Highway Capacity Capacity is the maximum number of vehicles that have a reasonable expectation of passing over a given section of lane or roadway in one direction or in both directions during one hour under prevailing road and traffic conditions. Hourly capacity is influenced by the following factors: ! ! ! ! ! ! !
Number of lanes; Carriageway width; Shoulder width; Gradients and their length; Truck and bus percentage of total traffic; Lateral obstructions on both or one side of the roadway; and Roadside friction
The degree of roadside friction impact on road capacity: None:
Few or no buildings along the roadside;
Light:
Buildings and/or road intersections along and close to the road, 100200 meters between these objects, pedestrians and non-motorized traffic observed occasionally;
Medium:
Scattered roadside development, 50-100 meters between buildings and/or road intersections, pedestrians and non-motorized traffic observed frequently;
Heavy:
Continuous roadside development with less than 50 meters between buildings and/or road intersections, pedestrians and non-motorized traffic tend to disrupt the motor vehicle traffic and reduce travel speed to below 35 km/hr even at low traffic volume.
Based on studies undertaken by the Planning Services of the Dept. of Public Works and Highways (1975-1977, 1979), the Philippine road capacities for varying roadway widths and flat terrain can be estimated as follows: Table 1: Basic hourly capacities according to road type (DPWH, 1979) Road Type Highway Highway Highway Highway Highway Highway Highway Urban Street Urban Street Urban Street Urban Street
Carriageway Width (m) 4.0 ! 4.1 - 5.0 5.1 - 5.5 5.6 - 6.1 6.2 - 6.5 6.6 - 7.3 2 x 7.0 6.0 ! 6.1 - 6.5 6.6 - 7.3 2 x 7.0
Roadside Friction None or Light None or Light None or Light None or Light None or Light None or Light None or Light Heavy Heavy Heavy Heavy
Basic Hourly Capacity (in PCU in Both Directions) 600 1,200 1,800 1,900 2,000 2,400 7,200 (Expressway) 1,200 1,600 1,800 6,700 1
Capacities under Philippine conditions are about 20 % higher than those reported in the U.S. Highway Capacity Manual of 1965. This could be due to the fact that the average passenger car unit on Philippine roads is smaller than its U.S. counterpart and may also be attributed to the behavior of local drivers (e.g., it is very common to drive quite close to the vehicle in front and roads with 2 lanes per direction are effectively used as if there were 3 lanes instead). The basic capacity reported in the U.S. HCM of 1965 is 2,000 pcu/hr for both directions of a 2-lane, flat, 7.3-meter road without roadway friction. As can be seen from Table 1, the Philippine corresponding capacity is 2,400 pcu/hr. For multi-lane highways in flat terrain, such as expressways, the lane capacities (if lane width is at least 3.5 meters) are estimated as follows: Table 2: Hourly capacities for multi-lane highways No. of lanes per direction Hourly capacity, pcu/lane
2 1,800
3 1,750
4 1,700
The DPWH has updated these basic hourly capacity values in their 2004 Highway Planning Manual and the new values are shown in Table 3. Table 3: Basic hourly capacity, PCU (DPWH, 2004) Carriageway width Single less than 4 meters 4.0 – 5.0 meters 5.1 – 6.0 meters 6.1 – 6.7 meters 6.8 – 7.3 meters 2 x 6.7 or 2 x 7.3 meters
Hourly PCU Rural 600 1,200 1,900 2,000 2,400 7,200
Urban 600 1,200 1,600 1,700 1,800 6,700
Passenger Car Equivalent Factors
The capacity is normally expressed in passenger car unit per hour (pcu/hr) and would depend on the so-called passenger car equivalent f actors (PCEF) of the different vehicle classes that compose the traffic. These factors express the impact of slowmoving vehicles and heavy vehicles interacting with gradients and length of gradients. The following formulas are used to estimate PCEF of different vehicle types: Table 4: PCEF formulas Vehicle Type
PCEF per vehicle
Cars, vans, jeeps
1.0
Jeepneys Motorcycles
2
Vehicle Type
PCEF per vehicle
Motor Tricycles Buses Trucks Note : Use 25 PCT instead of 100 PCT on multi-lane highways and expressways.
In all these formulas, the factors are defined as follows: •
RFRIC : roadside friction with values as follows: Table 5: RFRIC values Roadside Friction None Light Medium Heavy
•
RFRIC value 0 3.0 6.0 9.0
SWID : shoulder width with values as follows: Table 6: SWID values Shoulder Width (m) 0 – 0.5 0.6 – 1.9 2.0 or more; unpaved; in good condition 2.0 or more; paved; in good condition
•
SWID value 0 1.0 1.5 4.0
CWID : carriageway width with values as follows: Table 7: CWID values Carriageway Width (m) 4.0 4.1 – 5.0 5.1 – 5.5 5.6 – 6.1 6.2 – 6.5 6.6 – 7.3 2 x 7.0 Urban 6.1 – 6.5 Urban 6.6 – 7.3 Urban 2 x 7.0
•
CWID Value 0 1 2 3 4 5 6 7 8 9
PCT : gradients and length of gradients with values as follows: Table 8: GRA values Gradient, % 0 – 0.9 1.0 – 1.9 2.0 – 2.9
GRA Value 0.5 1.0 1.5 3
Gradient, % 3.0 – 4.9 5.0 – 6.9 7.0 – 8.9 > 10.0
GRA Value 3.0 6.0 8.0 10.0 2
PCT is computed as 0.05(GRA) for gradient length less than or equal to 400 2 meters and 0.07(GRA) for gradient length greater than 400 meters. •
PH : heavy vehicles, buses and trucks in % of total AADT. It is computed as follows:
Table 9: PCEF values (DPWH, Highway Planning Manual, 2007) Vehicle Type
Vehicle Type
1 2 3-5 6 7 8 9 10 11 12
Motor tricycle Passenger car Passenger and goods utility and small bus Large bus Rigid truck, 2 axles Rigid truck, 3 axles Truck semi-trailer, 3 and 4 axles Truck semi-trailer, 5+ axles Truck trailers, 4 axles Truck trailers, 5+ axles
Terrain Flat 1.5 1.0 1.5 2.0 2.0 2.5 2.5 2.5 2.5 2.5
Rolling 1.5 1.0 1.725 2.3 2.3 2.875 2.875 2.875 2.875 2.875
Mountainous 1.5 1.0 1.95 2.6 2.6 3.25 3.25 3.25 3.25 3.25
Level of Service
Following the definitions used by the DPWH Highway Planning Manual (2007) for the evaluation of road sections, the levels of service are as follows: Table 10: Level of Service (LOS) criteria Level of Service
Volume / Capacity Ratio
Description
A
less than 0.20
Free flow traffic
B
0.21 - 0.50
Relatively free flow traffic
C
0.51 - 0.70
Moderate traffic
D
0.71 - 0.85
Moderate / heavy traffic
E
0.86 - 1.00
Heavy traffic
F
greater than 1.0
Saturation traffic, stop and go movement
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Determination of AADT [Reference: Transport Training Center, Course Notes on Transportation and Traffic Technology, UP Diliman, 1983]
Traffic volume count stations: a) Seasonal stations – selected along major and national roads and mostly located near the district or city boundary. They are distributed and carefully selected throughout the country to cover the various types of highways and to reflect their traffic patterns. b) Control stations – selected along national roads and major arterials for the purpose of providing controls necessary to record volume counts on a common basis. These are required to establish seasonal and daily traffic volume characteristics, and factors for expanding data from single counts obtained at coverage stations. c) Coverage stations – are required to provide sufficient data to permit reasonably accurate estimates of ADT on each road segment.
Schedule of traffic count surveys on each type of station: a) Seasonal station – traffic counts are conducted over a 24-hour period, once in every month for a duration of 1 week (or more). The count is scheduled as follows: st st th st 1 week count – 1 to 7 day of the 1 month 2nd week count – 8 th to 14th day of the 2 nd month 3rd week count – 15 th to 21st day of the 3 rd month 4th week count – 22 nd up to the end of the 4 th month regardless of whether it exceeds 7 days b) Control station – traffic counts on control stations are conducted as follows: - 24 hours - 7 days continuous - Four cycles per year (quarterly) i. Select any 7 consecutive days such that none of them is a holiday ii. Saturday and Sunday are not considered as holidays iii. Commence count from 0600H (6:00 AM), 3 shifts of 8 hours each c) Coverage station – traffic counts on coverage stations are conducted as follows: - 12 hours although usually 16 hours if there is significant traffic during night-time - 2 weekdays continuous - 2 cycles per year (semestrally) i. Select any 2 weekdays such that none of them is a holiday ii. Commence count from 0600H (6:00 AM), 2 shifts of 8 hours each iii. Counting period of this station shall coincide with the counting period of its control station
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•
Determination of AADT of Seasonal Station:
Example: Where:
•
Estimate of AADT of Control Station:
Where:
•
ADT1 = average daily traffic for the 1st month ADT2 = average daily traffic for the 2nd month rd ADT3 = average daily traffic for the 3 month
ADTm1 = ADT of the control station in the 1 st quarter ADTm2 = ADT of the control station in the 2 nd quarter ADTm3 = ADT of the control station in the 3 rd quarter th ADTm4 = ADT of the control station in the 4 quarter
Estimate of AADT of Coverage Station:
Where:
V11 = expanded 24-hour volume of the coverage station for the 1 st day of month 1 V12 = expanded 24-hour volume of the coverage station for the 2 nd day of month 1
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st
V21 = expanded 24-hour volume of the coverage station for the 1 day of month 2 V22 = expanded 24-hour volume of the coverage station for the 2 nd day of month 2 To expand the 12-hour count at the coverage station, we use the ratio and proportion of the control station count as follows:
Where:
t12 = 12-hour count at coverage station T12 = 12-hour count at control station corresponding to the same 12 hours of the coverage station T24 = 24-hour count at control station
Also, st
DF11 = Daily Factor of the 1 day of month 1 DF12 = Daily Factor of the 2 nd day of month 1 DF21 = Daily Factor of the 1 st day of month 2 nd DF22 = Daily Factor of the 2 day of month 2 The Daily Factor is computed from:
SF = seasonal factor for the month under consideration
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