Two Lane Rural Highways Ch 6

1
Two Lane Rural Highways Ch 6

• The majority of our Nations highways paved
  highways are considered rural (80)
• Of these, 85 are two lane highways
• Thus the focus of many states is design and
  operational analysis of two lane highways

2
Primary Functions

• Mobility
• State and County primary highways, carry large
  numbers of users
• Access
• Low volume roadways, provide basic all-weather
  access to remote or sparsely-developed areas
• HCM has defined two classes of TLTW rural
  highways
• Class I (high speeds, intercity routes)
• Class II (scenic routes, recreational areas)

3
Passing Maneuvers

• Passing is a unique characteristic of two lane
  rural highways
• Directional flow has a major effect on
  operational performance
• As traffic in one direction increases, the demand
  for passing also increases in that direction
• If traffic is significant in the opposing
  direction, platoons begin to form and drivers are
  no longer able to choose their own travel speed

4
Passing Maneuvers

• Heavy vehicles also have a significant impact on
  operational performance of two lane highways
• Again, with the inability to pass slow moving
  vehicles, platoons can begin to form and
  operational performance deteriorate quickly
• Therefore, operational analysis is performed for
  the entire facility, not by direction of traffic
  flow

5
Two-lane Quality Definitions

• Average Travel Speed
• The segment length divided by the avg. travel
  time of ALL vehicles.
• Time Spent Following
• The avg of travel time that all vehicles are
  delayed due to inability to pass. This can be
  estimated by measuring the percentage of vehicles
  traveling at headways less than 5 seconds.
  weakness of MOE to date this factor has not
  been linked to the length of segment.
• Utilization of Capacity
• The ratio of the demand flow rate to the capacity
  of the facility.

6
Level of Service

• Determined by Average Travel Speed and Percent
  Time Spent Following for Class I facilities
• Determined only by Percent Time Spent Following
  for Class II facilities

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Ideal ConditionsRural two-lane highways

• Ideal Conditions

12 Lane Widths 6 Shoulders No No Passing
Zones 50/50 Directional Split Passenger Cars
Only 60 mph Design Speed Level Terrain
8
Free-Flow Speed

• Expected operating conditions can be estimating
  by knowing the free-flow speed
• Recommended that it be measured in the field when
  possible
• Can be estimated by the following when total flow
  levels greater than 200 pc/hr

Sm mean speed of the measured
sample, mph vfobserved flow rate for the period
of speed Sample, veh/hr fhvheavy vehicle
adjustment factor
9
Free-Flow Speed

• May also be estimated using technique from HCM

FFS free-flow speed for the facility,
mi/hr BFFS base free-flow speed for the
facility, mi/hr (typically use design speed or
speed limit) fLS adjustment for lane and
shoulder width, mi/hr (Table 6.16) fA
adjustment for access point density, mi/hr (Table
6.15)
10
Demand Flow Rate

• Need to adjust the hourly vehicle demand volume
  into demand flow rate (pc/hr)

vdemand flow rate, pc/hr V hourly demand
volume under prevailing conditions, veh/hr PHF
peak hour factor fHV adjustment for heavy
vehicle presence (Table 6.18) fG adjustment for
grades (Table 6.17)
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Demand Flow Rate

• Things to note
• When estimating average travel speed (ATS) and
  Percent Time Spent Following (PTSF) need to
  determine two different sets of adjustment
  factors
• When performing two-directional analysis, total
  volume is used to determine v when
  one-directional, two vs must be determined

12
Estimating ATS

• Once you have iteratively determined the demand
  flow rate (using your heavy vehicle and grade
  adjustment factors) next you can determine ATS
  and PTSF
• Again as before, you need to keep consistent with
  one-directional or two-directional analysis

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Example

• Class I 2-lane highway has rolling terrain and
  500 v/hr with PHF 0.94. 5 trucks, 2 buses, 6
  RVs. Determine the Average Travel Speed (ATS)
  and the Time Spent Following (PTSF).

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Example 6.7

• Now that we know the flow rates that are to be
  used to determine the LOS based on ATS and PTSF,
  continue the example.
• Additional information
• 11lanes
• 2shoulders
• 10 access points/mile
• 50 no-passing zones
• Base FFS55mph
• 60/40 directional split