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Chapter 23
Multilane Highways
23.1 Introduction
Increasing traffic flow has forced engineers to increase the number of lanes of highways in order to provide good manoeuvring facilities to the users. The main objectives of this lecture is to present the basics of multilane highway, its operational characteristics, capacity and level of service (LOS) concepts. An important parameter in the capacity and LOS analysis is the free flow speed. This also will be covered in the lecture.
23.2 Multilane Highways
A highway is a public road especially a major road connecting two or more destinations. A highway with at least two lanes for the exclusive use of traffic in each direction, with no control or partial control of access, but that may have periodic interruptions to flow at signalized intersections not closer than 3.0 km is called as multilane highway. Multilane highways exist in a number of settings, from typical suburban communities leading to central cities or along high- volume rural corridors that connect two cities or important activities generating a considerable number of daily trips.
23.2.1 Highway Classification
Although there are various ways of classification of highways; the most common one is based on the number of lanes. Thus highways may be classified as:
- Two lane highways.
- Three lane highway, and
- Four or more lane highway
23.2.2 Highway Characteristics
Multilane highways generally have posted speed limits between 60 km/h and 90 km/h. They usually have four or six lanes, often with physical medians or two-way right turn lanes. The traffic volumes generally varies from p15,000 - 40,000 vehicles per day. It may also go up to 100,000 per day with grade separations and no cross-median access. Traffic signals at major in- tersections are possible for multilane highways which facilitate partial control of access. Typical illustrations of multilane highway configurations are provided in Fig. ??.
23.3 Highway Capacity
An important operation characteristic of any transport facility including the multi lane high- ways is the concept of capacity. Capacity may be defined as the maximum sustainable flow rate at which vehicles or persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a specified time period under given roadway, geometric, traffic, environmental, and control conditions; usually expressed as vehicles per hour, passenger cars per hour, or persons per hour. There are two types of capacity, possible capacity and practical capacity. Possible capacity is defined as the maximum number of vehicles that can pass a point in one hour under prevailing roadway and traffic condition. Practical capacity on the other hand is the maximum number that can pass the point without unreasonable delay restriction to the average driver’s freedom to pass other vehicles.
The procedure for computing practical capacity for the uninterrupted flow condition is as follows:
- Select an operating speed which is acceptable for the class of highways the terrain and the driver.
- Determine the appropriate capacity for ideal conditions from table 1 shown below.
- Determine the reduction factor for conditions which reduce capacity (such as width of road, alignment, sight distance, heavy vehicle adjustment factor).
- Multiply these factors by ideal capacity value obtained from step 2.
Figure 23:1: LOS A
Figure 23:2: LOS B
easily absorbed without an effect on travel speed. Fig. 23:1 shows LOS A.
Level of Service B. Travel conditions are at free flow. The presence of other vehicles is noticed but it is not a constraint on the operation of vehicles as are the geometric features of the roadway and individual driver preferences. Minor disruptions are easily absorbed, although localized reduction in LOS are noted. Fig. 23:2 below shows LOS B.
Level of Service C. Traffic density begins to influence operations. The ability tomanoeuvre within the traffic stream is affected by other vehicles. Travel speeds show some reduction when free-flow speeds exceed 80 km/h. Minor disruptions may be expected to cause serious local deterioration in service, and queues may begin toform. Fig. 23:3 shows LOS C.
Level of Service D. The ability to manoeuvre is severely restricted due to congestion. Travel speeds are reduced as volumes increase. Minor disruptions maybe expected to cause serious
Figure 23:3: LOS C
Figure 23:4: LOS D
Figure 23:5: LOS E
local deterioration in service, and queues may begin toform. Fig. 23:4 shows LOS D.
Level of Service E. Operations are unstable and at or near capacity. Densities vary, de- pending on the free-flow speed. Vehicles operate at the minimum spacing for which uniform flow can be maintained. Disruptions cannot be easily dissipated and usually result in the for- mation of queues and the deterioration of service to LOS F. For the majority of multilane highways with free-flow speed between 70 and 100km/h, passenger-car mean speeds at capac- ity range from 68 to 88 km/h but are highly variable and unpredictable. Fig. 23:5 shows LOS E.
Level of Service F. A forced breakdown of flow occurs at the point where the numbers of vehicles that arrive at a point exceed the number of vehicles discharged or when forecast demand exceeds capacity. Queues form at the breakdown point, while at sections downstream they may appear to be at capacity. Operations are highly unstable, with vehicles experiencing brief periods of movement followed by stoppages. Travel speeds within queues are generally less than 48 km/h. Note that theterm LOS F may be used to characterize both the point of the breakdown and the operating condition within the queue. Fig. 23:6 shows LOS F.
23.5 Determination of Level of Service
The determination of level of service for a multilane highway involves three steps:
- Determination of free-flow speed
110 100 90 80 70 60 50 40 30 20 10 (^0) 0 400 800 1200 1600 2000 2400 Flow^ (pc/h/ln)
Speed (km/hr)
Figure 23:7: Speed-flow relationship on multilane highways (HCM, 2000)
1
Flow (pc/h/ln)
Density (pc/h/ln)
Free flow speed = 100 km/hr
Free flow speed = 70 km/hr Free flow speed = 80 km/hr Free flow speed = 90 km/hr 0
5
10
15
20
25
30
35
40
45
50
0 400 800 1200 1600 2000 2400
Figure 23:8: Density-flow relationships on multilane highways (HCM, 2000)
110 100 90 80 70 60 50 40 30 20 10 0 Flow (pc/h/ln)
Density = 25 pc/km/ln
Density = 22 pc/km/ln
Density = 7 pc/km/ln Density = 11 pc/km/ln Density = 16 pc/km/ln
(km/hr) Speed
0 400 800 1200 1600 2000 2400
Figure 23:9: Speed-flow curves with LOS criteria for multilane highways (HCM, 2000)
- A divided highway.
- Only passenger cars in the traffic stream.
- A free-flow speed of 90 km/h or more.
The total lateral clearance is 3.6 m, with minimum of 1.8 m on either sides of in the direction of travel. The clearnce distance is measured from the edge of the outer lane and is inclusive of the shoulder. If lateral clearance is more than 1.8 m, then it is considered as equal to 1.8 m. The average of all passenger-car speeds measured in the field under low volume conditions can be used directly as the free-flow speed if such measurements were taken at flow rates at or below 1400 pc/h/ln. No adjustments are necessary as this speed reflects the net effect of all conditions at the site that influence speed, including lane width, lateral clearance, type of median, access points, posted speedlimits, and horizontal and vertical alignment. Free-flow speed also can be estimated from 85th-percentile speed or posted speed limits, research suggests that free-flow speed under base conditions is 11 km/h higher than the speed limit for 65 km/h to 70 km/h speed limits and 8 km/h higher for 80 km/h to 90 km/h speed limits. Fig. 23:9 shows speed- flow curves with LOS criteria for multilane highways, here LOS is easily determined for any value of speed simply by plotting the point which is a intersection of flow and corresponding speed. Note that density is the primary determinant of LOS. LOS F is characterized by highly unstable andvariable traffic flow. Prediction of accurate flow rate, density, and speed at LOS F is difficult.
Table 23:4: Adjustment for lateral clearance(Source: HCM, 2000)
Four-Lane Highways Six-Lane Highways Total Lateral Reduction in FFS Total Lateral Reduction in FFS Clearance a (m) (km/h) Clearance a (m) (km/h) 3.6 0.0 3.6 0. 3.0 0.6 3.0 0. 2.4 1.5 2.4 1. 1.8 2.1 1.8 2. 1.2 3.0 1.2 2. 0.6 5.8 0.6 4. 0.0 8.7 0.0 6.
Table 23:5: Adjustment for median type(Source: HCM, 2000)
Median Type Reduction in FFS (km/h) Undivided highways 2. Divided highways 0.
Adjustment for lateral clearance is determined by first computing the total lateral decrease given as: T LC = LCL + LCR (23.2)
where, TLC = Total lateral clearance (m), LCL = Lateral clearance (m), from the right edge of the travel lanes to roadside obstructions (if greater than 1.8 m, use 1.8 m), and LCR= Lateral clearance (m), from the left edge of the travel lanes to obstructions in the roadway median (if the lateral clearance is greater than 1.8 m, use 1.8 m). Once the total lateral clearance is computed, the adjustment factor is obtained from Table 4. For undivided highways, there is no adjustment for the right-side lateral clearance as this is already accounted for in the median type. Therefore, in order to use Table 5 for undivided highways, the lateral clearance on the left edge is normally about 1.8 m. The access-point den- sity, for a divided roadway is found by dividing the total number of access points (intersections and driveways) on the right side of the roadway in the direction of travel being studied by the length of the segment in kilometres. The adjustment factor for access-point density is given in Table 6. Thus the free flow speed can be computed using equation 1 and applying all the
Table 23:6: Adjustment for Access-point density(Source: HCM, 2000)
Access Points/Kilometre Reduction in FFS (km/h) 0 0. 6 4. 12 8. 18 12. ≥ 24 16.
adjustment factors.
23.5.3 Determination of Flow Rate
The next step in the determination of the LOS is the computation of the peak hour factor. The fifteen minute passenger-car equivalent flow rate (pc/h/ln), is determined by using following formula: vp = (^) (P HF ∗ NV ∗ f HV ∗^ fp)^
where, vp is the 15-min passenger-car equivalent flow rate (pc/h/ln), V is the hourly volume (veh/h), P HF is the peak-hour factor, N is the number of lanes, fHV is the heavy-vehicle ad- justment factor, and fp is the driver population factor. PHF represents the variation in traffic flow within an hour. Observations of traffic flow consistently indicate that the flow rates found in the peak 15-min period within an hour are not sustained throughout the entire hour. The PHFs for multilane highways have been observed to be in the range of 0.75 to 0.95. Lower values are typical of rural or off-peak conditions, whereas higher factors are typical of urban and suburban peak-hour conditions. Where local data are not available, 0.88 is a reasonable estimate of the PHF for rural multilane highways and 0.92 for suburban facilities.
Besides that, the presence of heavy vehicles in the traffic stream decreases the FFS because base conditions allow a traffic stream of passenger cars only. Therefore, traffic volumes must be adjusted to reflect an equivalent flow rate expressed in passenger cars per hour per lane (pc/h/ln). This is accomplished by applying the heavy-vehicle factor (fHV ). Once values for ET and ER have been determined, the adjustment factors for heavy vehicles are applied as follows: fHV = (^) (1 + P^1 T (ET −^ 1) +^ PR(ER −^ 1)^
- A significant change in the density of access points
- Different speed limits
- The presence of bottleneck condition
In general, the minimum length of study section should be 760 m, and the limits should be no closer than 0.4 km from a signalized intersection.
- On the basis of the measured or estimated free-flow speed on a highway segment, an appropriate speed-flow curve of the same as the typical curves is drawn.
- Locate the point on the horizontal axis corresponding to the appropriate flow rate (vp) in pc/hr/ln and draw a vertical line.
- Read up the FFS curve identified in step 2 and determine the average travel speed at the point of intersection.
- Determine the level of service on the basis of density region in which this point is located. Density of flow can be computed as D = v Sp (23.5) where, D is the density (pc/km/ln), vp is the flow rate (pc/h/ln), and S is the aver- age passenger-car travel speed (km/h). The level of service can also be determined by comparing the computed density with the density ranges shown in table given by HCM. To use the procedures for a design, a forecast of future traffic volumes has to be made and the general geometric and traffic control conditions, such as speed limits, must be estimated. With these data and a threshold level of service, an estimate of the number of lanes required for each direction of travel can be determined.
23.5.5 Numerical Example
A segment of undivided four-lane highway on level terrain has field-measured FFS 74.0-km/h, lane width 3.4-m, peak-hour volume 1,900-veh/h, 13 percent trucks and buses, 2 percent RVs, and 0.90 PHF. What is the peak-hour LOS, speed, and density for the level terrain portion of the highway?
Solution
LOS can be calculated by computing the flow rate and free flow speed. Flow rate is vp = (^) (P HF ∗ N V∗ f HV ∗ f p) (23.6)
Since fHV is unknown it is calculated by
f HV =
(1 + P T (ET − 1) + P R(ER − 1)
vp = (^) (0. 90 ∗ 21900 ∗ 0. 935 ∗ 1) vp = 1129 pc/h/ln S = 74. 0 km/h (given) D = V p/S = 15. 3
Determine LOS: LOS determined from the speed-flow diagram. LOS = C
23.5.6 Numerical Example 2
A segment of an east-west five-lane highway with two travel lanes in each direction separated by a two-way left-turn lane (TWLTL) on a level terrain has- 83.0-km/h 85th-percentile speed ,3.6-m lane width, 1,500-veh/h peak-hour volume, 6 % trucks and buses, 8 access points/km (WB), 6 access points/km (EB), 0.90 PHF, 3.6-m and greater lateral clearance for westbound and eastbound. What is the LOS of the highway on level terrain during the peak hour?
Solution
Assume base FFS to be 3 km/h less than 85th percentile speed. BFFS=83.0-3=80.0 km/h Assume no RVs, since none is indicated. LOS can be calculated by knowing flow rate and free flow speed. Flow rate vp =
V
(P HF ∗ N ∗ f HV ∗ f p) (23.7) Since fHV is unknown it is calculated by
f HV = (^) (1 + P T (ET − 1) +^1 P R(ER − 1)
