FREEWAYS

 

LEVEL OF SERVICE

 

Density is the primary determinant of LOS on a freeway.  In other words, it is the Performance Criterion for freeways.   LOS thresholds for a basic freeway segment are defined in terms of density as follows:

LOS                 Density Range (pc/mi/ln)

              A                               0–11

              B                                > 11–18

              C                               > 18–26

  D                               > 26–35

              E                                > 35–45

              F                                > 45

 

The relationship between density, level of service, speed, and flow is shown in the following graph:

 

Notice that capacity = 2400 passenger cars per hour (per lane) in this figure.  The flow rate in this figure is represented by the variable Vp and the free flow speed in this figure is designated as FFS.   If we know, Vp and FFS we can use this graph to determine both the density and level of service.  Or we can use Vp and FFS to find the Speed (S) from this graph and then calculate density using the standard speed/flow/density equation D = Vp / S (a rearrangement of q = S x D where q is the same as Vp).  In either case, we must first determine FFS and Vp to calculate density and LOS.

 

 

DETERMINING FREE FLOW SPEED

 

The Free Flow Speed (FFS) is the mean speed of passenger cars measured during low to moderate flows (up to 1300 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 with guidelines provided in this chapter. The speed study should be conducted at a location that is representative of the segment when flows and densities are low (less than 1300 pc/h/ln).  Weekday off-peak hours are generally good times to observe low to moderate flow rates.

The speed study should measure the speeds of all passenger cars or use a systematic sample (e.g., every 10th passenger car). The speed study should measure passenger-car speeds across all lanes. A sample of at least 100 passenger-car speeds should be obtained. Any speed measurement technique that has been found acceptable for other types of traffic engineering speed studies may be used.

 

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.  This speed reflects the net effect of all conditions at the study site that influence speed, including those considered in this method (lane width, lateral clearance, interchange density, and number of lanes) as well as others such as speed limit and vertical and horizontal alignment. Speed data that include both passenger cars and heavy vehicles can

be used for level terrain or moderate downgrades but should not be used for rolling or mountainous terrain.

 

If field measurement of FFS is not possible, FFS can be estimated indirectly on the basis of the physical characteristics of the freeway segment being studied. The physical characteristics include lane width, number of lanes, right-shoulder lateral clearance, and interchange density:

BFFS = 70 mph for urban freeways and 75 mph for rural freeways. 

 

The first adjustment (fLW) relates to the effect of lane widths on Free Flow Speed.  Base conditions for a two-lane highway require lane widths of 12-ft or greater.

 

The second adjustment (fLC) relates to the effect of right shoulder lateral clearance on Free Flow Speed, and is dependent on the number of lanes in one direction.

 

 

Considerable judgment must be used in determining whether objects or barriers along the right side of a freeway present a true obstruction.  Such obstructions may be continuous, such as retaining walls, concrete barriers, or guardrails, or may be non-continuous, such as light supports or bridge abutments.  In some cases, drivers may become accustomed to certain types of obstructions, in which case their influence on traffic flow may be negligible.

 

The third adjustment (fN) relates to the effect of the number of lanes in one direction on Free Flow Speed.  It is only used for urban and suburban freeways, not rural freeways.

 

 

In determining number of lanes, only mainline lanes should be considered.  HOV lanes should not be included.

 

The fourth adjustment (fID) relates to the effect of interchange density on Free Flow Speed. 

The base interchange density is 0.5 interchange per mile, or 2 mile interchange spacing.  Base free-flow speed is reduced when interchange density becomes greater than this.  Interchange density is determined over a 6 mile segment of freeway (3 mile upstream and 3 mile downstream) in which the freeway segment is located.

 

DETERMINING DEMAND 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).  Four adjustments must be made to hourly demand volumes (V) to arrive at the equivalent passenger car flow rate (VP).  These adjustments are the PHF, number of lanes (N) in one direction, the heavy vehicle adjustment factor (fHV), and the driver population adjustment factor (fp). These adjustments are applied using the following equation:

 

 

Heavy Vehicle Adjustment (fHV)

 

Adjustment for the presence of heavy vehicles in the traffic stream applies to two types of vehicles: trucks and RVs.  Buses should not be treated as a separate type of heavy vehicle but should be included with trucks. The heavy-vehicle adjustment factor requires two steps. First, the passenger-car equivalency factors for trucks (ET) and RVs (ER) for the prevailing operating conditions must be found.   

 

Once values for ET and ER have been determined, the adjustment factor for heavy vehicles (fHV) is computed using the following equation where PT is the percentage of trucks in the traffic stream (expressed as a decimal) and PR is the percentage of RV’s in the traffic stream (also expressed as a decimal):

Driver Population Adjustment (fP)

 

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.  It is generally accepted that traffic streams with different characteristics (e.g., recreational drivers) use freeways less efficiently.  The adjustment factor fp is used to reflect this effect.  The values of fp range from 0.85 to 1.00. In general, the analyst should select 1.00, which reflects commuter traffic (i.e., familiar users), unless there is sufficient evidence that a lower value should be applied.