Basic Freeway Segments Capacity Analysis , 2000 Highway Capacity Manual, Chapter 23 Operational analysis of an existing freeway. Steps: 1. Volume adjustment: traffic volumes are adjusted with the following formula: p HV HF f * f * N * P V p V = With: V p = 15 minute flow rate in passenger cars per lane per hour V = hourly traffic volume PHF = Peak Hour Factor F p = driver familiarity, ranges from 0.85 to 1.0 (very familiar drivers) f HV = heavy vehicle adjustment ) 1 E ( P 1) - (E * P 1 1 f R R T HV − + + = With: P t = portion of trucks and buses P R = portion of recreational vehicles E T = passenger car equivalency for trucks and buses, found from Table 23-8 for extended segments or Table 23-9 (upgrade) or Table 23-11 (downgrade) for a specific segment E R = passenger car equivalency for recreational vehicles, from Table 23-8 (extended section) or 23-10 (upgrade) or = 1.2 for downgrade 2. Determine free flow speed. FFS = BFFS – f LW - f LC - f N - f ID With: FFS = free flow speed, measured speed or found with formula BFFS = base free flow speed, for example, design speed, 75 mph f LW = lane width adjustment found from Table 23-4 f LC = lateral clearance adjustment, found for table 23-5 f N = adjustment for number of lanes, found from Table 23-6 f ID = interchange density adjustment, found from Table 23-7 1. Determine level of service. Use Table 23-2. Design Analysis for a new freeway. In this case we want to find the number of lanes required. The process is the same as operational analysis, except you increase the number of lanes until you have an acceptable level of service, i.e. start with two lanes, check LOS, etc.
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Basic Freeway Segments Capacity Analysis, 2000 Highway Capacity Manual, Chapter 23 Operational analysis of an existing freeway. Steps:
1. Volume adjustment: traffic volumes are adjusted with the following formula:
pHVHF f *f * N * P
VpV =
With: Vp = 15 minute flow rate in passenger cars per lane per hour V = hourly traffic volume PHF = Peak Hour Factor Fp = driver familiarity, ranges from 0.85 to 1.0 (very familiar drivers) fHV = heavy vehicle adjustment
)1E(P 1) - (E * P 1
1fRR T
HV −++=
With: Pt = portion of trucks and buses PR = portion of recreational vehicles
ET = passenger car equivalency for trucks and buses, found from Table 23-8 for extended segments or Table 23-9 (upgrade) or Table 23-11 (downgrade) for a specific segment
ER = passenger car equivalency for recreational vehicles, from Table 23-8 (extended section) or 23-10 (upgrade) or = 1.2 for downgrade
2. Determine free flow speed.
FFS = BFFS – fLW - fLC- fN - fID
With: FFS = free flow speed, measured speed or found with formula BFFS = base free flow speed, for example, design speed, 75 mph fLW = lane width adjustment found from Table 23-4 fLC = lateral clearance adjustment, found for table 23-5 fN = adjustment for number of lanes, found from Table 23-6 fID = interchange density adjustment, found from Table 23-7
1. Determine level of service. Use Table 23-2.
Design Analysis for a new freeway. In this case we want to find the number of lanes required. The process is the same as operational analysis, except you increase the number
of lanes until you have an acceptable level of service, i.e. start with two lanes, check LOS, etc.
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