Highway Capacity Software (HCS) – Part II CE 4640 Fall 2002

Multilane Highways Generally have posted speed limits of 40 to 55 mph Usually have a total of 4 to 6 lanes Both directions of travel With medians, center lanes for left turns, or undivided Traffic volumes range from 15,000 to 40,000 veh/day

Free Flow Speed (FFS) Speed of traffic at low volume and low density Speed at which drivers feel comfortable traveling under the physical, environmental and traffic-control conditions on an uncongested section of multilane highway It is the starting point for analyzing capacity and LOS for uninterrupted-flow conditions

Measure FFS by conducting travel time runs during periods of low to moderate volume The upper limit for low volume for multilane highways is 1400 passenger cars per hour per lane (PCPHPL) FFS is the mean speed of cars under low to moderate traffic flow LOS is based on density Calculated by dividing the per lane volume by speed

Factors Affecting FFS Lane width Total lateral clearance Lane widths less than 12 feet reduce travel speeds but lane widths greater than 12 feet do not increase speeds Total lateral clearance Left side plus the right side along one direction of roadway 12 feet, or 6 feet x2, is considered the base condition

Factors Affecting FFS Median Type Undivided medians composed of a striped center lane Two way Left Turn Lanes composed of a full lane width Median composed of a raised curb, barrier or landscaping

Factors Affecting FFS Access Points Number of access points along the right side of the roadway The amount of activity at each point contributes to change in travel speeds

Factors Affecting FFS Other Factors Design speed Posted speed limit

Factors Affecting Flow Rate Traffic flow basis Peak 15 minute period flow during a peak hour Two volumes are required: peak hour volume peak 15 min volume within the peak hour To determine peak hour factor

Factors Affecting Flow Rates Heavy vehicles Number of trucks, buses, and recreation vehicles (RVs) Converted to an equivalent number of passenger cars Driver Population Commuter traffic Non-commuter traffic

Factors Affecting Flow Rate Level of Service (LOS) Multilane highway is characterized by three performance measures Density- passenger cars per mile per lane Primary performance measure for estimating LOS Speed- passenger car speed Volume to capacity ratio Density speed and volume are interrelated recall Q = kv Indicate how well the highway accommodates traffic flow.

LOS A- completely free flow conditions Operation of vehicles is completely unaffected by the presence of other vehicles Operation is only constrained by the geometric features of the highway and by driver preference Maneuverability within the traffic stream is good Any minor disruptions in the traffic stream are virtually unnoticeable, even without a change in travel speed

LOS B- also indicates free flow However, the presence of other vehicles becomes noticeable Average travel speeds are the same as in LOS A, but drivers have slightly less freedom to maneuver Any minor disruptions in the traffic stream are virtually unnoticeable

LOS C- the influence of traffic density on operations becomes marked Ability to maneuver within the traffic stream is clearly affected by other vehicles Minor disruptions can cause serious local deterioration in service and queues will form behind any disruption

LOS D- the ability to maneuver is severely restricted due to congestion Travel speed is reduced by the increasing volume Only minor disruptions can be absorbed without extensive queues forming and the service deteriorating

LOS E- represents operations at or near capacity, at an unstable level Vehicles are operating with the minimum spacing for maintaining uniform flow Disruptions cannot be dissipated easily often causing queues to form and service to deteriorate to LOS F. Travel speeds range between 42 to 55 mph on facilities with speed limits of 45 to 60 mph

LOS F-Represents forced or breakdown flow Occurs when vehicles arrive at a rate greater than the rate at which they are discharged Or when the forecasted demand exceeds the computed capacity of a planned facility Operations are at capacity and queues form behind these breakdowns Operations within queues are highly unstable with vehicles experiencing brief periods of movement followed by stoppages Travel speeds are generally less than 30 mph

Required Input Data and Estimated Values Use actual field data instead of default values when feasible. Lane width and Lateral Clearance Median: either divided or undivided No distinction among TWLTL, unpaved or landscaped medians Access Point Density Total number of active intersections and driveways on the right side of the road divided by the length of the facility Density should be averaged over a minimum of three miles

Default Values Development Type Default Value Access Points/mi (one side) Rural 8 0-10 Low Density Suburban 16 11-20 High Density Suburban 25 >20

Specific Grade or General Terrain Grade = 5-8%- Mountainous terrain Grade = 3-5% - Level Terrain Grade = 5-7% - Rolling terrain

Base FFS – 60 mph may be used for rural or suburban multilane highway

Multilane Highways

General Information

Free Flow Speed

Volume

Results

Example: Multilane Highway Telegraph Road from 12 Mile to Maple AM Peak Hour Divided Median Do not have field measured FFS Assume Base FFS = 50 mph Assume 12 foot lanes and 6 feet shoulders Assume Access point density of 15 per mile Assume 5% trucks and 0% RVs 3 lanes each direction Volume = 3600 vph NB and 3800 vph SB With PHF = 0.90 both directions Change the lane width to 11feet and see how your results are affected

Two Lane Highways Undivided roadway with two lanes, with one lane for each direction of travel Motorists in a platoon are subject to delay because they are unable to pass Passing requires the use of the opposing lane as sight distance and gaps in the opposing traffic stream permit

General Information

Input Data

Classification of Two Lane Highways Class I- motorists expect to travel at high speeds Major intercity routes Commuter routes Serve long distance trips Class II- motorists do not necessarily expect to travel at high speeds Scenic or recreational routes Serve short trips

Default Values for Percent No Passing Zones Terrain Type No Passing Zones (%) Level 20 Rolling 50 Mountainous 80

Default Values- same as for Multilane Highways Development Type Default Value Access Points/mi (one side) Rural 8 0-10 Low Density Suburban 16 11-20 High Density Suburban 25 >20

Other Default Values PHF Percent Heavy Vehicles 0.88 Rural 0.92 Urban Trucks Rural = 14% Urban = 2% RVs Rural = 4% RVs = 0%

Average Travel Speed- Calculations Based on Input Data

Free Flow Speed - Calculations Based on Input Data

Calculations and Results

Passing Lane Analysis: Input Data

Other Performance Measures

Example: Two Lane Highways Urban 2 lane highway Terrain = level Lane width and shoulder width are standards Segment length = 5 miles Class I Highway Make reasonable assumptions for PHF, % trucks and buses, and no passing zones Assume access points per mile = 12 Volumes Analysis direction = 500 vph Opposing direction = 400 vph Base FFS = 60 mph

Assume Lu = 3 miles Lp = 1 mile

FREEWAYS MODULE

f 1 2 f f 1 2 G=10.0 sec Y=4.0 AR = 1.0 G=40.0 sec Y=4.0 AR = 1.0 380 140 100 Main Street 100 280 470 f4 940 100 f3 180 G=5.0 sec Y=4.0 AR = 1.0 G=35.0 sec Y=4.0 AR = 1.0 First Street 180 200 760 Assume RTOR Volume is 10% of right turn volume Assume percent trucks (through traffic is 7% Assume PHF = 93%

Homework #10 Calculate optimal signal timing plan for the given intersection by hand Cycle length Phasing design Splits Assume appropriate yellow and all-red intervals Input your results in HCS Was your design optimal? Use HCS to fine tune an optimal design