Capacity Analysis • density is not meaningful since capacity is measured as the total of both directions of travel

  14. 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

  15. Capacity & LOS • LOS • Percent FFS – comparison of prevailing speed to FFS • T16.4 - LOS criteria

  16. 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

  17. 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

  18. 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

  19. Analysis • 2 types • Single directional analysis of general extended sections in level or rolling terrain (>= 2 mi) • Single direction analysis of specific grades

  20. 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

  21. 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

  22. Analysis • Find FFS • 2 lane rural hwy, 10.5’ lanes, 4’shoulders, 20 access points/mi, BFFS = 55mph

  23. 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)

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

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