Exam 2. Two-Lane Highway Capacity and LOS Analysis. Two-lane Highway Characteristics. LOS Considerations 10/24/2009

Transcription

1 Two-Lane Highway Capacity and LOS Analysis CE322 Transportation Engineering Ahmed Abdel-Rahim, Ph.D., P.E. Exam 2 Wednesday 10/28/2009 No class Monday 10/24/2009 Open-Book Open notes Exam 6 Problems Traffic Stream Models Queuing theory LOS (Freeway, Multi-Lane Highway, and Two-Lane Highways) Class website (Class notes, HWs, Suggested Problems) Two-lane Highway Characteristics One travel lane in each direction, and typically undivided Used for lower traffic volume situations Generally have free-flow speeds ranging between 45 and 65 mi/hr Have wide-ranging design standards May have occasional traffic signals or even stop-control In order to pass a vehicle, the opposing lane must be used (except for where passing lanes are provided) LOS Considerations Unlike the procedures for freeways and multilane highways, both directions must be considered in the level of service analysis, even if only interested in one direction Traffic in the opposing direction influences the passing opportunities for vehicles in the direction of interest Terrain can have a more significant effect due to restrictions on passing-sight distance 1

2 Service Measure(s) The service measure or measures is dictated by the functional classification of the two-lane highway segment. There are two classes defined for two-lane highways by the HCM. Classifications Class I: two-lane highways on which motorists expect to travel at high speeds (e.g., inter-city routes, primary arterials connecting major traffic generators, daily commuter routes, and primary links in state or national highway networks) and, Class II: two-lane highways on which motorists do not necessarily expect to travel at high speeds (e.g., scenic or recreational routes or pass through rugged terrain). These routes also generally serve shorter trip lengths than Class I routes. Level of Service Class I LOS Criteria LOS Percent Time-Spent- Average Travel Speed (ATS) Following (PTSF) mi/h km/h A 35 > 55 > 90 B 50 > 50 > 80 C 65 > 45 > 70 D 80 > 40 > 60 E > Class II LOS Criteria Table 6.21 Table 6.22 Percent Time-Spent-Following LOS (PTSF) A 40 B 55 C 70 D 85 E > 85 LOS Determination Procedure 1. Establish Base Conditions and Capacity 2. Estimate or Measure Free-Flow Speed 3. Calculate Analysis Flow Rate 4. Calculate Service Measure(s) and Determine LOS 2

3 Base Conditions Minimum lane widths of 12 ft Minimum shoulder widths of 6 ft 0% no-passing zones Traffic stream consisting of passenger cars only No direct access points along the roadway No impediments to through traffic due to traffic control or turning vehicles Level terrain, with grades no greater than 2% A 50/50 directional split of traffic Capacity Capacity for both directions combined is 3200 pc/h Capacity for a single direction is 1700 pc/h Measuring Free-Flow Speed Estimating Free-Flow Speed Free-flow speed is the mean speed of all vehicles measured under flow rates up to 200 pc/h (2-way). If speeds are measured under a flow rate > 200, use following adjustment equation: FFS = BFFS fls fa Eq V f FFS SFM f FFS = estimated free-flow speed in mi/h, S FM = mean speed of traffic measured in the field in mi/h, V f = observed flow rate, in veh/h, for the period when field data were obtained, and f HV = heavy-vehicle adjustment factor as determined by Eq HV Eq. 6.9 FFS = free-flow speed (mi/h), BFFS = estimated free-flow speed for base conditions, fls = adjustment for lane and shoulder width from Table 6.16 (mi/h), and fa = adjustment for access-point frequency from Table 6.15 (mi/h). 3

4 Lane Width and Shoulder Width Access Frequency Table 6.16 Table 6.15 Same as for multilane highways Calculate Analysis Flow Rate Grade Adjustment 1. Adjust for PHF 2. Adjust for grade 3. Adjust for vehicle mix V v p PHF f f v p = 15-min passenger-car equivalent flow rate (pc/h), V = hourly volume (veh/h), PHF = peak-hour factor, f G = grade adjustment factor, and f HV = heavy-vehicle adjustment factor. G HV Eq The grade adjustment factor accounts for the effect of terrain on the traffic flow. Table 6.17 Range of Two-Way Flow Average travel speed (ATS) Percent time-spentfollowing (PTSF) Rates (pc/h) Terrain Type Terrain Type Level Rolling Level Rolling > >

5 PCEs for Extended General Terrain Segments PCE values can be used for general terrain segments, just as for freeways and multilane highways. However, PCE values are different, and only given for level and rolling terrain classifications. Specific grade PCE values must be used for mountainous terrain. PCEs for Extended General Terrain Segments Table 6.18 Vehicle Range of Two-Way Average travel speed (ATS) Percent time-spentfollowing (PTSF) Type Flow Rates Terrain Type Terrain Type (pc/h) Level Rolling Level Rolling Trucks and Buses, ET > > RVs, ER > > Notes PCE values for specific grades not covered in this book. Refer to the HCM. Tables 6.17 and 6.18 are in terms of pc/h. However, until Eq is applied, flow rate is in units of veh/h. This can result in the need to use an iterative approach to arrive at the correct adjustment factors and final analysis flow rate. See Example 6.6. fhv and PHF calculated with same equations as for freeways and multilane highways. Calculate Service Measure(s) If the highway is Class I, both ATS and PTSF must be calculated. If the highway is Class II, only PTSF needs to be calculated. 5

6 Average Travel Speed Adjustment for effect of no-passing zones ATS FFS v p f np Eq ATS = estimated average travel speed in mi/h, for both directions of travel combined, FFS = free-flow speed in mi/h, as measured in the field and possibly adjusted by Eq. 6.9 or estimated from Eq. 6.10, v p = analysis flow rate in pc/h, as calculated from Eq. 6.11, and f np = adjustment factor for the percentage of no-passing zones, which is determined from Table Table 6.19 Percent Time-Spent-Following Combined Adjustment for effect of Directional Distribution and no-passing zones PTSF BPTSF / f d np Eq PTSF = percent time-spent-following for both directions of travel combined, BPTSF = base percent time-spent-following for both directions of travel combined, and f d/np = adjustment factor for the combined effect of the directional distribution of traffic and the percentage of no-passing zones (from Table 6.20) v p BPTSF e Eq Table

7 Determine LOS Use Table 6.21 for Class I Use Table 6.22 for Class II Practice Problems