Test 2 Flashcards
Traffic Design Standards
Design Standards for high speed, high volume, and highways are regulated by AASHTO. Local and lower speeds are established by local or state agencies.
Qualitative measure of operating conditions
Level of Service (LOS)
The traffic volume that can be supported by a street while keeping a desired level of service
Design Capacity
High volume, high mobility, low access freeways.
Principal Arterial
Less mobility than principal arterial, connects communities and or rural roads.
Minor Arterial
Links the local street systems with arterial and minor arterial streets.
Collector
Provides access from collector streets to individual properties in a development.
Local
Performed during project’s early planning stages
Estimates traffic
Identifies traffic access and determines if proposed project affects existing transportation networks
Traffic Impact Analysis
Projected peak hourly volume used for geometric design of highway
Accounts for change in demand over design period
Design Hour Volume (DHV)
How many hours in a year when a road experiences highest hourly volumes
30 hours
How much of the DHV is the AVT for urban highways
8-12 %
Traffic volume observed over a number of 24 hour periods
Does not show specific variations in volume during peak hours
Used to evaluate current traffic flows and plan for future transportation needs
Average Daily Traffic (ADT)
How are collector and local streets computed
Functions of:
number and type of dwelling units
and daily vehicle trips per type of dwelling unit
Single or one direction movement with either the origin or destination inside the study area
Trip Ends
Example:
Person drives to work and back home, how many trip ends?
2 Trip Ends
Volume of traffic vehicles that are using the road prior to development
Existing Traffic
Increase in traffic expected to occur in development
Future Background Traffic
Traffic derived from new development
Site Traffic
Sum of future and site traffic
Total future traffic
Rectangular Street Network
Used for mixed use developments
Pedestrian friendly
Grid (Street Layout)
Curvilinear street layout. Minimizes cut and fill
Cul-de-Sac Model
Points in a street network that allow vehicles to
change directions and enter onto different streets
Intersections
ncreased density of intersections can be used as a
means of slowing traffic. Concerns?
Congestion
Dangerous traffic conditions
Ideally, intersections will be spaced far enough apart
that waiting traffic does not back up into the
previous intersection
Spacing intersections between 100-150’ on low
volume streets can discourage “cut through” traffic
Intersection Spacing
Min approach angle
60-75 degrees
What is the most safe form of an intersection
4 way T shaped intersections
Normal crowned section that has a high point in the middle and slopes away symmetrically
Lanes
Typical pavement Cross Slope for drainage
1/4 inch per foot (2%)
Increase separation distance between opposing traffic
Refuge for pedestrians
Not commonly found on local or collector streets
Medians
Collect and convey storm water
Separate traffic lanes from sidewalks
Curb and Gutter
Gutter cross slope
1 inch per foot (8.3%)
Area between the sidewalk and back of the curb that is typically a grass strip underneath which utility lines are located
Sidewalk and utility strips
Typical width for sidewalk and utility strips
2-4 inches
5-6’ minimum for tree planting
Sidewalk width for development density
4’ for moderate density
6-10’ for high pedestrian traffic
Sidewalk cross slope facing towards street
1/4 inch per foot (2%)
In low density areas shoulder might be used instead of curbs and gutters
Shoulders with 6’ width of gravel and drainage ditches at the top to collect runoff
What connects the road section to the natural terrain
Side Slopes
2H:1V max slope
4H:1V or greater is common
What is street layout defined by
Site topography
Points of connection
Location of proposed streets next to adjacent land
What should street layout provide?
Pedestrian and driver safety
Max lot yield
Looks and site attraction
An arc may be defined by its radius or this Degree?
Degree of Curvature (DA)
Delta in a horizontal curve
Central Angle
Theta in a horizontal curve
Degree in radians
Distance traveled while the driver is reacting plus the distance after brakes are applied
Stopping Sight Distance (SSD)
Functions of SSD
Reaction time
Street grade
Weather (Friction)
Street Curvature
SSD on horizontal curves
For curves with a clear line of sight. Sight distance is the cord distance and the stopping distance is measured along the arc of the centerline.
When expressed as a function of SSD can be used to evaluate obstructions in a drivers line of sight
Middle Ordinate (MO)
Where are Vertical curves centered
ON the PVI and are parabolic and equal on the tangent length
SSD on crest vertical curves
Main controlling factor a minimum curve length.
Assumes G is 0 since SSD could terminate
SSD on sag vertical curves
Overriding factor is headlight projection distance
How to create Parcels
Create them as site first
How to make alignments fit with parcels for right of way
Move them to the site
Minimum lot width
Number of setbacks + Building Width
Minimum lot depth
Front Setback + Building Depth + Rear Depth
T / F
A parcel will not have the minimum lot width, depth, and frontage while maintaining the
minimum lot size.
True
Who regulates water systems
Virginia Department of Health for system layouts and flow and pressure requirements
What is the larger demand for water flow, domestic or fire demands?
Fire Demands
What dictates fire flow requirements?
Building size and construction materials
Water Supply Design Considerations
Pressure
Flow
Velocity
Storage
Layout
Pressure Ranges
Low-20 PSI
High- 80 PSI
Why does pressure for water need to be adjusted?
Too high can damage the system and too low makes it harder to fight fires.
Water Supply System Forces
Hydrostatic
Hydrodynamic
Water Supply System Thrust Restraints
Prevents pipe separation and damage with concrete thrust blocks and restrained joints.
Water Supply System Velocity
Low- 2 fps
High- 10 fps
Water Supply System Layout
Grid or looped layout is preferred over a branched layout
Water Supply System Taps
Taps are new connections to existing water lines. Tapping sleeves and valves connect with needing water systems to be shutdown.
How much horizontal separation from wastewater lines?
10’
How much vertical separation from wastewater lines
18’’
What happens when spacing requirements for water lines cannot be meet?
An encasement, structural support for the sewer line and or more resilient pipe material is needed.
Pipe Materials
DIP
PVC
Copper
Pipe Size
6-20 inches
Can be 3/4 ‘’ onsite
Fire Hydrants
Have two 2.5’’ hose connections and a 4.5’’ bumper connections
Located 2-3’ behind the curb with the pumper connection facing the street
Spacing frequency depends on houses and lots
Blind Flange / Cap
Terminates a pipe to end flow and is used at dead ends
Bends
Change alignment
Two 45 degree bends are preferred over a 90 degree
Reducers
Transitions pipes between diameters to either reduce or increase diameter
Fire Department Connection (FDC)
Supplement water supply for sprinklers with an adjacent fire hydrant 50’ away
Tees
Intersection of pipes at 90 degree angle. Allows for branches to connect
Water Meters
Measure Domestic Water Consumption
5/8’’ for single family residential
1’’ for commercial family
Valves
Isolate sections of the system and control flow directions
Air Release Valves
Water is comprised of 2-3% air by volume.
This air can dissociate itself from the water, and
collect in the water line’s high point(s). ARV located at local high points along the
waterline
Blow off Valves
Used to dewater lines for maintenance. Located at low points and terminal ends.
Post Indicator Valve (PIV)
Displays open when valve is open. Used for fire lines.
Backflow Preventer
Prevents stagnation and contaminated water and measures water.
Conduit for conveying wastewater
Sanitary Sewer
Liquid conveyed in a sewer
Sewage
Conduit connecting an individual building to a main sewer
Lateral
Main sewers receiving flow from large areas
Submain, main, trunk
Access point to sewer
Manhole
How much of sewage is composed of water
99%, wastewater can be treated as water in calculations
What defines sewer strength
Biological Oxygen Demand
Composition of Suspended Solids
BOD
measure of amount of organic matter present that will oxidize
SS
content is typically more than 80% of organic matter
Provide for conservative design by
accounting for periods when flows from multiple sources
are likely to coincide
Peak Factors
Lateral and Submains Peaking Factor
400% of computed average design flow
Sewer Line Spacing
10’ horizontal and 18’’ vertical separation from water lines
50’ distance between sewer line and drinking water supply wells
If distance is not met a concrete or PVC sleeve might be used
Sewer Line System- Gravity
Always try to design for gravity use site conditions and topography
Sewer Line System- Pump Station and Force Main
When gravity is not possible. Pressured system that is more expensive and requires infrastructure maintenance.
Where are public mains located
Public rights of way
dedicated easements
Wastewater Cleaning Velocity
2 fps prevents the deposition of soils
Which angles to avoid in wastewater design
Acute Angles in flow direction
Minimum diameters
Mains- 8”
Individual Service Connections- 6”
Sewer Pipes
PVC Pipes
What is SDR
Standard Dimension Ratio
Ratio of pipe diameter / Wall Thickness
Higher SDR equals thinner pipe thickness
Manhole material and size
Precast Concrete
48” to 60” standard diameters
Manhole Placement
Terminal End of 8’’ or greater sewer line
Every change in alignment or diameter
Max spacing 300’-500’ is relative to pipe diameter
Sewer Cleanouts
Provides maintenance access to sewer instead of manholes for Private Systems
Placed 75’ (max) apart or at bends
Drop Manholes
Drop manholes are used
when there is more than 2
feet of difference between
the invert elevations of
connecting sewers.
Purpose?
Reduce slope & turbulence
Maintenance
Who deals with onsite treatment
VDH
Line that connects points of equal elevation above some datum
Contour Lines
Vertical elevation difference between contour lines
Contour Intervals usually 2 ft
Contour Line Rules
Contour Lines never cross each other. Lines are always closed off. Locally closed contours depict hills or depressions.
Water Flow in Contour Lines
Water flows from higher to lower elevations and perpendicular to contour lines. For ditches V points upstream and ridges point downstream.
Who handles wastewater and storm water design
DEQ
Water consumption rate for residential and sewage rate
100 gallons per person a day in Virginia that varies on facility
Main peak factor for sewage
400%
Hydraulic Radius for half full or full pipe
Diameter/4 (ft)
Major and Minor Contour Lines
Major contours are placed every 5 intervals and minor contours are in between
How is slope expressed
H : V
Percentage: Change in V over Change in H
Two hill like elevations on a topographic map
Saddle
High to low elevation figures to move flow
Channels
Road geometry
Symmetrically crowned
Surface model comprised of triangles. The triangles are
formed by connecting surface points. The elevation at any point along a
triangle leg can be computed by interpolating between the elevation of
the end point vertices.
Triangle Irregular Network (TIN)
Linear surface features that depict an abrupt change in elevation
Break lines
Ex- curbs and buildings
Can TIN triangles cross over on break lines
No TIN triangles can not overlap with break lines
What is a similar street geometry topographically?
Ridge going down
Who regulates wastewater
VDEQ- Sewage collection and treatment
VDEH- Onsite sewage treatment
