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Chapter 24
Freeway Operations
24.1 Introduction
Freeways are generally considered as the most efficient type of highway for the movement of vehicles. Freeways were originally intended to serve longer trips of generally regional and interurban character. Traffic on freeways differs from that on city streets and rural roads in that it moves at higher speeds (depending on traffic conditions, design standards, etc.), more smoothly, and at much larger rates of flow. Speed limits are generally higher on freeways, and are occasionally non-existent. Because higher speeds reduce decision time, freeways are usually equipped with a larger number of guide signs than other roads, and the signs themselves are physically larger. Guide signs are often mounted on overpasses or overhead gantries so that drivers can see where each lane goes. Access to freeways is typically provided only at grade-separated interchanges, though lower-standard right-in/right-out access can be used for direct connections to side roads. In many cases, sophisticated interchanges allow for smooth, uninterrupted transitions between intersecting freeways and busy arterial roads. However, sometimes it is necessary to exit onto a surface road to transfer from one freeway to another. To reduce the probability that high-speed freeway traffic will have to slow down for slower same-direction traffic, access to freeways is usually limited to drivers of motor vehicles that are powerful enough to maintain a certain minimum speed. This chapter basically describes the capacity and level of service of the freeways. Then weaving in freeways is discussed briefly.
24.2 Basic freeway segment
Freeway is defined as a divided highway with full control of access and two or more lanes for exclusive use of traffic in each direction. This means two things. First, adjoining property owners do not have a legal right of access, meaning that they cannot connect their lands to
Not to scale
Basic Freeway Segment Basic Freeway Segment
Figure 24:1: Basic freeway segment
the highway by constructing driveways, although frontage roads provide access to properties adjacent to a freeway in many places. When an existing road is converted into a freeway, all existing driveways must be removed and access to adjacent private lands must be blocked with fences or walls. Second, traffic on a freeway is “free-flowing”. All cross-traffic (and left- turning traffic) is relegated to overpasses or underpasses, so that there are no traffic conflicts on the main line of the highway which must be regulated by traffic lights, stop signs, or other traffic control devices. Achieving such free flow requires the construction of many overpasses, underpasses, and ramp systems. The advantage of grade-separated interchanges is that freeway drivers can almost always maintain their speed at junctions since they do not need to yield to vehicles crossing perpendicular to mainline traffic. A freeway is composed of three elements:
- Basic freeway segment (outside the influence of ramps and weaving areas)
- Ramp junctions (for merging and diverging operations)
- Weaving areas (for vehicles to cross each other’s path)
Fig. 24:1 shows the basic freeway segment. We can see in Fig.24:1 that a basic freeway segment is independent of the ramps and weaving areas and the flow in such section occurs smoothly at the much larger rates. Merging and diverging of traffic occurs where on-or-off ramps join the basic freeway segment. Weaving occurs when vehicles cross each other’s path while travelling on freeway lanes. The exact point at which basic freeway segment begins or ends- that is, where the influence of weaving areas and ramp junctions has dissipated- depends on local conditions, particularly the level of service operating at the time. If traffic flow is light, the influence may be negligible, whereas under congested conditions, queues may be extensive.
- Clearance of 1.8 m between the edge of the travel lanes and the nearest obstructions or objects at the roadside and in the median,
- Free-flow speed of 120 km/h for freeways,
- Only passenger cars in the traffic stream (no heavy vehicles),
- Level terrain,
- No no-passing zones on two-lane highways, and
- No impediments to through traffic due to traffic control or turning vehicles.
Base conditions for intersection approaches include the following:
- Lane widths of 3.6 m,
- Level grade,
- No curb parking on the approaches,
- Only passenger cars in the traffic stream,
- No local transit buses stopping in the travel lanes,
- Intersection located in a noncentral business district area, and
- No pedestrians
Roadway conditions- Roadway conditions include geometric and other elements. In some cases, these influence the capacity of a road; in others, they can affect a performance measure such as speed, but not the capacity or maximum flow rate of the facility. Roadway factors include the following:
- Number of lanes,
- The type of facility and its development environment,
- Lane widths,
- Shoulder widths and lateral clearances,
- Design speed,
- Horizontal and vertical alignments, and
- Availability of exclusive turn lanes at intersections.
Traffic conditions- Traffic conditions that influence capacities and service levels include ve- hicle type and lane or directional distribution.
Vehicle type - The entry of heavy vehicles - that is, vehicles other than passenger cars (a category that includes small trucks and vans) - into the traffic stream affects the number of vehicles that can be served. Heavy vehicles are vehicles that have more than four tires touching the pavement. Trucks, buses, and recreational vehicles (RVs) are the three groups of heavy vehicles.
- They are larger than passenger cars and occupy more roadway space; and
- They have poorer operating capabilities than passenger cars, particularly with respect to acceleration, deceleration, and the ability to maintain speed on upgrades.
Directional and Lane Distribution - In addition to the distribution of vehicle types, two other traffic characteristics affect capacity and level of service: directional distribution and lane dis- tribution. Each direction of the facility usually is designed to accommodate the peak flow rate in the peak direction. Typically, morning peak traffic occurs in one direction and evening peak traffic occurs in the opposite direction. Lane distribution also is a factor on multilane facilities. Typically, the shoulder lane carries less traffic than other lanes.
Control conditions- For interrupted-flow facilities, the control of the time for movement of specific traffic flows is critical to capacity and level of service. The most critical type of control is the traffic signal. The type of control in use, signal phasing, allocation of green time, cycle length, and the relationship with adjacent control measures affect operations. Stop signs and yield signs also affect capacity, but in a less deterministic way. A Impact of control conditions traffic signal designates times when each movement is permitted; however, a stop sign at a two-way stop-controlled intersection only designates the right-of-way to the major street. The capacity of minor approaches depends on traffic conditions on the major street. An all-way stop control forces drivers to stop and enter the intersection in rotation. Capacity and operational characteristics can vary widely, depending on the traffic demands on the various approaches.
driver’s perception of those conditions. Safety is not included in the measures that establish service levels.
- LOS A LOS A describes free-flow operations. Free-flow speeds prevail. Vehicles are almost completely unimpeded in their ability to maneuver within the traffic stream. The effects of incidents or point breakdowns are easily absorbed at this level.
- LOS B LOS B represents reasonably free flow, and free-flow speeds are maintained. The ability to maneuver within the traffic stream is only slightly restricted, and the general level of physical and psychological comfort provided to drivers is still high. The effects of minor incidents and point breakdowns are still easily absorbed.
- LOS C LOS C provides for flow with speeds at or near the FFS of the freeway. Freedom to maneuver within the traffic stream is noticeably restricted, and lane changes require more care and vigilance on the part of the driver. Minor incidents may still be absorbed, but the local deterioration in service will be substantial. Queues may be expected to form behind any significant blockage.
- LOS D LOS D is the level at which speeds begin to decline slightly with increasing flows and density begins to increase somewhat more quickly. Freedom to maneuver within the traffic stream is more noticeably limited, and the driver experiences reduced physical and psychological comfort levels. Even minor incidents can be expected to create queuing, because the traffic stream has little space to absorb disruptions.
- LOS E At its highest density value, LOS E describes operation at capacity. Operations at this level are volatile, because there are virtually no usable gaps in the traffic stream. Vehicles are closely spaced leaving little room to maneuver within the traffic stream at speeds that still exceed 80 km/h. Any disruption of the traffic stream, such as vehicles entering from a ramp or a vehicle changing lanes, can establish a disruption wave that propagates throughout the upstream traffic flow. At capacity, the traffic stream has no ability to dissipate even the most minor disruption, and any incident can be expected to produce a serious breakdown with extensive queuing. Maneuverability within the traffic stream is extremely limited, and the level of physical and psychological comfort afforded the driver is poor.
Figure 24:2: LOS A
Figure 24:3: LOS B
• LOS F
LOS F describes breakdowns in vehicular flow. Such conditions generally exist within queues forming behind breakdown points. Breakdowns occur for a number of reasons:
- Traffic incidents can cause a temporary reduction in the capacity of a short segment, so that the number of vehicles arriving at the point is greater than the number of vehicles that can move through it.
- Points of recurring congestion, such as merge or weaving segments and lane drops, experience very high demand in which the number of vehicles arriving is greater than the number of vehicles discharged.
- In forecasting situations, the projected peak-hour (or other) flow rate can exceed the estimated capacity of the location. In all cases, breakdown occurs when the ratio of existing demand to actual capacity forecast demand to estimated capacity exceeds 1.00. The figure 2 given below gives a better idea of the LOS classification done on the basis of density of the traffic stream.
24.5 Determination of LOS
A basic freeway segment can be characterized by three performance measures: density in terms of passenger cars per kilometer per lane, speed in terms of mean passenger-car speed, and
volume-to-capacity (v/c) ratio. Each of these measures is an indication of how well traffic flow is being accommodated by the freeway. The measure used to provide an estimate of level of service is density. The three measures of speed, density, and flow or volume are interrelated. If values for two of these measures are known, the third can be computed. The steps involved in calculation of LOS are -
- Calculation of flow rate (vp)
- Calculation of average passenger car (S)
- Calculation of density (D) and determining LOS
24.5.1 Calculating Flow rate (vp)
The hourly flow rate must reflect the influence of heavy vehicles, the temporal variation of traffic flow over an hour, and the characteristics of the driver population. These effects are reflected by adjusting hourly volumes or estimates, typically reported in vehicles per hour (veh/h), to arrive at an equivalent passenger-car flow rate in passenger cars per hour (pc/h). The equivalent passenger-car flow rate is calculated using the heavy-vehicle and peak-hour adjustment factors and is reported on a per lane basis (pc/h/ln). The flow rate can be given as-
vp = (^) P HF × N V× f HV ×^ fP
Where, V = hourly volume P HF = peak hour factor (.80 -.95) N = no. of lanes fHV = heavy vehicle adjustment factor fP = driver population factor
Peak hour factor (PHF) The peak-hour factor (PHF) represents the variation in traffic flow within an hour. Observa- tions 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. 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. Field data should be used, if possible, to develop PHFs representative of local conditions.
Heavy vehicle adjustment factor (fHV ) Freeway traffic volumes that include a mix of vehicle types must be adjusted to an equivalent flow rate expressed in passenger cars per hour per lane. This adjustment is made using the factor fHV. Once the values of ET and ER are found, the adjustment factor, fHV , is determined by using equation given below -
fHV = 11 + PT (ET − 1) + PR(ER − 1) (24.2)
Where, ET , ER = passenger car equivalents for truck buses and recreational vehicles (RV’s) in traffic stream respectively PT , PR = proportion of truck/buses and recreational vehicles in traffic stream.
Adjustments for heavy vehicles in the traffic stream apply for three vehicle types: trucks, buses, and RVs. There is no evidence to indicate distinct differences in performance between trucks and buses on freeways, and therefore trucks and buses are treated identically. The factor fHV is found using a two-step process. First, the passenger-car equivalent for each truck/bus and RV is found for the traffic and roadway conditions under study. These equiv- alency values, ET and ER, represent the number of passenger cars that would use the same amount of freeway capacity as one truck/bus or RV, respectively, under prevailing roadway and traffic conditions. Second, using the values of ET and ER and the proportion of each type of vehicle in the traffic stream (PT and PR), the adjustment factor fHV is computed.
24.5.2 Calculating Average passenger car speed (S)
The average passenger car speed depends on the free flow speed (FFS) and flow rate as calcu- lated earlier and can be given as - For, 90 ≤ F F S ≤ 120 and vp ≤ (3100 − 15 F F S),
S = F F S (24.3)
For, 90 ≤ F F S ≤ 120 and (3100 − 15 F F S) ≤ vP ≤ (1800 + 5F F S)
S = F F S − [1/28(23F F S − 1800(vp^20 + 15F F SF F S −^ − 1300 3100 )26] (24.4)
The average of all passenger-car speeds measured in the field under low- to moderate- volume conditions can be used directly as the FFS of the freeway segment.
Concept of free flow speed (FFS)
Table 24:1: Adjustment for Lane Width Lane Width (m) Reduction in Free-Flow Speed, fLW (km/h) 3.6 0. 3.5 1. 3.4 2. 3.3 3. 3.2 5. 3.1 8. 3.0 10.
Table 24:2: Adjustment for Lateral Clearance Right Shoulder Reduction in Free-Flow Speed, fLC (km/h) Lateral Lanes in One Direction Clearance (m) 2 3 4 ≥ 5 ≥1.8 0.0 0.0 0.0 0. 1.5 1.0 0.7 0.3 0. 1.2 1.9 1.3 0.7 0. 0.9 2.9 1.9 1.0 0. 0.6 3.9 2.6 1.3 0. 0.3 4.8 3.2 1.6 1. 0.0 5.8 3.9 1.9 1.
Adjustment for Number of Lanes Freeway segments with five or more lanes (in one direction) are considered as having base con- ditions with respect to number of lanes. When fewer lanes are present, the BFFS is reduced. Table 24:3 provides adjustments to reflect the effect of number of lanes on BFFS. In determin- ing number of lanes, only mainline lanes, both basic and auxiliary, should be considered.
Adjustment for Interchange Density The base interchange density is 0.3 interchanges per kilometer, or 3.3-km interchange spacing. Base free-flow speed is reduced when interchange density becomes greater. Adjustments to reflect the effect of interchange density are provided in Table 24:4. Interchange density is determined over a 10-km segment of freeway (5 km upstream and 5 km downstream) in which the freeway segment is located. An interchange is defined as
Table 24:3: Adjustment for Number of Lanes Number of Lanes (One Direction) Reduction in Free-Flow Speed, fN (km/h) ≥ 5 0. 4 2. 3 4. 2 7.
Table 24:4: Adjustment for Interchange Density Interchanges per Kilometer Reduction in Free-Flow Speed, fID (km/h) ≤ 0.3 0. 0.4 1. 0.5 2. 0.6 3. 0.7 5. 0.8 6. 0.9 8. 1.0 9. 1.1 10. 1.2 12.
having at least one on-ramp. Therefore, interchanges that have only off-ramps would not be considered in determining interchange density. Interchanges considered should include typical interchanges with arterials or highways and major freeway-to-freeway interchanges.
24.5.3 Calculation of Density and determining LOS
Level of service on the basis of density can be calculated using the equation 24.
D = v Sp (24.6)
Where, D = density (pc/km/ln) vp= flow rate (pc/h/ln)
Step 2 Find fHV using equation 24.2 as given below -
fHV = (^) 1 + P^1 T (ET −^ 1) +^ PR(ER −^ 1) fHV = (^) 1 + 0.05(2^1. 5 − 1) + 0 fHV = 0. 930
Step 3 Putting the value of fHV to get value of vp -
vp = 2000(0.92)(2)(0.930)(1.00) vp = 1 , 169 pc/h/ln
Step 4 Compute free-flow speed from equation 24.5 as given -
F F S = BF F S − fLW − fLC − fN − fID
Putting the respective values of adjustment factors we get F F S as
F F S = 120 − 3. 1 − 3. 9 − 0. 0 − 3. 9 F F S = 109. 1 km/h
Step 5 Determine the density using the equation 24.6 as -
D = v Sp
Since, 90 ≤ F F S ≤ 120 and vp ≤ (3100 − 15 F F S) we can take S = F F S (from equation 24.3). Keeping values of vp and S we can get the value of density as -
D = (^1091169). 1 = 10. 7 pc/km/ln
Step 6 Find Level of service, for the calculated value of density we can get the level of service from the LOS table. i.e for D = 10.7 pc/km/ln we get LOS = B
24.5.5 Example Problem 2
A new suburban freeway is designed in the level terrain. Peak hour volume is 4,000 veh/h and the flow consists of 15% trucks and 3% recreational vehicles (RV’s). The traffic is commuter type with peak hour factor 0.85 and interchange density as 0.9 interchanges per kilometer. Lane width is proposed to be 3.6 m with lateral clearance of 1.8 m. How many lanes are needed to provide LOS C during the peak hour?
Solution Assumptions: Assume BF F S of 120 km/h. Since the freeway is being designed in a suburban area assume that the number of lanes affects free-flow speed. For commuter traffic we can take fp = 1.00. We can get the corresponding values of adjustment factors from the tables as - fLW = 0, fLC = 0, fID = 8.1 and fN = 4.8.
Step 1 Convert volume (veh/h) to flow rate (pc/h/ln) using equation 24.1 as given below -
vp = (^) P HF × N V× f HV ×^ fP vp = 4000
- 85 × N × fHV × 1. 00
Step 2 Find fHV using equation 24.2 as given below -
fHV = 1 1 + PT (ET − 1) + PR(ER − 1) fHV = 1 1 + 0.15(1. 5 − 1) + 0.03(1. 2 − 1) fHV = 0. 925
Step 3 Consider a four lane option, for four lane N = 2, keeping value of fHV and N in equation 24. we get vp as -
vp = 4000(0.85)(2)(0.925)(1.00) vp = 2 , 544 pc/h/ln
Four lane option is not acceptable 2544 pc/h/ln exceeds capacity of 2400 pc/h/ln. Here 2400 pc/h/ln is the capacity of a single lane under standard conditions.
D
A
B
C
Figure 24:8: Simple weaving segment
the exception of guide signs). Weaving segments are formed when a merge area is closely fol- lowed by a diverge area, or when an on-ramp is closely followed by an off-ramp and the two are joined by an auxiliary lane. Weaving segments require intense lane-changing maneuvers as drivers must access lanes appro- priate to their desired exit points. Thus, traffic in a weaving segment is subject to turbulence in excess of that normally present on basic freeway segments. The turbulence presents special operational problems and design requirements. Fig. 24:8 shows the simple weaving segment formed by a single merge point followed by a single diverge point. Multiple weaving segments may be formed where one merge is followed by two diverge points or where two merge points are followed by one diverge point.
24.6.1 Weaving configurations
The most critical aspect of operations within a weaving segment is lane changing. Weaving vehicles, which must cross a roadway to enter on the right and leave on the left, or vice versa, accomplish these maneuvers by making the appropriate lane changes. The configuration of the weaving segment (i.e., the relative placement of entry and exit lanes) has a major effect on the number of lane changes required of weaving vehicles to successfully complete their maneuver. There is also a distinction between lane changes that must be made to weave successfully and additional lane changes that are discretionary (i.e., are not necessary to complete the weaving maneuver). The former must take place within the confined length of the weaving segment, whereas the latter are not restricted to the weaving segment itself. There are three major categories of weaving configurations: Type A, Type B, and Type C.
Type A weaving configuration The identifying characteristic of a Type A weaving segment is that all weaving vehicles must make one lane change to complete their maneuver successfully. All of these lane changes occur across a lane line that connects from the entrance gore area directly to the exit gore area. Such a line is referred to as a crown line. Type A weaving segments are the only such segments to
B
C
D
A
Figure 24:9: Ramp Weave C
B D
A
Figure 24:10: Major Weave
have a crown line. The most common form of Type A weaving segment is shown in Fig. 24:9. The segment is formed by a one-lane on-ramp followed by a one-lane off-ramp, with the two connected by a continuous auxiliary lane. The lane line between the auxiliary lane and the right-hand freeway lane is the crown line for the weaving segment. All on-ramp vehicles entering the freeway must make a lane change from the auxiliary lane to the shoulder lane of the freeway. All freeway vehicles exiting at the off-ramp must make a lane change from the shoulder lane of the freeway to the auxiliary lane. This type of configuration is also referred to as a ramp-weave. Fig. 24: illustrates a major weaving segment that also has a crown line. A major weaving segment is formed when three or four of the entry and exit legs have multiple lanes. As in the case of a ramp-weave, all weaving vehicles, regardless of the direction of the weave, must execute one lane change across the crown line of the segment.
Type B weaving configuration Type B weaving segments are shown in Figs. 24:11 to 24:13. All Type B weaving segments fall into the general category of major weaving segments in that such segments always have at least three entry and exit legs with multiple lanes (except for some collector distributor configurations). It is the lane changing required of weaving vehicles that characterizes for the Type B configuration:
- One weaving movement can be made without making any lane changes, and
- The other weaving movement requires at most one lane change.
