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Level of service and Capacity
Two-Lane Rural Highway
Overview
• Two-lane highways are unique among facilities providing for uninterrupted flow in that the directional flows strongly interact. Passing is accomplished by use of the opposing travel lane, and thus, volume in one direction influences capacity and operations in the other.
Passing on two-lane rural highways
• Passing sight distance = d1 + d2 + d3 + d4
• d1 = distance traveled during perception reaction time and during the initial acceleration to the point of encroachment on the left lane.
• d2 = distance traveled while the passing vehicle occupies the left lane.
• d3 = distance between the passing vehicle at the end of its maneuver and the opposing vehicle.
• d4 = distance traveled by an opposing vehicle for two-thirds the time the passing vehicle occupies the left lane.
Ideal Conditions• 12-foot lanes• 6-foot shoulders• level terrain• 100% passing sight distance available or no no-passing
zones• all passenger cars in the traffic stream• 50/50 directional distribution of traffic
Two-Lane HighwayCapacity and Level of Service
Two-lane Highways - Description One lane per travel direction, Passing a slower vehicle requires use of
opposing lane, Motorists must adjust their speed as
volume increases and the ability to pass decreases,
Class I facilities are those on which motorists expect to travel at relatively high speeds,
Class I highways connect major intercity routes, major traffic generators, or may constitute daily commuter routes or primary links in state or national highway network
Class I facilities serve long distance trips or connect facilities serving long distance trips,
Class II highways are those on which motorists do not expect to travel at high speed,
Class II highways serve scenic or recreational trips,
Class II highways often serve short trips, or the beginning and end portions of long trips
LOS on Two-lane Highways - Definition Level of service A – Good maneuverability,
and ease of absorption of minor disruptions.
Level of Service B – Good Maneuverability, and Minor absorption are easily absorbed although localized impacts may occur.
Level of Service C – Some restrictions on maneuverability exist. Minor disruptions may cause serious deterioration in service and queues.
Level of Service D – Severe restriction in maneuverability due to congestion. Minor disruptions may cause serious deterioration in service and queues.
Level of service E – Operations are unstable, at or near capacity. Disruptions can not be easily dissipated and result in long queues.
Level of service F – Flow breaks down. Operation is highly unstable (stop and go traffic).
Level of Service
• PTSF represents the average percentage of travel time that a vehicle spends in platoons due to its inability to pass.
• PFFS based on the comparison of prevailing to free flow speed.
• For Class I highways, mobility is paramount; hence percent time spent following, PTSF, and average travel speed, ATS, convey the LOS.
• For Class II highways mobility is less critical and PTSF conveys the level of service.
• For class III (a category defined by FDOT for 2-lane highways traversing small rural communities with low travel speed and signals) highways, PFFS , is the level of service variable.
• Two lane highways can be analyzed directionally either as general extended sections or as specific grades.
Level of ServiceDirectional Segment Methodology
• Use extended segment methodology for level and rolling terrain
• Use specific grades for grades of 3% or more that are at least 0.6 mi long.
LOS-Directional Extended Segment Method
Level of Service –Computation• Step 1. Compute the free flow speed
LOS Directional-Extended Segment Method
Step 2. Compute the flow rate
LOS - Directional Method
• If grade steep enough such that some heavy vehicles crawl, then:
Level of ServiceDirectional Segment Methodology
• Step 3. Compute the average travel speed, ATS
LOS-Directional Extended Segment Method
• Step 4. Compute percent time spent-following, PTSF
LOS - Directional Segment Methodology
• Step 5. Compute the LOS (according to Two-lane Tables)
LOS-Directional Extended Segment Method
• Step 5. Determine the level of service, LOS
LOS – ET, ER, ATS Computation, Extended Directional Segment Method
LOS-fnp Directional Extended Segment Method
LOS-Directional Extended Segment Method
LOS - fg, Specific Grade, ATS Computation, Directional Segment
LOS - ET, Specific Grade, ATS Computation, Directional Segment
LOS - fg, Specific Grade, PTSF Computation, Directional Segment
LOS - ET, Specific Grade, PTSF Computation, Directional Segment
LOS - ER, Specific Grade, PTSF Computation, Directional Segment
LOS - Specific Grade Directional Segment Method
LOS - fnp, ATS Computation, Directional Segment
LOS - fnp, PTSF Computation, Directional Segment
LOS Directional Segment with Passing Lane
Level of Service –Computation• Step 1. Compute the PSTF and ATS for the normal cross section without the
passing lane
• Step 2. Divide the directional segment into four regions: – Upstream of passing lane– Passing lane (including lane addition and lane drop tapers)– Downstream of passing lane (within effective length)– Downstream of passing lane (beyond effective length)
Passing lane segments
LOS Directional Segment with Passing Lane
• Step 3. Compute Percent Time Spent Following
LOS Directional Segment with Passing Lane (truncated)
• If analysis section is truncated prior to reaching the full extent of the passing lane effective length,
LOS Directional Segment with Passing Lane
• Step 4. Compute Average Travel Speed
