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 –d 1 +d 2 +d 3 +d 4 d 1 = Distance traveled during PR time + initial accel to point of encroachment into left lane d 1 = 1.47t 1 *(S-m+at1/2) –t 1 = 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 d 2 = Distance traveled while passing vehicle is in left lane d 2 = 1.47St 2 –t 2 = time passing vehicle occupies left lane d 3 = Distance between passing vehicle at end of maneuver and opposing vehicle d 3 = 100 – 300 feet d 4 = Distance traversed by opposing vehicle for 2/3 time the passing vehicle occupies the left lane or 2/3 d 2 d 4 = 2/3 d 2

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 = S m + 0.00776(v f /f HV ) S m = mean spd of sample, v f = observed flow rate

21. Analysis FFS = BFFS – f LS – f A BFFS can be taken as –(Cl 1 – 55-65mph, Cl 2 – 45 – 50mph, Cl 3 – 40 -50mph) –Design spd –f LS = lane & shoulder width T 16.5 –f A = 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*f HV *f G ) –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 f HV = 1/(1+P T (E T -1) + P R (E R -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+P TC *P T (E TC -1) +(1-P TC )*P T (E T - 1)+P R (E R -1)) T16.14 P TC = % of trucks at crawl speed

27. Estimating Average Travel Speed ATS d = FFS – 0.00776(v d +v o )-f npA v d = demand flow rate in direction of analysis v o = demand flow rate in opposing direction of analysis f npA = adjustment to ATS for no passing zones in study area – T 16.15

28. Determining PTSF PTSF d = BPTSF d +f npP (v d /(v d +v o )) BPTSF d = 100(1-exp(av d b )) f npP = 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 ATS d, PTSF d 2) Find the 4 subsegments of the segment –L u = subsegment upstream of passing lane –L pl = subsegment that is the passing lane including tapers –L de = effective downstream length of the passing lane T14.23 –L d = 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 –L de 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 –L d = L – (L u +L pl +L de ) –L de, L d 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 L d = L u = L de f pl = are selected from T14.26