Chapter 3 characteristics of highway components

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CHARACTERISTICS OF
THE DRIVER, THE PEDESTRIAN, THE VEHICLE, THE ROAD
Fayaz Rashid, MSc
Taxila Institute of Transportation Engineering (TITE)
COMPONENTS OF THE HIGHWAY MODE OF
TRANSPORTATION
Main components: Driver, the pedestrian, the vehicle, and the road
• Bicycle/Bikes are important too (design of urban highways and streets)
• Characteristics and limitations.
• Interrelationship
Road therefore must be designed to accommodate a wide range of vehicle
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DRIVER CHARACTERISTICS
• Transportation engineers problem
Varying skills and perceptual abilities of drivers on the highway (Sight and hearing vary
considerably across age groups)
Abilities may also vary for individual (alcohol, fatigue, time of day)
Criteria for design must be compatible
Use of mean value may not be enough for large numbers.
Percentile (higher the percentile wider will be range)
THE HUMAN RESPONSE PROCESS
Evaluation and reaction to information
Action taken by drivers by information they obtain from certain stimuli that they see or
hear
Information is usually;
Of Short time (continually changing information)
Visual (most of the information received)
Can be hearing perception.
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VISUAL RECEPTION
Visual reception (visual acuity, peripheral
vision, color, glare and depth perception.)
Visual Acuity:
• Ability to see fine details of an object..
Represented by the visual angle which is
the reciprocal of the smallest pattern detail
in minutes of arc that can be resolved
https://www.slideshare.net/FarhanaAdi/visual-acuity-2
ɸ=2arctan( L /2D)
L= diameter of the target (letter or symbol)
D=distance from the eye to target in the same units as L
For example, the ability to resolve a pattern detail with a visual acuity of one minute of
arc (1/60 of a degree) is considered the normal vision acuity (20/20 vision).
Normal vision acuity (20/20 vision)
Snellen
chart/
Eye
chart
Arc minute: angular measurement
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Static and dynamic visual acuity
Static - when both Driver and object are stationary (background brightness, contrast, and time are
factors which affect it).
Optimal time for identification is 0.5 and 1.0 seconds.
Dynamic - detect relatively moving objects. Drivers will see clearly devices within 12o cone.
Periphery– outer boundary
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GLARE VISION AND RECOVERY
Great importance during night driving and for old people.
Glare is the loss of visual performance or discomfort produced by an intensity of light in the
visual field greater than the intensity of light to which the eyes are adapted. Decrease of
visibility and causes discomfort.
• Direct : when relatively bright light appears in the individual’s field of vision.
• Specular: when the image reflected by the relatively bright light appears in the field of
vision
Simply put, glare occurs when too much light enters your
eye and interferes with your eye's ability to manage it.
Age has a significant effect on the sensitivity to glare.
Glare recovery the time required by a person to recover from the effects of glare after passing the
light source. i-e, 3 sec moving from dark to light and 6 sec when moving from light to dark.
To reduce Glare effects:
• Location of street lighting should be well in design – higher mounting heights, positioning
lighting supports away.
• By reducing luminaire brightness and by increasing the background brightness .
• Restricting light so that there would be minimum interference with the visibility of driver.
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DEPTH PERCEPTION
Particularly important in two-way lane during passing maneuvers, ( head-on crashes may
result due to poor judgment)
• Important Ability of the human eye to differentiate between objects is fundamental to
this phenomenon. But Not good in estimating absolute values (speed, distance, size, and
acceleration).
• Affects the ability of a person to estimate speed and distance.
Traffic control devices are standard in size, shape, and color as it aids in distance
estimation and helps colour blind drivers.
Hearing perception
Ear receives sound stimuli (important in warning sounds like Ambulance, traffic police car)
Can be corrected by a hearing aid.
PERCEPTION-REACTION PROCESS
• Perception: control device, warning sign, or object on the road
• Identification :driver identifies the object & understands the stimulus
• Emotion: decides on action (brake ,change lane etc.)
• Reaction/volition: driver actually executes the action.
Perception-reaction time or PIEV: time elapses during each of these sub processes
varies among individuals, vary for same person as the occasion changes.
Importance: In determination of braking distances Minimum sight distance
required on a highway.
Factors : situation complexity , environmental conditions, age, tiredness, drugs,
stimulus is expected or unexpected.
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Triggs and Harris described this phenomenon in detail. They noticed that 85th-percentile
time to brake varied from 1.26 to over 3 seconds. Reaction time for design purposes is
large enough to include reaction times for most drivers using the highways.
AASHTO specify 2.5 seconds for stopping-sight distances (about 90 percent of drivers)
Not suitable for unexpected or complex conditions such as grade interactions, ramp
terminals or when signals are unexpected – reaction time increases b 35%.
distance traveled during perception-reaction time
Example 3.1
A driver with a perception-reaction time of 2.5 sec is driving at 65 mi/h when she observes that an
accident has blocked the road ahead. Determine the distance the vehicle would move before the driver
could activate the brakes. The vehicle will continue to move at 65 mi/h during the perception-reaction
time of 2.5 sec.
Solution:-
convert mi/h to ft /sec:
v = 65 mi/h = (65 x 5280/3600) ft/sec
v = 95.55 ft/sec
(1mile = 5280 ft)
Distance traveled: D = vt
95.55 x 2.5 = 238.9 ft
Where v= velocity and t= time.
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OLDER DRIVERS’ CHARACTERISTICS
• As one grows older sensory, cognitive, and physical functioning ability declines(less safe
than young )
• Declining abilities reduced visual acuity, ability to see at night, flexibility ,motion
range, reduced muscle strength, narrower visual fields
A higher probability of being injured when involved in a crash
Higher crash risk : greater sensitivity to glare, higher reaction times, and For example;
greater sensitivity to glare, longer recovery time, failure to respond to obstacles etc.
For making decisions on highway design and operational characteristics (consider diminished
characteristics of older drivers).
Pedestrian characteristics
Pedestrian
“A person walking rather than travelling in a vehicle”
Pedestrian characteristics relevant to traffic and highway engineering practice
because it has influence on design and location of pedestrian control devices.
Control devices
o Special pedestrian signals
o Safety zones and islands at intersections
o Pedestrian underpasses
o Elevated walkways
o Crosswalks
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Characteristics includes:
o Visual
o Hearing
o Walking
Walking characteristics play a major part in the design of controls.
For example, the design of an all-red phase, which permits pedestrians to cross an
intersection with heavy traffic, requires knowledge of the walking speeds of
pedestrians.
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• The traffic engineer is responsible for designing a safe & suitable facilities for all
pedestrians.
• Special considerations should be given to small children, senior citizens,
physically handicapped and blind.
• For blind pedestrian special control devices should be installed for example ring
bell signal that conveys to pass on red phase.
• For handicapped pedestrian Ramps are also now being provided at intersection
curbs to facilitate the crossing of the intersection by the occupant of a wheelchair.
• Average walking speed of the handicapped pedestrian can vary from a low of 1.97
ft /sec to 3.66 ft /sec.
Bicyclists And Bicycles
• The basic human factors discussed for the automobile driver also apply to the
bicyclist/bike riders, particularly with respect to perception and reaction.
• Unlike the automobile driver, the bicyclist is not only the driver of the bicycle, but
he/she also provides the power to move the bicycle.
• The bicycle and the bicyclist therefore unite to form a system, thus requiring that
both be considered jointly.
• In Pakistan, 2.3 million motorbikes are induced on roads every year i-e, about
7500/day.
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Bicyclists And Bicycles Characteristics
• Three Classes of Bicyclists (A, B, And C) have been identified in the guide for the development
of Bicycle Facilities by AASHTO.
i. Experienced or advanced bicyclists are within class A
ii. Less experienced bicyclists are within class B
iii. Children riding on their own or with parents are classified as C
• Class A bicyclists typically consider the bicycle as A motor vehicle and can comfortably ride in
traffic.
• Class B bicyclists prefer to ride on neighborhood streets and are more comfortable on
designated bicycle facilities, such as bicycle paths.
• Class C bicyclists use mainly residential streets that provide access to schools, recreational
facilities, and stores.
Bicyclists And Bicycles Characteristics
For Designing
• In designing urban roads and streets, it is useful to consider the feasibility of
incorporating bicycle facilities that will accommodate class B and class C bicyclists.
• The bicycle, like the automobile, also has certain characteristics that are unique.
• For example, based on the results of studies conducted in Florida, Pein suggested the
minimum design speed for bicycles on level terrain is 20 mi/h.
i. But downgrade speeds can be as high as 31 mi/h, while upgrade speeds can be as low
as 8 mi/h.
ii. Pein also suggested that the mean speed of bicycles when crossing an intersection
from a stopped position is 8 mi/h and the mean acceleration rate is 3.5 ft/𝒔𝒆𝒄 𝟐
.
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Vehicle Characteristics
Criteria for the geometric design of highways are partly based on the static,
kinematic, and dynamic characteristics of vehicles.
• Static characteristics include the weight and size of the vehicle.
• kinematic characteristics involve the motion of the vehicle without considering
the forces that cause the motion.
• Dynamic characteristics involve the forces that cause the motion of the vehicle.
Vehicle Characteristics
• Since nearly all highways carry both passenger-automobile and truck traffic, it is
essential that design criteria take into account the characteristics of different types
of vehicles.
• A thorough knowledge of these characteristics will aid the highway and/or traffic
engineer in designing highways and traffic-control systems that allow the safe and
smooth operation of a moving vehicle, particularly during the basic maneuvers of
passing, stopping, and turning.
• Therefore, designing a highway involves the selection of a design vehicle, whose
characteristics will encompass those of nearly all vehicles expected to use the
highway in future.
• The characteristics of the design vehicle are then used to determine criteria for
geometric design, intersection design, and sight-distance requirements.
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Static Characteristics
The size of the design vehicle for a highway is an important factor in the determination
of design standards for several physical components of the highway.
• These include lane width, shoulder width, length and width of parking bays, and
lengths of vertical curves.
• The axle weights of the vehicles expected on the highway are important when
pavement depths and maximum grades are being determined.
• For many years, each state prescribed by law the size and weight limits for trucks
using its highways, and in some cases local authorities also imposed more severe
restrictions on some roads.
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Truck Type Allowed on National Highways Allowed on Motorways
2 AX SINGLE (BEDFORD) 20 17.5
2 AX SINGLE (HINO/
NISSAN)
23 17.5
3 AX TANDEM 32 27.5
3 AX SINGLE 32 29.5
4 AX SINGLE TANDEM 42 39.5
4 AX TANDEM SINGLE 42 39.5
4 AX SINGLE 44 41.5
5 AX SINGLE TRIDEM 51 48.5
5 AX TANDEM TANDEM 52 49.5
5 AX SINGLE SINGLE
TANDEM
54 51.5
5 AX TANDEM SINGLE
SINGLE
54 51.5
6 AX TANDEM TRIDEM 61 58.5
6 AX TANDEM SINGLE
TANDEM
64 61.5
Permissible Gross weight of vehicles in Tons NHA Pakistan
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The federal regulations specify the overall maximum gross weight for a group of
two or more consecutive axles should be determined by following equation.
W=500
𝑳𝑵
𝑵 𝟏
+ 𝟏𝟐𝑵 + 𝟑𝟔
where
W= overall gross weight
L = the extreme of any group of two or more consecutive axles (ft)
N= number of axles in the group under consideration
Example 3.2 Estimating Allowable Gross Weight of a Truck
A 5-axle truck traveling on an interstate highway has the following axle characteristics:
Distance between the front single axle and the first set of tandem axles =20 ft
Distance between the first set of tandem axle and the back set of tandem Axles= 48 ft
If the overall gross weight of the truck is 79,500 lb, determine whether this truck
satisfies federal weight regulations.
Solution: Although the overall gross weight is less than the maximum allowable of
80,000 lb, the allowable gross weight based on the axle configuration
should be checked.
Use Eq.
W=500 + 12𝑁 + 36
W=500 + 12𝑋4 + 36
W=74000 lb
which is less than the allowable of 80,000 lb. The truck therefore satisfies the federal
truck weight regulations.
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12 ft 48 ft
• The static characteristics of vehicles expected to use the highway are factors that
influence the selection of design criteria for the highway.
• It is therefore necessary that all vehicles be classified so that representative static
characteristics for all vehicles within a particular class can be provided for design
purposes.
• AASHTO has selected four general classes of vehicles:
i. Passenger cars
ii. Buses
iii. Trucks
iv. Recreational vehicles
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• Included in the passenger-car class are sport/utility vehicles, minivans, vans, and
pick-up trucks.
• Included in the bus class are intercity motor coaches and city transit, school, and
articulated buses.
• Within the class of trucks are single-unit trucks, truck tractor-semitrailer
combinations, and trucks or truck tractors with semitrailers in combination with full
trailers.
• Within the class of recreational vehicles are motor homes, cars with camper trailers,
cars with boat trailers, and motor homes pulling cars.
• A sport utility vehicle (SUV)
is a generic marketing term for
a vehicle similar to a station
wagon, but built on a light-
truck chassis it is usually
equipped with four-wheel
drive for on- or off-road
ability, and with some
pretension or ability to be used
as an off-road vehicle.
The passenger-car class (sport/utility vehicles)
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• A van is a kind of vehicle used for transporting goods or groups of people.
The passenger-car class (vans)
• A van is a kind of vehicle used for transporting goods or groups of people.
The passenger-car class (minivans)
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• A pickup truck is a light motor vehicle with an open-top rear cargo area (bed) which is
almost always separated from the cab to allow for chassis flex when carrying or pulling
heavy loads
The passenger-car class (pickup truck )
• A coach (also motor coach) is a large motor vehicle for conveying passengers on
excursions and on longer distance express coach scheduled transport between cities - or
even between countries.
The bus class (intercity motor coaches)
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• Lahore city transit buses are Speedo Buses
The bus class (intercity motor coaches)
• School bus is a type of bus designed and manufactured for student transport:
carrying children and teenagers to and from school and school events.
The bus class (school bus)
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• An articulated bus is a bus which is articulated, essentially meaning it
bends in the middle. For example Islamabad metro buses.
The bus class (articulated bus)
It is usually a single-deck design, and comprises two rigid sections linked
by a pivoting joint.
The Truck Class
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The Truck Class
The recreational vehicles class (motor homes)
Recreational vehicle or RV is, in North America, the usual term for a Motor vehicle or
trailer equipped with living space and amenities found in a home.
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The recreational vehicles class
(cars with camper trailers)
(cars with boat trailers)
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(motor homes pulling cars)
• A total of 19 different design vehicles have been selected to represent the
different categories of vehicles within all four classes.
• Table 3.2 shows the physical dimensions for each of these design vehicles, and
figure 3.1 shows examples of different types of trucks.
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AASHTO also has suggested the following guidelines for selecting a
design vehicle:
• For a parking lot or series of parking lots, a passenger car may be used.
• For intersections on residential streets and park roads, a single-unit truck
could be considered.
• For the design of intersections of state highways and city streets that
serve bus traffic but with relatively few large trucks, a city transit bus may
be used.
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• For the design of intersections of highways with low-volume county and
township/ local roads with average annual daily traffic (AADT) of 400 or less, a
large school bus with a capacity of 84 passengers or a conventional bus with a
capacity of 65 passengers may be used. The selection of the bus type depends on
the expected frequency of each of the buses on the facility.
• For intersections of freeway ramp terminals and arterial highways, and for
intersections of state highways and industrial streets with high traffic volumes, or
with large truck access to local streets, the wb-20 (wb-65 or 67) may be used.
Minimum turning radii
• In carrying out the design of any of the intersections the minimum turning
radius for the selected design vehicle traveling at a speed of 10 mi/h Should be
provided.
• Minimum turning radii at low speeds (10 mi/h or less) are dependent mainly
on the size of the vehicle.
• The turning-radii requirements for single-unit (SU) truck and the wb-20 (wb-
65 and wb-67) design vehicles are given in figures 3.2 and 3.3 respectively.
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Minimum turning radii
The turning-radii requirements for other vehicles can be found in
AASHTO’s policy on geometric design of highways and streets.
3.6.2 KINEMATIC CHARACTERISTICS
Definition
kinematic characteristics involve the motion of the vehicle without considering the
forces that cause the motion.
The primary element among kinematic characteristics is the acceleration capability of
the vehicle
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Importance
Acceleration capability is important in several traffic operations, such as
passing maneuvers and gap acceptance.
Also, the dimensioning of highway features such as freeway ramps and
passing lanes is often governed by acceleration rates.
Acceleration is also important in determining the forces that cause motion.
Therefore , a study of the kinematic characteristics of the vehicle primarily
involves a study of how acceleration rates influence the elements of motion,
such as velocity and distance.
We therefore review in this section the mathematical relationships among
acceleration, velocity, distance, and time.
Mathematical relationships
Let us consider a vehicle moving along a straight line from point 0 to point
m, a distance x in a reference plane T. The position vector of the vehicle
after time t may be expressed as
o m
x
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Mathematical relationships
The velocity and acceleration for m may be simply expressed as:
o m
x
Acceleration Assumed Constant
Two cases are of interest: (1) acceleration is assumed constant; (2) acceleration is a
function of velocity.
When the acceleration of the vehicle is assumed to be constant,
The constants C1 and C2 are determined either by the initial conditions on velocity
and position or by using known positions of the vehicle at two different times.
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Acceleration as a Function of Velocity
The assumption of constant acceleration has some limitations,
because the accelerating capability of a vehicle at any time t is related to the
speed of the vehicle at that time (ut).
The lower the speed, the higher the acceleration rate that can be obtained.
Figures 3.4a and 3.4b show maximum acceleration rates for passenger cars and
tractor-semitrailers at different speeds on level roads.
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Acceleration as a Function of Velocity
One model that is used commonly in this case is:
 After derivation, the velocity and the position are as follows:
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3.6.3 DYNAMIC CHARACTERISTICS
 Several forces act on a vehicle while it is in motion:
1. air resistance,
2. grade resistance,
3. rolling resistance, and
4. curve resistance
1. Air Resistance
 A vehicle in motion has to overcome the resistance of the air in front of it as well as
the force due to the frictional action of the air around it.
 The force required to overcome these is known as the air resistance and is related
to the cross-sectional area of the vehicle in a direction perpendicular to the
direction of motion and to the square of the speed of the vehicle.
 Claffey has shown that this force can be estimated from the formula
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2. Grade Resistance
When a vehicle moves up a grade, a component of the weight of the vehicle acts
downward, along the plane of the highway.
This creates a force acting in a direction opposite that of the motion. This force is
the grade resistance.
A vehicle traveling up a grade will therefore tend to lose speed unless an
accelerating force is applied.
Note:
grade resistance = weight X grade, in decimal.
3. Rolling Resistance
 There are forces within the vehicle itself that offer resistance to motion.
 These forces are due mainly to frictional effect on moving parts of the vehicle, but
they also include the frictional slip between the pavement surface and the tires.
 The sum effect of these forces on motion is known as rolling resistance.
 The rolling resistance depends on the speed of the vehicle and the type of
pavement.
 Rolling forces are relatively lower on smooth pavements than on rough pavements.
 The rolling resistance force for passenger cars on a smooth pavement can be
determined from the relation
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Where:
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4. Curve Resistance
When a passenger car is maneuvered to take a curve, external forces act on the
front wheels of the vehicle.
 These forces have components that have a retarding effect on the forward motion
of the vehicle.
The sum effect of these components constitutes the curve resistance.
This resistance depends on the radius of the curve, the gross weight of the vehicle,
and the velocity at which the vehicle is moving.
It can be determined as:
Power requirements
Power is the rate at which work is done.
It is usually expressed in horsepower (a U.S. unit of measure),
where 1 horsepower is 746 W.
The performance capability of a vehicle is measured in terms of the horsepower the
engine can produce to overcome air, grade, curve, and friction resistance forces
and put the vehicle in motion.
Figure 3.6 shows how these forces act on the moving vehicle.
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Power requirements
Power requirements
 The power delivered by the engine is:
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Braking distance
The action of the forces (shown in Figure 3.6) on the moving vehicle and the effect of
perception-reaction time are used to determine important parameters related to the
dynamic characteristics of the vehicles.
These include the braking distance of a vehicle and the minimum radius of a circular
curve required for a vehicle traveling around a curve with speed u where u > 10 mi/h
Braking distance
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A similar equation could be developed for a vehicle traveling uphill , in which case
the following equation is obtained.
A general equation for the braking distance can therefore be written as:
AASHTO recommends the coefficient of friction to be a/g and a to be 11.2
ft/s2 , then braking distance becomes:
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Similarly, it can be shown that the horizontal distance traveled in reducing the speed
of a vehicle from U1 to U2 in mi/h during a braking maneuver is given by:
The distance traveled by a vehicle between the time the driver observes an object
in the vehicle's path and the time the vehicle actually comes to rest is longer than the
braking distance, since it includes the distance traveled during perception-reaction
time. This distance is referred to in this text as the stopping sight distance S and is
given as
it is the perception-reaction(in seconds)
and u is the velocity in ft/sec at which the vehicle was traveling when the brakes
were applied.
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Estimation of Velocities
 It is sometimes necessary to estimate the speed of a vehicle just before it is
involved in a crash.
 This may be done by using the braking-distance equations if skid marks can be
seen on the pavement.
 The steps taken in making the speed estimate are as follows:
Step 1. Measure the length of the skid marks for each tire and determine
the average. The result is assumed to be the braking distance Db of the
vehicle.
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Step 2: Determine the coefficient of friction f by performing trial runs at
the site under similar weather conditions, using vehicles whose tires are in a
state similar to that of the tires of the vehicle involved in the accident. This
is done by driving the vehicle at a known speed Uk and measuring the
distance traveled Dk while braking the vehicle to rest. The coefficient of
friction fk can then be estimated by using:
Alternatively, a value of 0.35 for a/g can be used for fk
Step 3: Use the value of fk obtained in step 2 to estimate the unknown
velocity uu just prior to impact; that is, the velocity at which the vehicle
was traveling just before observing the crash. This is done by using Eq.
3.26.
If it can be assumed that the
application of the brakes reduced
the velocity uu to zero, then uu may
be obtained from
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However, if the vehicle involved in the accident was traveling
at speed U1 when the impact took place and the speed U1 is
known , then using Eg. 3.24, the unknown
speed Uu just prior to the impact may be obtained from
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MINIMUM RADIUS OF A CIRCULAR CURVE
When a vehicle is moving around a circular curve, there is an inward radial force
acting on the vehicle, usually referred to as the centrifugal force.
 There is also an outward radial force acting toward the center of curvature as a
result of the centripetal acceleration.
In order to balance the effect of the centripetal acceleration, the road is inclined
towards the center of the curve.
 The inclination of the roadway toward the center of the curve is known as super
elevation.
The centripetal acceleration depends on the component of the vehicle’s weight along
the inclined surface of the road and the side friction between the tires and the
roadway.
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• The minimum radius of a circular curve R for a vehicle traveling at u mi/h can be
determined by considering the equilibrium of the vehicle with respect to its moving
up or down the incline.
• If a is the angle of inclination of the highway, the component of the weight down
the incline is wsina,and the frictional force also acting down the incline is wf cos a.
The centrifugal force fc is
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• When the vehicle is in equilibrium with respect to the incline (that is, the vehicle moves
forward but neither up nor down the incline), we may equate the three relevant forces
and obtain
• Where fs = coefﬁcient of side friction and (u2/g) = r(tan a + fs). This gives
• Tan a, the tangent of the angle of inclination of the roadway, is known as the rate of
super elevation e. Equation can therefore be written as
• Again, if g is taken as 32.2 ft/sec2 and u is measured in mi/h, the minimum radius R is
given in feet as
• There are, however, stipulated maximum values that should be used for either e or fs. For
highways located in rural areas with no snow or ice, a maximum super elevation rate of
0.10 generally is used. For highways located in areas with snow and ice, values ranging
from 0.08 to 0.10 are used. For expressways in urban areas, a maximum super elevation
rate of 0.08 is used.
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ROAD CHARACTERISTICS
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SIGHT DISTANCE
• Sight distance is the length of the roadway a driver can see ahead at any particular
time.
• The sight distance should be such that at design speed, there is enough time to avoid a
collision after an object is observed.
• Two types are
1.Stopping sight distance
2.Passing sight distance
STOPPING SIGHT DISTANCE
• The minimum sight distance required for a driver to stop a vehicle after seeing an
object in the vehicle’s path.
• This distance is the sum of the distance traveled during perception-reaction time
and the distance traveled during braking.
Where
• u=speed in mi/h
• a=deceleration rate
• g=acceleration due to gravity(32.2 ft/s2)
• G=grade
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• Highways be designed such that sight distance along the highway is at least equal to the
SSD.
• The ssd requirements dictate the minimum lengths of vertical curves and minimum radii for
horizontal curves.
• The value in the table 3.4 are for horizontal alignment and grade zero.
• On upgrades, the ssds are shorter; on downgrades, they are longer.
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DECISION SIGHT DISTANCE
• Distance required for a driver to detect an unexpected or otherwise difficult-to-perceive
information source or hazard in a roadway environment that may be visually cluttered,
recognize the hazard of its threat potential, select an appropriate speed and path, and
initiate
and complete the required safety maneuvers safely and efficiently.
• The decision sight distances depend on the type of maneuver required to avoid the
hazard on the road, and also on whether the road is located in a rural or urban area.
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PASSING SIGHT DISTANCE
• The passing sight distance is the minimum sight distance required on a two-lane, two-
way highway that will permit a driver to complete a passing maneuver without colliding
with an opposing vehicle and without cutting off the passed vehicle.
• The passing sight distance will also allow the driver to successfully abort the passing
maneuver if he so desires.
• Only single passes are considered.
• In order to determine the minimum passing sight distance, certain assumption have to be
made regarding the movement of the passing vehicle during a passing maneuver.
ASSUMPTIONS REGARDING THE VEHICLE
1.The vehicle being passed (impeder) is traveling at a uniform speed.
2.The speed of the passing vehicle is reduced and is behind the impeder as the
passing section is entered.
3.On arrival at a passing section, some time elapses during which the driver
decides whether to undertake the passing maneuver.
4.If the decision is made to pass, the passing vehicle is accelerated during the
passing maneuver, and the average passing speed is about 10 mi/h more than
the speed of the impeder vehicle.
5.A suitable clearance exists between the passing vehicle and any opposing
vehicle when the passing vehicle reenters the right lane.
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• The minimum passing sight distance is the total of four
components
• d1 = distance traversed during perception-reaction time and
during initial acceleration to the point where the passing
vehicle just enters the left lane
d2 = distance traveled during the time the passing vehicle is
traveling in the left lane
d3 = distance between the passing vehicle and the opposing
vehicle at the end of the passing maneuver
d4 = distance moved by the opposing vehicle during two
thirds of the time the passing vehicle is in the left lane (usually
taken to be 2/3 d2)
• The distance d1 is given as
• Where
• t1=initial maneuver time
• a= average acceleration rate(mi/h/sec)
• u=average speed of passing vehicle(mi/h)
• m=difference between speeds of passing and impeder vehicles
• The distance d2 is obtained from
• Where
• t2=time spent by passing vehicle in left lane
• u=average speed(mi/h)
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• The clearance distance d3 between the passing vehicle and the opposing vehicle at the
completion of the passing maneuver has been found to vary between 100 ft and 300 ft.
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Chapter 3 characteristics of highway components

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