highway capacity design

Highway Capacity DesignChapter 2

Based on Highway Capacity Manual 2000 Transport Research Board

Waruna Jayasooriya

• Although speed is a major concern of drivers as related to

service quality, freedom to maneuver within the traffic

stream and proximity to other vehicles are equally

noticeable concerns.

• These qualities are related to the density of the traffic

stream.

• The LOS are defined to represent reasonable ranges in the

three critical flow variables; speed, density, and flow rate.

Level of Service

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 repercents 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 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 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.

LOS E describes operation at capacity at its highest density value. 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..

LOS F describes breakdowns in vehicular flow. Such conditions generally exist within queues forming behind breakdown points.

Figure 2.2

LOS A

LOS C

LOS D

LOS E

LOS B

The capacity of a two-lane highway is 1700 pc/h for each direction.

For both directions of combined travel, the capacity will not exceed 3200 pc/h.

Capacity

LOS criteria for pedestrian walkway

For Class I highways:Efficient mobility is more importantLOS is defined in terms of both percent time

spent following and average travel speed.

For Class II highways:Mobility is less criticalPercent time spent following without

considering avg. travel speed.

Level of Service (LOS)

The difference between these LOS assessments represent the difference in motorist expectations for Class I and Class II

Level of Service (LOS) cont…

Step 1 :Collect Field data & computations

Step 2A : Average travel speed

calculation

Determination of grade adjustment factor

Computation of fHV for an average travel

speed

Computation of Vp

Calculation of highest directional flow rate &

check against to capacity

Compute free flow speed

Compute average travel speed

Step 2B : percent time-spent-following

Compute base percent time-spent-following

Compute percent time-spent-following

Determine LOS

Calculation of current LOS of the road Methodology

Two-way volume (based on 2 hour traffic count)

% of trucks and buses% of recreational vehiclesDirectional split% of no passing zonesNo. of access point per km length

Field data

Table 1: Traffic flow considering both directions according to collected traffic data

Traffic survey data

Time

Perio

d

Moto

r

Cycle

3

whee

l

Car Utility

(RV)

Light Medi Heavy Multi

Axle

Mini

Bus

Larg

e

Bus

Servic

e

Vehicl

e

Tract

or

Bicycl

e

Total

2.15-

2.30

2.30-

2.45

2.45-

3.00

3.00-

3.15

3.15-

3.30

3.30-

3.45

3.45-

4.00

4.00-

4.15

Table 1: Traffic flow considering both directions according to collected traffic data

Traffic survey data

Time

Perio

d

Moto

r

Cycle

3

whee

l

Car Utility

(RV)

Light Medi Heavy Multi

Axle

Mini

Bus

Larg

e

Bus

Servic

e

Vehicl

e

Tract

or

Bicycl

e

Total

2.15-

2.30

71 42 52 59 10 12 5 2 6 3 1 0 1 264

2.30-

2.45

75 33 57 60 9 5 5 0 5 7 1 1 2 260

2.45-

3.00

78 41 43 40 8 5 9 0 0 4 0 0 4 232

3.00-

3.15

61 35 64 57 5 6 9 1 0 6 0 0 5 249

3.15-

3.30

74 25 64 56 3 4 10 0 0 8 0 1 5 250

3.30-

3.45

66 48 54 70 3 8 14 2 1 5 1 0 5 277

3.45-

4.00

69 31 52 53 2 7 12 0 1 6 0 0 9 242

4.00-

4.15

71 34 57 58 8 5 10 0 0 8 3 1 1 256

Table 2: Traffic data arrange according to considering number of vehicle to both directions

Calculation from survey data

Time To Talawathugoda To Pannipitiya Total

2.15- 2.30117 147 264

2.30- 2.45111 150 261

2.45- 3.00104 128 232

3.00- 3.15104 145 249

3.15-3.30120 130 250

3.30-3.45116 161 277

3.45-4.00104 138 242

4.00-4.15117 140 256

Table 3: Hourly traffic flows to the both direction

Time

Interval

To

Talawathugod

a

To

Pannipitiya

Total Directional

Split

2.15-3.15 436 570 1006 43/57

2.30-3.30 339 553 892 38/62

2.45-3.45 444 564 1008 44/56

3.00-4.00 444 574 1018 44/56

3.15-4.15 457 569 1025 45/55

Time

Perio

d

Moto

r

Cycle

3

whee

l

Car Utilit

y

Light Medi Heav

y

Multi

Axle

Mini

Bus

Larg

e Bus

Service

Vehicle

Trac

tor

3.15

-

4.15

280 138 227 16 24 46 2 2 27 4 2 20

Table 4: Peak hour distribution of vehicle

Terrain condition (based on vertical profile eg. Level or Rolling)

Geometric data (existing avg. pavement width, shoulder width)

Speed study ( at representative location, sample of at least 100 vehicles with systematic sampling. eg. Avg of every 10th vehicle)

Geometric data

Section Lane width(m) Shoulder width(m)

1 2.87 2.57

2 2.64 2.46

3 2.61 1.87

4 2.59 1.88

Total 2.68 8.78

Table 1 : Extracted data from Auto CAD drawing

Step A : Average travel speed

calculation

Determination of grade adjustment

factor

Computation of fHV for an average

travel speed

Computation of Vp

Calculation of highest directional flow

rate & check against to capacity



Step B : percent time-spent-following

Compute base percent time-spent-

following

Compute percent time-spent-following

Determine LOS


Determination of grade adjustment factor - fG

Computation of fHV for an average travel speed

Step 3 : Computation of passenger-car equivalent flow rate - Vp

Highest directional flow rate = maximum dir. Split * Vp

Check flow rates against to capacity values of 1700 pc/h or 3200 pc/h

Calculation of highest directional flow rate & check


Compute average travel

speed

Step 2B : percent time-

spent-following

Compute base percent

time-spent-following

Compute percent time-

spent-following

Determine LOS

Step 2B : percent time-spent-following

Computation of fHV for a percent

time-spent-following fG and check

Compute base percent time-spent-

following

Compute percent time-spent-

following

Determine LOS


Computation of fHV for a percent time-spent-following

Computation of passenger-car equivalent flow rate - Vp



Calculation of highest directional flow rate & check for time spent following

Compute base percent time-spent-following & percent time-spent-following

Determine LOS

Graphical method…

Consider a Class I two-lane highway segment with the following facts;1600 veh/h(two way volume)14% trucks and buses4% RVs89.2 km/h free flow speed FFS0.95 PHFRolling terrain3.4 m lane width10 km length1.2m shoulder width50% no-passing zones50/50 directional split12 access points/km

What is the two-way segment LOS for the peak hour?

Question

Computation of fHV for an average travel speed

Step 3 : Computation of passenger-car equivalent flow rate - Vp



Calculation of highest directional flow rate & check

Computation of fHV for a percent time-spent-following

Computation of passenger-car equivalent flow rate - Vp



Calculation of highest directional flow rate & check for time spent following

Compute base percent time-spent-following & percent time-spent-following

Determine LOS

Graphical method…

Avg. travel speed: fG=0.99 FHV=0.931 VP=1837 pc/h Check VP*max dir. split < 1700 pc/h & VP<3200 pc/h ATS=65.1 km/h

Time spent following fG=1.00 fHV=1.00 VP=1684 pc/h Check VP*max dir. split < 1700 pc/h & VP<3200 pc/h BPTSF=77.2% Fd/np=4.8% PTSF=82%

LOS = E

Answers

A segment of two lane Gampaha - Idigolla road carries a peak hour volume of 780 vph and has a 4% grade. The 11.75 km length segment, has operational speed of 54 kmph with two lanes of 3.3m wide and the shoulders of 0.6 m. 40% of the road segment has passing prohibition. The directional split of traffic is 70/30 with 12% trucks, 8% buses and 80% passenger cars in the traffic stream. The effects of other vehicles may be ignored.

At what level of service will the road operate during peak periods, if the flow rate for the peak 15 minutes total for both directions is 87% of the total flow rate for the peak hour?

What is the maximum volume that can be?

Example 1:

Existing level of service of the roadProposed centerline of the road with setting-out

details of curves and cross sectionsDrainage design includes cross drainage and lea

dawn drain design.Pavement design (Provide the cross sections

details)Safety audits of the existing road and proposed

design improvements.Level of service of the road at the end of the design

periodEstimate & prepare cost details for tendering

Highway Design Project

Drawing file which includes plan view of a trace

HCM 2000 with chapter 20 hard copyHSR for unit costing.xls file which contains culvert details

Details that you may have…

1:50,000 topo-sheets for catchment area calculationsTRL road note 31 or AASTHO guidelines for

pavement designRoad Safety Audit reports

Design Manual for Roads and Bridges Volume 5 Section 2

Determine Existing LOSTraffic forecasting for end of the design periodDetermine the required LOS at the end of design

periodback calculation to present condition based on

future LOSBased on

Highway Capacity Manual 2000Traffic forecasting based on realistic dataPresent road geometry with certain modification

Capacity Design

Audits should conduct for existing road segment & designed road segment.

Identify existing safety drawbacksProposed road cross-sectionPlan view with necessary road safety

detailing

Road Safety Audit

Drainage DesignDetermine the cross drains and parallel drain

sizes with necessary calculations

Evaluate axle-load values base on the forecasted traffic

Evaluate baring capacity by following necessary laboratory tests

Determine the layer thickness based on road note 31 or following AASHTO guideline

Pavement Design

Figure out existing geometric parametersHorizontal alignment and profile correction

base on the design speed(posted, safer, etc..)Provide final design drawings with supportive

calculations

Geometric Design

Cost for the reconstructionBased on profile correctionBased on horizontal alignment correction

Cost for the land acquisition & compensationCost for the drainage

Cost estimation & tendering

Ref # Date Group

Highway Design Project

Civil 3D Field workInception

reportInterim report

Final presentation & report submission

1 17th Sep Group A & B Group A & B

2 24th Sep A1,A2,A3,A4,A5,A6 A4,A5,A6 A1,A2,A3

3 1st Oct B1,B2,B3,B4,B5,B6 B4,B5,B6 B1,B2,B3 A1,A2,A3

4 8th Oct A1,A2,A3,A4,A5,A6 A4,A5,A6 A1,A2,A3 B1,B2,B3

5 15th Oct B1,B2,B3,B4,B5,B6 B4,B5,B6 B1,B2,B3

6 29th Oct A1,A2,A3,A4,A5,A6 A1,A2,A3 A4,A5,A6

7 5th Nov Reading week

8 12th Nov A1,A2,A3,A4,A5,A6 B1,B2,B3 B4,B5,B6 A4,A5,A6 A1,A2,A3

9 19th Nov B1,B2,B3,B4,B5,B6 A1,A2,A3 A4,A5,A6 B4,B5,B6 B1,B2,B3

10 26th Nov A1,A2,A3,A4,A5,A6 B1,B2,B3 B4,B5,B6 A4,A5,A6

11 3rd Dec B1,B2,B3,B4,B5,B6 B4,B5,B6

12 10th Dec B1,B2,B3,B4,B5,B6 A1,A2,A3,A4,A5,A6

13 17th Dec B1,B2,B3,B4,B5,B6

14 24th Dec

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