Chapter 1 - Lecture 2 Notes

8/9/2019 Chapter 1 - Lecture 2 Notes

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Chapter 2, Lecture 2: Design Characteristics Part 2

TRAFFIC CHARACTERISTICS

General Characteristics

The design of a highway and its features should be based upon explicit consideration ofthe traffic volumes and characteristics to be served.

Traffic data for a road or section of road are generally available or can be obtained fromfield studies. The data collected by State or local agencies include traffic volumes for:

Traffic volume by days of the year

Traffic volume by time of the day

Traffic distribution of vehicles by typeTraffic distribution of vehicles by weight

Vlu!e Average Daily Traffic

The ADT is defined as the total volume during a given time period (in whole days)greater than one day and less than one year divided by the number of days in that time

period.

Peak Hour Traffic

Traffic volumes for an interval of time shorter than a day more appropriately reflect the

operating conditions that should be used for design. !n nearly all cases a practical and

ade"uate time period is one hour.

#$$T %xhibit &'&

*ased on research it is recommended that the hourly traffic volume that should generally

be used in design is the +,th highest hourly volume of the year abbreviated as +, -.

Directinal Distri"utin

/or two'lane rural highways the design hourly volume is the total traffic in both

directions of travel. !n the design of highways with more than two lanes and on two'laneroads where important intersections are encountered or where additional lanes are to be

provided later 0nowledge of the hourly traffic volume for each direction of travel is

essential.

A multilane highway with a high percentage of traffic in one direction during the pea0

hours may need more lanes than a highway having the same ADT but with a lesser percentage of directional traffic.



The directional distribution of traffic on multilane facilities during the design hour

(DD-) should be determined by ma0ing field measurements on the facility under

consideration or on parallel and similar facilities.!n designing intersections and interchanges the volumes of all movements occurring

during the design hour should be 0nown.

C!psitin # Tra##ic

ehicles of different si1es and weights have different operating characteristics that should

be considered in highway design. *esides being heavier truc0s are generally slower and

occupy more roadway space. 2onse"uently truc0s have a greater individual effect on

highway traffic operation than do passenger vehicles. The effect on traffic operation ofone truc0 is often e"uivalent to several passenger cars.

/or uninterrupted traffic flow as typically found in rural areas the various si1es and weights of

vehicles as they affect traffic operation can be grouped into passenger cars or truc0s.

/or traffic'classification purposes truc0s are normally defined as those vehicles having

manufacturer3s gross vehicle weight (45) ratings of 6 ,,, 0g 78,,, lb9 or more and

having dual tires on at least one rear axle.

/or design purposes the percentage of truc0 traffic during the pea0 hours should be

determined. Data on traffic composition should be determined by traffic studies. Truc0traffic should be expressed as a percentage of total traffic during the design hour.

Pr$ectin # Future Tra##ic De!an%s

any highway engineers believe the maximum design period is in the range of ;< to &6years. Therefore a period of &, years is widely used as a basis for design.

7Traffic cannot usually be forecast accurately beyond this period on a specific facility

because of probable changes in the general regional economy population and landdevelopment along the highway which cannot be predicted with any degree of

assurance.9

Spee%

The speed of vehicles on a road or highway depends in addition to capabilities of the

drivers and their vehicles upon five general conditions:

the physical characteristics of the highway

the amount of roadside interferencethe weather

the presence of other vehicles



the speed limitations (established either by law or by traffic control devices).

Operating Speed

=perating speed is the speed at which drivers are observed operating their vehicles during

free'flow conditions. The <th percentile of the distribution of observed speeds is themost fre"uently used measure of the operating speed associated with a particular location

or geometric feature.

Running Speed

The speed at which an individual vehicle travels over a highway section is 0nown as its

running speed. The running speed is the length of the highway section divided by the

running time re"uired for the vehicle to travel through the section. The average runningspeed is the sum of the distances traveled by vehicles on a highway section during a

specified time period divided by the sum of their running times.

Design Speed Design speed is a selected speed used to determine the various geometric design features

of the roadway. =nce the design speed is selected all of the pertinent highway featuresshould be related to it to obtain a balanced design. Some design features such as

curvature superelevation and sight distance are directly related to and vary appreciably

with design speed. =ther features such as widths of lanes and shoulders and clearancesto walls and rails are not directly related to design speed but they do affect vehicle

speeds. Therefore wider lanes shoulders and clearances should be considered for higher

design speeds. Thus when a change is made in design speed many elements of the

highway design will change accordingly.

A design speed of >, mph should be used for freeways expressways and other arterial

highways in rural areas.

4iven an overall range in design speeds of ;< to >< mph used in geometric design it is

desirable to select design speeds in increments of < mph.

?rban arterial streets should be designed and control devices regulated where practical

to permit running speeds of &, to 6< mph. !t follows that the appropriate design speeds

for arterials should range from +, to @, mph.

Tra##ic Fl& Relatinships

Traffic flow conditions on roadways can be characteri1ed by the volume flow rate

expressed in vehicles per hour the average speed in 0ilometers per hour 7miles per hour9and the traffic density in vehicles per 0ilometer 7vehicles per mile9.

#$$T %xhibit &'+,



HIGH'A( CAPACIT(

General Characteristics

The term capacityB is used to express the maximum hourly rate at which persons or

vehicles can reasonably be expected to traverse a point (i.e. a uniform section of a lane

or a roadway) during a given time period under prevailing roadway and traffic conditions.

Capacit) as a Design Cntrl

Design Service Flow Rate Versus Design Volume

The design volume is the volume of traffic proCected to use a particular facility during the

design year which is usually ;, to &, years in the future. Design volumes are estimatedin the planning process and are often expressed as the expected traffic volume during a

specified design hour.

Design service flow rate is the maximum hourly flow rate of traffic that a highway with particular design features would be able to serve without the degree of congestion falling

below a pre'selected level.

easures of !ongestion

umerous measures of the overall service provided by a roadway section have been

suggested including safety freedom to maneuver the ratio of traffic volume to capacity

(vEc) operating speed average running speed and others. !n the case of signali1ed

intersections the stopped delay encountered by motorists is a commonly used measure of congestion.

This maximum rate of flow for any given facility is defined as its capacity. As capacity isapproached flow becomes more unstable because available gaps in the traffic stream

become fewer and fewer. At capacity there are no usable gaps in the traffic stream and

any conflict from vehicles entering or leaving the facility or from internal lane changingmaneuvers creates a disturbance that cannot be effectively damped or dissipated.

/or uninterrupted flow the principal measures are speed density and flow.

/or interrupted flow in the vicinity of intersections average stopped'time delay is the

principal measure of effectiveness used.

Accepta"le Degrees of !ongestion

The degree of congestion that should not be exceeded during the design year on a proposed highway can be realistically assessed by:

(;) determining the operating conditions that the maCority of motorists will accept as

satisfactory



(&) determining the most extensive highway improvement that the governmental

Curisdiction considers practical

(+) reconciling the demands of the motorist and the general public with the financesavailable to meet those demands.

Some principles or guidelines that should aid in arriving at such decisions include:;. The highway should be so designed that when it is carrying the design volume the

traffic demand will not exceed the capacity of the facility even during short intervals of

time.

&. The design volume per lane should not exceed the rate at which traffic can dissipate

from a standing "ueue.

+. Drivers should be afforded some choice of speed. The latitude in choice of speed should

be related to the length of trip.

6. =perating conditions should be such that they provide a degree of freedom from driver tension that is related to or consistent with the length and duration of the trip.

<. There are practical limitations that preclude the design of an ideal freeway.

@. The attitude of motorists toward adverse operating conditions is influenced by their

awareness of the construction and right'of'way costs that might be necessary to provide

better service.

Factrs *ther Than Tra##ic Vlu!e that A##ect *perating Cn%itins

Hig#way FactorsSome factors that can3t be avoided but have impacts on operating conditions include:

!ntersections (even unsignali1ed)Development adCacent to the highway

Sharp curves

Steep grades

Alignment

/or traffic traveling at any given speed the better the roadway alignment the more trafficit can carry.

$eaving Sections5eaving sections are highway segments where the pattern of traffic entering and leaving

at contiguous points of access results in vehicle paths crossing each other. !t is generally

accepted that a reduction in operating speed of about < mph below that for which the

highway as a whole operates can be considered a tolerable degree of congestion forweaving sections.

#$$T %xhibit &'++



Ramp Terminals

Famps and ramp terminals are features that can adversely influence operating conditions

on freeways if the demand for their use is excessive or if their design is deficient. The

loss in efficiency is a function of the volume of traffic entering or leaving ramps the

distance between points of entry and exit and the geometric layout of the terminals.

Traffic Factors

Traffic streams are usually composed of a mixture of vehicles: passenger cars truc0s

buses and occasionally recreational vehicles and bicycles. /urthermore traffic does not

flow at a uniform rate throughout the hour day season or year.

Le+els # Ser+ice

The -2 defines the "uality of traffic service provided by specific highway facilities

under specific traffic demands by means of a level of service. The level of servicecharacteri1es the operating conditions on the facility in terms of traffic performance

measures related to speed and travel time freedom to maneuver traffic interruptions and

comfort and convenience.

#$$T %xhibit &'+;

!n heavily developed sections of metropolitan areas conditions may ma0e the use of

level'of'service D appropriate for freeways and arterialsG however this level should be

used sparingly and at least level'of'service 2 should be sought.

#$$T %xhibit &'+&

Design Ser+ice Fl& Rates

=nce a level of service has been selected it is desirable that all elements of the roadway

are designed consistent to this level. This consistency of design service flow rate resultsin near'constant freedom of traffic movement and operating speed and flow interruptions

due to bottlenec0s can be avoided.

Chapter 1 - Lecture 2 Notes

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