lecture 9 Flashcards
Cross section
meaning when we have cross-section view of the road
We have principal cross section elements like
Travel lanes
shoulders
medians
ask about :We have secondary elements such as
ask about :Median barrier (guard rails) roadside barriers curbs sidewalk cross-slopes side slopes drainage channel right of way
Road way
portion of highway including shoulders without median for vehicular use
A divided highway has two or more roadways
Traveled way
the portion of roadway for the movement of vehicles , exclusive of shoulders
page 6 lecture 9
Draw it and memorise it
page 9 and 10
also draw and memorise it
What is a shoulder
A shoulder is the portion of ““roadway””” tht is adjacent with the traveled lane that is needed for capacity and safety reasons
Shoulders Safety advantages (4 is enough)
#Vehicles can stop because of mechanical difficulties and emergencies. #motorists can stop to consult road maps # evasive maneuvers to avoid potential crashes and reduce their sverity #Discharge for storm water from travel lanes which reduce splashes # increased sight distance for vehicles entering the roadway #recovery area to regain control of a vehicle
Shoulders Capacity advantages
# Provide greater effective turning radii for trucks # Provided space for pedestrian and bicycle use # Provided space for bus stops # Provided space for occasional encroachment of vehicles # Provided space for mail delivery trucks # provided space for detouring traffic during construction
Shoulders maintenance advantage
# snow removal and storage # clearance is provided for signs and guardrails # clearance is provided for portable maintenance signs.
Shoulders dimensions
#Minimum width = 0.6 m (for difficult terrain and low volume highways) # On rural collectors : shoulder width = 1.8 - 2.4 m # On highway with heavy traffic : shoulder width = 3.6 m
Shoulders characteristic
# should be contrasting with travel lanes in color or texture # Can have a median shoulder for divided highwat
Median, what are
Section of divided highway which separates lanes in opposing directions
Median functions
#Stopping during emergency #Refugee for pedestrians #Storage for left turning and U turning vehicles
Medians types (Medians can be)
#Raised (urban arterials) #Flush/Striped #Depressed(free way)
Medians dimensions
#Width vary between 0.6 and 24 meters including median shoulders. # AASHTOO recommends a minimum of 3m for 4 lanes urban free way #Paint-striped median is between 0.6 and 1.2 m.
Frontage roads or what is known as service roads. // Functions:
# Control access to arterials # Segregate loacl trafffic fro the higher speed through-traffic #Used on all types of highways # Provide more favorable access for commercial and residential development than the faster moving arterial street and helps preserve the safety and capacity of the arterial.
Secondary elements:
Roadside and median barriers function
Prevent vehicles that left the traveled way from hitting an object that has greater crash severity potential than the barrier itself
-Roadside barrier: protect vehicles from roadside hazards
Median barrier: prevent crossing from one side to another.
Secondary elements:
Typical roadside barriers
guardrails
Crash cushions
Secondary elements:
roadside barriers
Guardrails
Longitudinal barriers to prevent vehicles from leaving roads
Secondary elements:
roadside barriers
Typical location of guardrails
# Outside of sharp edges # At section with high fills
Secondary elements:
roadside barriers
Crash Cushions function
decelerate errant vehicles to a stop
Secondary elements:
Median barriers
Concrete - W section.
Secondary elements:
Curbs
Raised structures made of cement used mainly on urban highways not to be used on high-speed highway.
Secondary elements:
Curbs purposes
# roadway edge delineation # discouraging vehicles from leaving the roadway #Delineation of pedestrian walkways and protection of pedestrians #Minimizing collision between vehicles and structures #Control of drainage #Discourage cars from driving close to the wall.
Secondary elements:
Sidewalks
#Usually in urban areas only. #Minimum width of 1.2 m in residential areas. #Minimum width 1.2-2.4 m or more in commercial areas #Proper attention should be given to the needs of persons with disabilities whose means of mobility are dependent upon wheelchairs and other devices ( for example we can provide curb ramps in sidewalks.)
Secondary elements:
Cross slopes (Highway without median)
see pics
Pavement is sloped from the middle downwards to both sides of the highway implies rounded crown cross slopes.
Secondary elements:
Cross slopes (Highway with median)
see pics
#Each roadway is crowned in each direction of travel ( Here surface water is drained quickly away but requires more drainage facilities, it is preferred for areas with heavy rain.) # Slope whole pavement in one direction
Secondary elements:
Cross Slopes
Each roadway is crowned separately
see pics
Advantages
#Rapidly draining the pavement during rainstorm.
#Difference between high and low points in the cross section is minimal
Disadvantages
#More underground drainage lines and inlets are needed.
# Treatment of intersections is more difficult because of
the number of high and low points on the cross section
Secondary elements:
Cross Slopes
Roadways with unidirectional cross slopes
see pics
advantages # provide more comfort to drivers when they change lanes # more economical, provide more savings in drainage structures and minimize drainage across the inner, and simplify treatment of intersecting streets # Outer lanes which are used by most traffic are more free of surface water
Rates of cross slopes
AASHTO recommends 1.5% - 2%
Shoulder cross slope: 2- 6% to provide adequate drainage.
Secondary elements:
Side Slopes
#It provides stability of earthworks # Gradient of side slope= 1V:3H or flatter #In cases of high cut or fill we can use 1V:3H # Retaining wall may be utilized for steeper slopes
Secondary elements:
Right of way
#The ROW is the total land area to be acquired for constructing the road and all its elements
Secondary elements:
Right of way width
The width should accommodate the following:
– All elements of highway cross-section
– Public utility facilities along highway
– Any planned widening
#If 2-lane urban collector: 12-18 m
#If 4-lane arterial (2 lanes in each direction): 25
m (desirable)
