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<p>Chapter Six</p> <p>12</p> <p>Chapter 6</p> <p>Highway Capacity and Level of Service Analysis</p> <p> Seek to measure Highway Performance (Congestion). Needed for: Highway construction</p> <p> Congestion relief</p> <p>Problems:</p> <p> Wide variation of highway conditions:</p> <p> Different roadway types (Freeway, multilane, etc.)</p> <p> Number of lanes and width of lanes</p> <p> Vehicle mix (cars vs. trucks)</p> <p> Shoulder widths</p> <p> Temporal variation in traffic flow over the day and over the peak hour</p> <p>6.1 LEVEL OF SERVICE CONCEPT</p> <p> The level of service provides a qualitative ranking of the traffic operational conditions experienced by users of a facility Highway Capacity Manual defines the LOS categories for freeways and multilane highways as follows:</p> <p>Level of Service Afree-flow conditions (traffic operates at free flow speeds as defined in Chapter 5). Individual users are virtually unaffected by the presence of others in the traffic stream. Freedom to select desired speeds and to maneuver within the traffic stream is extremely high. The general level of comfort and convenience provided to drivers is excellent.</p> <p>Level of Service Ballows speeds at or near free-flow speeds, but the presence of other users in the traffic stream begins to be noticeable. Freedom to select desired speeds is relatively unaffected, but there is a slight decline in the freedom to maneuver within the traffic stream relative to LOS A.</p> <p>Level of Service Cspeeds at or near free-flow speeds, but the freedom to maneuver is noticeably restricted (lane changes require careful attention on the part of drivers). The general level of comfort and convenience declines significantly at this level. Disruptions in the traffic stream, such as an incident (for example, vehicular accident or disablement), can result in significant queue formation and vehicular delay. In contrast, the effect of incidents at LOS A or LOS B are minimal, and cause only minor delay in the immediate vicinity of the event.</p> <p>Level of Service Dconditions where speeds begin to decline slightly with increasing flow. The freedom to maneuver becomes more restricted and drivers experience reductions in physical and psychological comfort. Incidents can generate lengthy queues because the higher density associated with this LOS provides little space to absorb disruption in the traffic flow.</p> <p>Level of Service ELOS E represents operating conditions at or near the roadways capacity. Even minor disruptions to the traffic stream, such as vehicles entering from a ramp or vehicles changing lanes, can cause delays as other vehicles give way to allow such maneuvers. In general, maneuverability is extremely limited and drivers experience considerable physical and psychological discomfort.</p> <p>Level of Service Fdescribes a breakdown in vehicular flow. Queues form quickly behind points in the roadway where the arrival flow rate temporarily exceeds the departure rate, as determined by the roadways capacity (Chapter 5). Vehicles typically operate at low speeds in these conditions and are often required to come to a complete stop, usually in a cyclic fashion. The cyclic formation and dissipation of queues is a key characterization of LOS F.</p> <p>LOS A</p> <p>LOS B</p> <p>LOS C</p> <p>LOS D</p> <p>LOS E</p> <p>LOS F6.2 LEVEL OF SERVICE DETERMINATION</p> <p>6.2.1 Base Conditions lane widths, lateral clearances, access frequency (non access controlled highways), terrain; traffic stream conditions such as the effects of heavy vehicles (large trucks, buses and RVs), and driver population characteristics.Process:1. Free-Flow SpeedThe free-flow speed (FFS) is the mean speed of traffic as measured when flow rates are low to moderate (specific values are given under the individual sections for each roadway type). 2. Analysis Flow Rate</p> <p>The highest volume in a 24-hour period (the peak-hour volume) is used for V in traffic analysis computations. However, this hourly volume is adjusted for:</p> <p> The temporal variation of traffic demand within the analysis hour, the impacts due to heavy vehicles and, characteristics of the driving population.3. Determine LOS</p> <p>6.3 BASIC FREEWAY SEGMENTS</p> <p>6.3.1 Base Conditions and Capacity</p> <p> 3.6 m (12-ft) (minimum lane widths,</p> <p> 1.8 m (6-ft) minimum right-shoulder clearance between edge of travel lane and objects (utility poles, retaining walls, etc.) that influence driver behavior,</p> <p> 0.6 m (2-ft) minimum median lateral clearance,</p> <p> only passenger cars in the traffic stream,</p> <p> 5 or more lanes in each travel direction (urban areas only),</p> <p> 3 km (2-mi) or greater interchange spacing,</p> <p> level terrain (no grades greater than 2%), and</p> <p> a driver population of mostly familiar roadway users.</p> <p>Capacity:Table 6.2 The Relationship Between Free-Flow Speed and Capacity on Basic Freeway SegmentsFree-Flow Speed(mi/h)Capacity(pc/h/ln)Free-Flow Speed(km/h)Capacity(pc/h/ln)</p> <p>7524001202400</p> <p>7024001102350</p> <p>6523501002300</p> <p>602300902250</p> <p>552250</p> <p>6.3.2 Service Measure</p> <p> The service measure for basic freeway segments is density. Density, as discussed in Chapter 5, is typically measured in terms of passenger cars per mile per lane (pc/mi/ln) (pc/km/ln), and therefore provides a good measure of the relative mobility of individual vehicles in the traffic stream. Recalling Eq. 5.14 from Chapter 5,</p> <p>q = uk</p> <p>(6.1)Where:</p> <p>q = flow in veh/h, u = speed in mi/h (km/h) and, k = density in veh/mi (veh/km).</p> <p> So density is therefore calculated as flow divided by speed. Once the flow and speed values have been determined according to the given conditions, a density can be calculated and then referenced in Table 6.1 or Fig. 6.2 to arrive at a level of service for the freeway segment.</p> <p>Table 6.1 LOS Criteria for Basic Freeway SegmentsCriteriaLOS</p> <p>ABCDE</p> <p>FFS = 75 mi/h</p> <p>Maximum density (pc/mi/ln)1118263545</p> <p>Average speed (mi/h)75.074.870.662.253.3</p> <p>Maximum v/c0.340.560.760.901.00</p> <p>Maximum service flow rate (pc/h/ln)8201350183021702400</p> <p>FFS = 70 mi/h</p> <p>Maximum density (pc/mi/ln)1118263545</p> <p>Average speed (mi/h)</p> <p>Maximum v/c0.320.530.740.901.00</p> <p>Maximum service flow rate (pc/h/ln)7701260177021502400</p> <p>FFS = 65 mi/h</p> <p>Maximum density (pc/mi/ln)1118263545</p> <p>Average speed (mi/h)</p> <p>Maximum v/c0.300.500.710.891.00</p> <p>Maximum service flow rate (pc/h/ln)7101170168020902350</p> <p>FFS = 60 mi/h</p> <p>Maximum density (pc/mi/ln)1118263545</p> <p>Average speed (mi/h)</p> <p>Maximum v/c0.290.470.680.881.00</p> <p>Maximum service flow rate (pc/h/ln)6601080156020202300</p> <p>FFS = 55 mi/h</p> <p>Maximum density (pc/mi/ln)1118263545</p> <p>Average speed (mi/h)</p> <p>Maximum v/c0.270.440.640.851.00</p> <p>Maximum service flow rate (pc/h/ln)600990143019102250</p> <p>Note: The exact mathematical relationship between density and v/c has not always been maintained at LOS boundaries because of the use of rounded values. Density is the primary determinant of LOS. The speed criterion is the speed at maximum density for a given LOS. LOS F is characterized by highly unstable and variable traffic flow. Prediction of accurate flow rate, density, and speed at LOS F is difficult.</p> <p>Figure 6.1 Basic Freeway Segment Speed-Flow Curves and Level of Service Criteria.How reasonable is using just density to measure LOS?</p> <p>Car drivers believe worse level of service if:</p> <p> Higher density</p> <p> Only 4-lane freeway</p> <p> Between 21 and 65 years old</p> <p> College graduate</p> <p> Do not use freeway every day</p> <p> Do not regularly drive to work</p> <p> Lower average speed</p> <p> Higher standard deviation for speed</p> <p> Higher flow</p> <p> Higher percentage of trucks</p> <p> Bad visibility (fog etc.)</p> <p> Higher standard deviation of headways</p> <p>Truckers believe worse level of service if:</p> <p> Drive less than 40,000 miles/year</p> <p> Higher density</p> <p> Only 4-lane freeway</p> <p> Higher standard deviation for speed Higher standard deviation of headways</p> <p> Lower percentage of trucks</p> <p>6.3.3 Determining Free-Flow Speed</p> <p>The FFS for basic freeway segments is the mean speed of passenger cars operating in flow rates up to 1300 passenger cars per hour per lane (pc/h/ln). If the FFS is to be estimated rather than measured, the following equation can be used, which accounts for the roadway characteristics of lane width, right-shoulder lateral clearance, number of lanes, and interchange density.</p> <p>FFS = BFFS fLW fLC fN fID</p> <p>(6.2)Where:</p> <p>FFS = estimated free-flow speed in mi/h (km/h),</p> <p>BFFS = estimated free-flow speed, in mi/h (km/h), for base conditions,</p> <p>fLW = adjustment for lane width in mi/h (km/h),</p> <p>fLC = adjustment for lateral clearance in mi/h (km/h),</p> <p>fN = adjustment for number of lanes in mi/h (km/h), and</p> <p>fID = adjustment for interchange density in mi/h (km/h).</p> <p>The BFFS is assumed to be :</p> <p> 110 km/h (70 mi/h) for freeways in urban areas, and 120 km/h (75 mi/h) for freeways in rural areas. </p> <p>Lane Width Adjustment</p> <p> When lane widths are narrower than the base 3.6m (12 ft), the adjustment factor fLW is used to reflect the impact on free-flow speed. Table 6.3 Adjustment for Lane WidthLane Width (ft)Reduction in Free-Flow Speed, fLW (mi/h)</p> <p>120.0</p> <p>111.9</p> <p>106.6</p> <p>Lateral Clearance AdjustmentWhen obstructions are closer than 1.8 m (6 ft) (at the roadside) from the traveled pavement the adjustment factor fLC is used to reflect the impact on FFS. Table 6.4 Adjustment for Right-Shoulder Lateral Clearance</p> <p>Right-Shoulder Lateral Clearance (ft)Reduction in Free-Flow Speed, fLC (mi/h)</p> <p>Lanes in One Direction</p> <p>234( 5</p> <p>(</p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p>Number of Lanes Adjustment</p> <p>A freeway segment with five or more lanes in a direction is the base condition with respect to free-flow speed. Adjustments in FFS for fewer directional lanes are shown in Table 6.5.</p> <p>Table 6.5 Adjustment for Number of Lanes on Urban FreewaysNumber of Lanes(One Direction)Reduction in Free-Flow Speed, fN</p> <p>mi/hkm/h</p> <p>( 50.00.0</p> <p>41.52.4</p> <p>33.04.8</p> <p>24.57.3</p> <p>Note: For all rural freeway segments, fN is 0.0.</p> <p>Interchange Density Adjustment</p> <p>When interchanges are spaced more frequently than one every two miles (one every 3.2 km), the traffic disturbances generated from merging and diverging movements will reduce the FFS. </p> <p>Table 6.6 Adjustment for Interchange Density</p> <p>Interchanges per MileReduction in Free-Flow Speed, fID (mi/h)</p> <p>( 0.500.0</p> <p>0.751.3</p> <p>1.002.5</p> <p>1.253.7</p> <p>1.505.0</p> <p>1.756.3</p> <p>( 2.007.5</p> <p>FFS = BFFS fLW fLC fN fID</p> <p>(6.2)6.3.4 Determining Analysis Flow Rate</p> <p>The analysis flow rate is calculated using the following equation.</p> <p>(6.3)Where:</p> <p>vp = 15-min passenger-car equivalent flow rate (pc/h/ln),</p> <p>V = hourly volume (veh/h),</p> <p>PHF = peak-hour factor,</p> <p>N = number of lanes,</p> <p>fHV = heavy-vehicle adjustment factor, and</p> <p>fp = driver population factor.</p> <p> Peak-Hour Factor</p> <p>To account for this varying arrival rate, the peak 15-minute vehicle arrival rate within the analysis hour is used for practical traffic analysis purposes.</p> <p>(6.4)Where:</p> <p>PHF = peak-hour factor,</p> <p>V = hourly volume for hour of analysis,</p> <p>V15 = maximum 15-min flow rate within peak hour, </p> <p>4 = number of 15-min periods per hour.SF= service flow is the actual rate of flow for the peak 15-min period expanded to an hourly volume and expressed in vehicles per hour.Equation 6.4 indicates that the further the PHF is from unity, the more peaked or nonuniform the traffic flow during the hour. Example, consider two roads both of which have a peak-hour volume, V, of 1800 veh/h. The first road has 600 vehicles arriving in the highest 15-min interval and the second road has 500 vehicles arriving in the highest 15-min interval. The first road has a more nonuniform flow, as indicated by a PHF of 0.75 [1800/(600 SYMBOL 180 \f "Symbol" 4)] that is further from unity than the second roads PHF of 0.90 [1800/(500 SYMBOL 180 \f "Symbol" 4)].</p> <p> Heavy Vehicle Adjustment</p> <p> Large trucks, buses and recreational vehicles have performance characteristics (slow acceleration and inferior braking) and dimensions (length, height, and width) that have an adverse effect on roadway capacity. The adjustment factor fHV is used to translate the traffic stream from base to prevailing conditions. The fHV correction term is determined by first getting the passenger-car equivalent (PCE) for each large truck, bus, and/or recreational vehicle in the traffic stream. These values represent the number of passenger cars that would consume the same amount of roadway capacity as a single large truck, bus, or recreational vehicle. These passenger-car equivalents are denoted ET for large trucks and buses, and ER for recreational vehicles, and are a function of roadway grades because steep grades will tend to magnify the poor performance of heavy vehicles as well as the sight distance problems caused by their larger dimensions (the visibility afforded to drivers in vehicles following heavy vehicles). For segments of freeway that contain a mix of grades, an extended segment analysis can be used as long as no single grade is steep enough or long enough to significantly impact the overall operations of the segment. As a guideline, an extended segment analysis can be used for freeway segments where no single grade that is less than 3% is more than 0.5 mi (0.8 km) long, or no single grade that is 3% or greater is longer than 0.25 mi (0.4 km). If an extended segment analysis is used, the terrain must be generally classified according to the following definitions:</p> <p>Level Terrain Any combination of horizontal and vertical alignment permitting heavy vehicles to maintain approximately the same speed as passenger cars. This generally includes short grades of no more than 2%.</p> <p>Rolling Terrain Any combination of horizontal and vertical alignment that causes heavy vehicles to reduce their speed substantially below those of passenger cars but does not cause heavy vehicles to operate at their limiting speed for the given terrain for any significant length of time or at frequent intervals (not having Fnet(V) = 0 due to high grade resistance as illustrated in Fig. 2.6).</p> <p>Mountainous Terrain Any combination of horizontal and vertical alignment that causes heavy vehicles to operate at their limiting speed for significant distances or at frequent intervals.</p> <p>Table 6.7 Passenger Car Equivalents (PCEs) for Extended Freeway Segments</p> <p>FactorType of Terrain</p> <p>LevelRollingMountainous</p> <p>ET (trucks and buses)</p> <p>ER (RVs)</p> <p> Any grade that does not meet the conditions for an extended segment analysis must be analyzed as a separate segment because of its significant impact on traffic operations. In these cases, grade-specific PCE values must be used.Table 6.8 Passenger Car Equivalents (PCEs) for Trucks and Buses on Specific UpgradesUpgrade(%)Length(mi)ET</p> <p>Percentage of Trucks and Buses</p> <p>2...</p>


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