BASIC FREEWAY SEGMENTS CAPACITY ANALYSIS
Freeway segments that are outside of the influence of ramps or weaving sections. -- The physical qualities of a basic freeway section such as lane width, shoulder clearances and density of interchanges (on and off ramps).
Ramp
Weaving Section
The base conditions under which the full capacity of a basic freeway segment is achieved are good weather, good visibility, and no incidents or accidents. For the analysis procedures in this chapter, these base conditions are assumed to exist. If any of these conditions fails to exist, the speed, LOS, and capacity of the freeway segment all tend to be reduced.
Minimum
lane widths of 3.6 m, right-shoulder of 1.8 m; Minimum median of Minimum right-shoulder 0.6 m, Traffic stream composed entirely of passenger cars, Five or more lanes for one direction, Interchange spacing at 3 km or greater, Level terrain, with grades no greater than 2 percent, A driver population composed principally of regular users of the facility. These base conditions represent a high operating level, with a free-flow speed (FFS) of 110 km/h or greater.
The methodology does not apply to or take into account (without modification by the analyst) the following:
• Special lanes reserved for a single vehicle type, such as high-occupancy vehicle (HOV) lanes, truck lanes, and climbing lanes, • Extended bridge and tunnel segments, • Segments near a toll plaza, • Facilities with free-flow speeds below 90 km/h or in excess of 120 km/h.
Methodology (according to U.S. HCM 2000) FFS
BFFS
BFFS Adjustment
Compute FFS
Lane width Input Geometric data FFS or BFFS
Median type Interchange density
Determine
Lateral Clearance
LOS
Volume Volume Adjustment Peak-hour Factor Number of Lanes Driver population Heavy vehicles
Compute flow rate
FFS is the mean speed of passenger cars measured during low to moderate flows (up to 1,300 pc/h/ln). For a specific segment of freeway, speeds are virtually constant in this range of flow rates.
Two methods can be used to determine the FFS of a basic freeway segment: field measurement and estimation. The field-measurement procedure is provided for users who prefer to gather these data directly. However, field measurements are not required for application of the method. If field-measured data are used, no adjustments are made to the free-flow speed.
If field measurement of FFS is not possible, FFS can be estimated indirectly on the measurement is not possible basis of the physical characteristics characteristics of the freeway segment being studied. The physical characteristics include lane width, number of lanes, right-shoulder lateral clearance, and interchange density.
where FFS = free-flow speed (km/h); BFFS = base free-flow speed, 110 km/h (urban) or 120 km/h (rural); f LW = adjustment for lane width from Exhibit 23-4 f LC = adjustment for right-shoulder lateral clearance from Exhibit 23-5 f N = adjustment for number of lanes from Exhibit 23-6 f ID = adjustment for interchange density from Exhibit 23-7
Exercise:
FFS adjustment
An existing six-lane freeway in an urban area has the following physical characteristics: 3.5-m lanes 0.6-m lateral clearance on outer shoulders interchange density of 1 interchange per 0.5 km. • •
•
Calculate the free-flow speed for this section of freeway.
Solution:
FFS adjustment
where f LW = adjustment for lane width = 1.0 f LC = adjustment for right-shoulder lateral clearance = 2.6 f N = adjustment adjustment for number number of lanes lanes = 4.8 f ID = adjustment for interchange density = 12.1
FSS =110 – 1.0 – 2.6 – 4.8 – 12.1 =
vp = 15-min passenger-car equivalent flow rate (pc/hr/ln) V = hourly volume (veh/hr) PHF = peak hour factor N = number of lanes f HV = heavy vehicle adjustment factor f p = driver population factor
On freeways, typical PHFs range from 0.80 to 0.95. Lower PHFs are characteristic of rural freeways or off-peak conditions. Higher factors are typical of urban and suburban peak-hour conditions.
ET , ER = passenger car equivalents for trucks or buses (T) and recreational vehicles (RV) in the traffic stream (refer Exhibit 23-8) PT , PR = proportion of truck/buses and RVs in the traffic stream
*Note* In Malaysia there are no recreational vehicles. Therefore, neglect PR and ER .
A six lane freeway has a flow of 3500 vehicles. This flow consists of 180 trucks per hour, 200 RVs per hour, 350 passenger buses per hour and the remainder of passenger. Calculate the heavy vehicle adjustment factor for a 1.8 km section of this freeway that has a +4% grade.
The percentage of trucks is 5% [(180/3500) x 100], buses is 10% [(350/3500) x 100] and RVs is 6%. ET = 3.0 3.0 (Exhibit 23-9) ER = 3.0 (Exhibit 23-10) PT, = 5 5 + 10 = 15% PR = 6% Therefore, = 0.704
The traffic stream characteristics that are the basis of this methodology are representative of regular drivers in a substantially commuter traffic stream or in a stream in which most drivers are familiar with the facility . In this case, the driver population factor is 1.00.
Ranges between 0.85 to 1.00.
1. Define Define and segmen segmentt the the freew freeway ay facilit facility y as approp appropriat riate. e. 2. Estima Estimated ted or field field meas measure ured d FFS, FFS, an an appro appropria priate te spee speed-fl d-flow ow curve of the same shape as the typical curves (Exhibit 23-3) is constructed. On the basis of the flow rate, v p, and the constructed speed-flow curve, an average passenger-car speed is read on the y-axis of Exhibit 23-3. 3. Calculate density using following formula; D = Vp/S where
D = density (pc/km/ln), v p = flow rate (pc/h/ln), and S = average passenger-car speed (km/h). 4. LOS of the basic freeway segment is then determined by comparing the calculated density with the density ranges in Exhibit 23-2.
Existing four-lane freeway, freeway, rural area, very restricted geometry, rolling terrain, 110-km/h speed limit. Given information: Two lanes in each direction with 3.3-m lane width Two terrain. 5 percent trucks and Rolling terrain. 0.92 PHF, lateral clearance, 0.6-m lateral interchanges/km 0.6 interchanges/km traffic, 2,000-veh/h peak-hour volume and commuter traffic, What is the LOS during the peak hour? ho ur?
New suburban freeway is being designed. How many lanes are needed to provide LOS D during the peak hour?
4,000 veh/h (one direction), 0.85 PHF, Level terrain, 0.9 interchanges per kilometer, 15 percent trucks, 3 percent RVs, and 3.6-m lane width, 1.8-m lateral clearance.
The Freeway Existing six-lane freeway in a growing urban area with the following f ollowing information:
√ 5,000 veh/h (one direction, existing); √ 6 lanes, Level terrain,10 percent trucks √ 5,600 veh/h (one direction, in 3 years); and √ Beyond 3 years, traffic grows at 4 percent √ FFS = 110 km/h (measured in field) and 0.95 PHF; (i) (i) What What is is the the curr curren entt LOS LOS duri during ng the the pea peak k hour hour?? (ii) (ii) What What LOS will will occur occur in 3 years? years? (iii) When should should a fourth lane be added in each direction direction to avoid an excess of demand over capacity?
New urban facility being planned with a forecast opening-day AADT of 75,000 veh/day. i)
What What is the the min minim imum um numb number er of lane lanes s nee neede ded d to to provide at least LOS D during the peak hour on opening day? ii) What What are are the speed speed and densit density y of of traf traffic fic for the proposed number of lanes?
75,000 veh/day (both directional), K=0.090, Directional Split (55/45), FFS = 110km/h (field), PHF = 0.9, Rolling terrain, 10 percent trucks.
