Main Components of the Highway Mode of

Transportation

[Chapter 3]

Dr. TALEB M. AL-ROUSAN

Introduction The Four Main Components of the highway mode of

transportation are: The driver The pedestrian The vehicle The road The bicycle (becoming important in the design or

urban highways and streets) Its essential to know the characteristics and

limitations of these components and the interrelationship between them.

Their characteristics are also important when traffic engineering measures (e.g. traffic control devices) are to be used in the highway mode.

Driver Characteristics Skills and perceptual abilities (hear, see,

evaluate, react) of drivers vary on the highway.

These abilities may also vary under different conditions (influence of alcohol, fatigue, and time of the day).

Design criteria should be compatible with drivers varying characteristics.

85th or 95th percentile are usually selected for design criteria (Note: as % increase, wider range covered).

Driver Characteristics/ Human Response Process

1- Visual reception Drivers receive information from all senses, but more

than 90 percent is received visually. The principal characteristics of the eye are:

Visual acuity: ability to see fine details of an object (static & dynamic)

peripheral vision: ability to see objects beyond the cone of clearest vision.

color vision: ability to differentiate among colors. (color blindness).

glare vision and recovery: due to bright light (3 sec from dark to light and 6 sec from light to dark).

depth perception: affects the ability of person to estimate speed and distance (important on two-lane highways during passing maneuvers).

Driver Characteristics/ Human Response Process

2- Hearing perception Important only when warning

sounds given out by emergency vehicles.

Loss of some hearing ability is not a serious problem since it can be normally corrected by a hearing aid.

Perception Reaction process Perception: driver see a control device,

warning sign, or object on the road. Identification: driver identifies the objective

and understand the stimulus. Emotion: driver decide what action to take

in response to the stimulus ( to step on the brake, to swerve, or change lane)

Reaction or volition: driver execute the action decided on during emotion sub-process.

Perception Reaction process The elapsed time in perception reaction

process is known as Perception-reaction time.

This time is important in determining: The braking distance, which dictates the

minimum sight distance required on a highway, and

The length of the yellow phase at signalized intersections

The perception reaction time varies among individuals and may vary for the same person as the occasion changes.

Perception Reaction Time Changes in perception reaction time depend on:

Situation Complexity Existing environmental conditions: familiarity,

visibility Driver age Condition of the driver (fatigued/ rested, under

influence of drugs and/or alcohol,…) Stimulus is expected or not.

AASHTO recommends 2.5 seconds for stopping sight distance (90% of drivers under most highway conditions)

Note: unexpected conditions may require higher PRT

Older Drivers

Important characteristics Slower information processing Slower reaction time Slower decision making Visual deterioration Hearing deterioration Difficulty to judge time, speed and

distance Side effects of prescription drugs

Pedestrian Characteristics Pedestrian Characteristics relevant to traffic and

highways engineering practice include those of the driver (visual and hearing).

In addition: Walking characteristics: affect for example all-

red phase. Walking speed according Highway Capacity

Manual is 4.0 ft/sec and can be reduced to 3.3 ft/sec when % of elderly pedestrians in more than 20%).

Pedestrian characteristics affect the design and location of pedestrian control devices such as: special signals, safety zones, islands at intersections, pedestrian underpasses, elevated walkways, and cross walks).

Bicyclists and Bicycles Human factors are same for drivers with respect to

perception and reaction. Note: bicyclists are not only the drivers of the bicycles but

also provide the power to move the bicycle, thus should be considered jointly.

AASHTO classifies three classes: A: Experienced or advanced bicyclists: consider the bicycle as

a motor vehicle and can ride in traffic. B: Less experienced bicyclists: prefer to ride on neighborhood

streets and more comfortable on designated bicycle facilities. C: Children riding on their own or with parents: use residential

streets that provide access to schools, recreational facilities, and stores.

In designing urban roads and streets it is useful to consider the feasibility of incorporating bicycle facilities that will accommodate classes B and C.

Min design speed at level terrain is 20 mi/h, 31mh/h on down grade, 8 mi/h crossing intersection from stop position, and mean acceleration rate is 3.5 ft.sec2

Vehicle Characteristics Criteria for the geometric design of highways are

partly based on the vehicles following characteristics: Static: weight and size. kinematic: motion of the vehicle. Dynamic: forces that cause the motion of the vehicle.

Knowledge of these characteristics will aid the highway and traffic engineer in designing highways and traffic control systems that allow safe and smooth operations during maneuvers of passing, stopping, and turning.

In designing a highway, A design vehicle is selected. The characteristics of the design vehicle are used to

determine criteria for geometric design, intersection design, and sight-distance requirements

Static Characteristics Size of design vehicle affects design standards for

several physical components of the highway. Lane width Shoulder width Length and width of parking bays Length of vertical curves

The axle weight of the vehicles are important in determining: Pavement thickness (depth) Maximum grade

See Table 3.1 for range of max. allowable values of length and weight of vehicles.

Vehicle Static Characteristics Height

Overall height – influences vertical clearance Clearance for overpass & bridges (always use

consistent measurements) Driver eye height – influences sight distance Center of gravity height – influences rollover

threshold (higher CG leads to higher risk) Width – influences cross-section elements Length – influences vehicle storage areas (turn

bays, parking, etc.) Configuration – influences alignment design

Static Characteristics Cont. AASHTO selected three general classes:

Passenger cars ( compact, sub-compact, all light vehicles, and all light delivery trucks (vans and pickups)).

Trucks (single unit truck, truck tractor-semitrailer, and truck tractors with semi trailers in combination with full trailers).

Buses/recreational vehicles ( single unit buses, motor homes, passenger cars or motor homes pulling trailers or boats

See Table 3.2 for design vehicle dimensions.

Turning Paths

Min. turning radii at low speeds depend mainly on the size of the vehicle.

The boundaries are delimited by the outer trace of the front overhang and the path of the inner rear wheel.

Trucks and buses require more generous designs than passenger cars.

See Figures 3.2 and 3.3 for min. turning path for passenger car and WB-62 design vehicle, respectively.

Turning radii requirements are found in AASHTO policy of geometric design of highways and streets.

Kinematic characteristics

Acceleration characteristics affect: passing maneuvers and gap acceptance Dimensions of highway features (free

way ramps and passing lanes Determining the forces that cause

motion. (velocity and distance).

Operational Characteristics Weight-to-Horsepower Ratio:

influences acceleration/deceleration of vehicles Influences speed maintenance on grades

Acceleration/deceleration Influences sight distance, speed change lane

Speed maintenance Influences length and steepness of grades

Passenger cars usually not a concern.

Dynamic Characteristics

Several forces act on a vehicle while it is in motion resisting the tractive force delivered from the vehicle engine: Air resistance Grade resistance Rolling resistance and friction Curve resistance

Rolling Resistance Rollover Threshold

Definition: maximum lateral acceleration that can be achieved without causing rollover

Influences horizontal design alignment Influences roadside design materials. Usually problem for vehicles with high CG, however, vehicle

with lower CG can also rollover more often. Rollover Threshold for Trucks

Very high CG Loads may be uneven Effects of liquids (gas, oil, etc.) in emergency maneuvers Important design problems for freeway entrances and exit

ramps The maximum rollover threshold should be around 0.3 g for

horizontal curve design (85th percentile is 0.4 g)

Minimum Radius of Circular Curve When a vehicle is moving a round a

circular curve, there is an inward radial force acting on the vehicle (centrifugal force).

There is also outward radial force imagined by the driver as a result of the centripetal acceleration acting toward the center of the curvature.

In order to balance the effect of centripetal acceleration, the road is inclined toward the center of the curve (Superelevation).

Superelevation The min. 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 : (α) is the angle of inclination of the highway. Wsin α = component of weight down the incline Wfs cos α = frictional force acting down the incline Fc = (W ac) / g = centrifugal force

where: ac = acceleration = u2/RW = weight of the vehicleg = acceleration of gravityfs = coefficient of side friction (see

Table 3.3)

Superelevation Cont. When vehicle is in equilibrium with respect to the

incline, we may equate three relevant forces and obtain:

[(W u2)/gR] cos α = Wsin α + Wfs cos α[u2/g] = R (tan α + fs )R = u2/[g (tan α + fs )]

tan α = tangent of the angle of inclination

=rate of superelevation (e)R = u2/[g (e + fs )]

If (g= 32.2 ft/sec2), and (u in mi/h), and (R in feet), then:

R = u2/[15 (e + fs )]

Factors Controlling Superelevation There are max. values for e and fs

Max Rate of superelevation (e) is affected by: Location of Highway (urban or rural):

rural highway with no snow or ice (e = 0.10), Rural highway with snow & ice (e= 0.08 - 0.10) Expressway in urban area (e= 0.08) Local urban roads usually not superelevated due to low

speeds. Weather conditions (rain or snow) Distribution of slow moving traffic within the traffic

stream. Values used for side friction (fs) generally vary with

design speed (lower for higher speeds) and superelevation. See Table 3.3

Relationship between vehicular and facility characteristics

Vehicular characteristicsRelated facility characteristics

LengthParking stall lengthTransit station platform length

WidthLane widthParking stall widthLateral clearance

heightVertical clearanceMinimum vertical curve length

Wheel base (turning radius)Lateral clearance on curvesIntersection edge radii

WeightStructural design of surfaceStructural design of guidewayStructural design of bridges

Acceleration / decelerationMaximum gradeMinimum vertical curve lengthHorizontal curve radius

SpeedHorizontal curve radiusMinimum vertical curve lengthMaximum superelevation

liftRunway length

Road Characteristics The characteristics of the highway are related to

stopping and passing because they have more direct relation to the characteristics of the driver and the vehicle discussed earlier.

Sight Distance: the length of the roadway a driver can see a head at any particular time.

The sight distance available at each point of the highway must be such that when a driver is traveling at the highway’s design speed, adequate time is given, after an object is observed in the vehicles path, to make the necessary evasive maneuvers without colliding with the object.

Types of sight distance: Stopping Sight Distance (SSD). Passing Sight Distance (PSD).

Stopping Sight Distance (SSD) The minimum sight distance required for a

driver to stop a vehicle after seeing an object in the vehicle’s path without hitting that object.

SSD = SUM (distance travel during perception reaction time + distance traveled during braking)

It is essential that the highway be designed such that the sight distance along the highway is at least equal to the SSD.

SSD requirements dictates: Min radii for horizontal curves. Min length of vertical curve

Stopping Sight Distance (SSD) Table 3.4 shows SSD’s for different design speeds for

horizontal alignment and zero grade. On upgrades, SSD’s are shorter On downgrades, SSD’s are longer

SSD = 1.47 u t + [u2 / (30 [(a/g) ± G)])] SSD = stopping sight distance (ft)u = vehicle speed (mi/h)t = perception-reaction time (sec) (2.5 sec mostly)G = grade percent (+ uphill, -ve down hill)g = acceleration of gravity (32.2 ft/sec2)a = deceleration of the vehicle when the brakes are applied

(11.2 ft /sec2 most comfortable)

Decision Sight Distance When stimulus is un expected for the driver, SSD’s in

Table 3.4 become inadequate. When the driver is expected to make unusual

maneuvers, longer SSD’s are usually required since the perception reaction time is much longer.

The longer sight distance is the decision sight distance.

Decision sight distance: distance required for a driver to detect an unexpected or other-wise different 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.

Decision Sight Distance

It depends on: Type of maneuver required to avoid the

hazard on the road. Road location (urban or rural) See Table 3.5 fro recommended

decision sight distance values for different avoidance maneuvers which can be used for design.

Passing Sight Distance Is the min sight distance required on a two-

lane, two-way highway that will permit a driver to complete a passing maneuver without colliding with opposing vehicle and without cutting off the passed vehicle.

AASHTO has made some assumptions regarding the movement of the passing vehicle during a passing maneuver in order to develop a minimum passing sight distance.

Passing Sight Distance 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 (= 2/3 d2).

Passing Sight Distance Determination

d1 =1.47 t1 [u – m + ((a t1)/2)]d1 = (ft)t1 = time for initial maneuver (sec)a = average acceleration rate (mi/h/sec)u = average speed of passing vehicle (mi/h)m = difference in speeds of passing and impeder vehicles

d2 =1.47 u t2

d2 = (ft)t2 = time passing vehicle is traveling in left lane (sec)u = average speed of passing vehicle (mi/h)

Passing Sight Distance Determination

d3 =distance between the passing vehicle and the opposing vehicle at the end of the passing maneuver

d3 = found to vary between 100 and 300 ft.

Table 3.6 shows components of safe passing sight distance on two-lane highways for design purposes only and cannot be used for marking passing and no-passing zones on completed highways.

Values used for marking passing zones are obtained from different assumptions and are much shorter.

Table 3.7 shows values recommended for this purpose (although recent studies showed that these are inadequate)