1. Road Design Basic Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Basic Road Design
“If you don't know where you are going, any road will get you there.”
Lewis Carroll
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2. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Design
Horizontal alignment of a road defines its location and orientation in plan view.
Vertical alignment of a road addresses its shape in profile.
Every road must take into consideration the horizontal and vertical changes.
The curves left and right are referred to as the horizontal alignment.
The change in elevation, such as when a car climbs or descends a hill, is the vertical alignment.
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3. Road Design Examine the existing topographical data.
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Design
Examine the existing topographical data.
The first step in road design is to know the existing topography in the area of the road.
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4. Road Design Decide on the type of road and road characteristics.
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Design
Decide on the type of road and road characteristics.
Freeway
Expressway
Arterial
Collector
Local
Cul-de-sac
A freeway is a road designed for high speeds and has no at-grade intersections.
An expressway is a divided highway with partial control on access (e.g., limited number of intersections).
Arterial roads are moderate-to-high capacity roads which can carry high volumes of traffic but at lower speeds than highways and expressways. They have intersections with collector and local streets with very few or no residential access directly onto the street. Commercial areas are often located on arterial roads. Arterial roads typically have limited residential entrances.
A collector road is a low or moderate-capacity road that leads traffic to activity areas within a community.
A local road (side road) is low speed and typically found in residential neighborhoods.
A cul-de-sac is a dead end street with only one ingress/egress. These are typically created to limit through traffic.
©iStockphoto.com
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5. Freeway Road designed for high speeds
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Freeway
Road designed for high speeds
Possesses no at-grade intersections
Intersections are separated by grade or connected by interchanges
A freeway is a road designed for high speeds. A freeway has no at-grade intersections. This is a picture of a freeway in Montgomery County, Pennsylvania.
Design speeds vary from mph (sometimes 80 mph in flat rural areas).
This picture shows an intersecting road that is elevated (which is referred to as a grade change) over the freeway.
US. Department of Transportation
Federal Highway Administration
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6. Expressway Divided highway designed for high volumes of traffic
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Expressway
Divided highway designed for high volumes of traffic
Partial control access
An expressway is a divided highway with partial control on access (e.g., limited number of intersections).
This picture shows an expressway in Connecticut. It allows easy access to development along the road but has limited stops for through traffic. Expressways can have speeds of 50 mph (up to 70 mph in some rural locations).
US. Department of Transportation
Federal Highway Administration
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7. Arterial Road Road designed for moderate or high volumes
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Arterial Road
Road designed for moderate or high volumes
Lower speeds than highways and expressways
Intersections with collector and local streets
Commercial areas often located on arterial roads
Arterial roads are moderate-to-high capacity roads which can carry high volumes of traffic but at lower speeds than highways and expressways. They have intersections with collector and local streets. They provide very few or no residential access directly onto the street. Commercial areas are often located on arterial roads.
This picture shows an arterial road in Connecticut. It allows easy access to developments along the road, but has limited stops for through traffic. Some rural principal arterial roads can support speeds up to 70 mph, but most have lower posted speeds.
US. Department of Transportation
Federal Highway Administration
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8. Collector Street Low or moderate capacity road
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Collector Street
Low or moderate capacity road
Leads traffic to activity areas within a community
Intersects with arterial roads and local roads
A collector road is a low or moderate-capacity road that leads traffic to activity areas within a community such as community centers, libraries, schools, etc.
Collector roads typically have design speeds of 20 – 40 mph.
The picture on the left shows a representative collector road in a residential area in Greenbelt, Maryland.
The picture on the right shows a representative urban collector road in Lambertville, New Jersey.
US. Department of Transportation
Federal Highway Administration
US. Department of Transportation
Federal Highway Administration
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9. Local Road Side road Low capacity Low speed
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Local Road
Side road
Low capacity
Low speed
Typically located in residential neighborhoods
A local road (side road) is low speed and typically found in residential neighborhoods.
Local roads typically have a design speed of 20 – 40 mph.
This photo shows a local street in Montgomery County, Maryland.
US. Department of Transportation
Federal Highway Administration
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10. Cul-de-sac Dead end street with only one ingress/egress
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Cul-de-sac
Dead end street with only one ingress/egress
Created to limit through traffic
A cul-de-sac is a dead end street with only one ingress/egress. These are typically created to limit through traffic.
wikimedia
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11. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Intersections
Streets shall be designed to intersect at right angles whenever possible.
Many road codes will not allow streets to intersect at angles less than 75 degrees.
A school bus has a difficult time turning and swings out over the other lane when the radius is less than 90 degrees.
75º min.
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12. Intersections Modern roundabouts are becoming more common.
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Intersections
Modern roundabouts are becoming more common.
Roundabouts can reduce delay and increase safety.
Roundabouts are designed to slow entering traffic and allow all traffic to flow through the junction freely and safely.
This roundabout is in Cecil County, Maryland.
US. Department of Transportation
Federal Highway Administration
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13. Point of Intersection (PI)
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Point of Intersection (PI)
The point at which two or more roads intersect PI
Notice in the photograph that all of the road intersections are at 90 degree angles.
©iStockphoto.com
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14. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sight Distance
The distance from which an object at eye level remains visible
Road codes require different sight distances at intersections depending upon the design speed
Takes into account average perception and reaction times, braking distance, and condition of pavement
A driver must be able to see a sufficient distance ahead to avoid a potential collision with other cars, pedestrians, and objects in the roadway.
An example of a sight distance might be 125 ft at a speed of 20 mph.
Higher speed limits require greater site distances.
[click] The photograph shows a road with a short sight distance because a driver cannot see past the next curve as he approaches.
©iStockphoto.com
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15. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Horizontal Curves
Horizontal curves connect intersecting straight sections of roads
Point where the straight sections intersect is also called a PI
The larger the radius, the flatter the curve
Most municipalities require a radius greater than 125 feet for a 20 mph street
This picture shows three horizontal curves.
R = 125’ min PI
©iStockphoto.com
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16. Elements of a Simple Circular Curve
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Elements of a Simple Circular Curve
PC represents the Point of Curvature. This is where the curve begins.
PT stands for Point of Tangency.
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17. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Length of a Curve
The distance from PC to PT is the length of the curve, which is calculated using the formula below.
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18. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Degree of Curvature, D
Like the radius, the degree of curvature is a measure of the sharpness of a curve.
The degree of curvature is the angle formed by two radii drawn from the center of the circle to the ends of a chord 100 feet in length for a given radius.
It is fairly easy to lay out (using surveying equipment) a curve with a small radius – it is similar to laying out a curved track for a model railroad. When the radius is large (e.g., a real railroad track), the process is more difficult. The center of the curve is located a great distance away from the track location.
An alternate method of laying out a large radius curve involves using the Degree of Curvature. R D
100‘ R
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19. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Curve Data
Complete curve data must be calculated for every curve on the road.
Complete curve data includes the radius, central angle, length of curve, and degree of curvature.
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20. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Station Numbers
In order to define the location of a road, stations numbers are used.
Station 1+50 would be 150 feet from the beginning of the road.
Station 3+25 would be 325 feet from the beginning of the road.
The distance from Sta to Sta is feet.
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21. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Station Numbers
All significant points (PI, PC, and PT) must be identified on the road plan.
The curve length is used to find the station number of each PC and PT.
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22. Vertical Road Alignment
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Vertical Road Alignment
Roads must be aligned to the topography of the site for smooth, safe driving.
To prevent abrupt changes in grade, vertical curves are used.
Unlike horizontal curves, vertical curves are parabolic arcs.
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23. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Crest Vertical Curves
A crest vertical curve is used at the top of a summit.
The length of the curve is determined by two factors: stopping sight distance and rider comfort.
If the curve is too short, then cars approaching the summit could become airborne.
Drivers also need enough time to stop in case of debris on the road.
S =Sight distance
Object
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24. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Sag Vertical Curves
A sag vertical curve is used at the bottom of a curve.
Sag curves are also designed for sight distance and rider comfort.
The sight distance is determined by the illuminated area from the average car headlight beam.
If the curve is too short, then a car will bottom out.
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25. Elements of a Vertical Curve
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Elements of a Vertical Curve
A vertical curve has two slopes or grade lines.
The two grade lines are called the back tangent (g1) and forward tangent (g2).
The intersection point is the point of vertical intersection. (PVI).
The beginning of the curve is the point of vertical curve (PVC).
The end of the curve is the point of vertical tangent (PVT). g2 g1
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26. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Design
To create the vertical profile of the road, lines are projected from the station points and an exaggerated vertical scale is created. Elevation data from existing station points are plotted to a graph.
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27. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Design
A straight line is plotted through the points. Vertical curves are added between slopes. The station numbers for PVC and PVT are computed.
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28. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Profile
This is an example of a road profile that includes a crest vertical curve and a sag vertical curve. The road surface is shown as a solid line. The existing grade is shown.
NEED Permission
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29. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Charts
Important data for the construction of the road is contained in a road chart.
Stations every 100 feet as well as all important points are included on the chart.
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30. Road Chart Setting up a chart for survey data and control points
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Chart
Station
Transit line for Project Road from Lead The Way Boulevard (N 90º W)
0+00
Begin Project
78.68’ N3º37’W FROM BM to centerline of Project Road
Setting up a chart for survey data and control points
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31. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Chart BM
Station
Transit line for Project Road from Lead The Way Boulevard (N 90º W)
0+00
Begin Project
78.68’ N3º37’W FROM BM to centerline of Project Road
STA
P.C to the left
Curve Data
R = 125’
Δ=59º
L =
D=45.83º
90.0’
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32. Road Chart BM 90.0’ 83.23’ Station
Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Chart
Station
Transit line for Short Cut Drive from Project Road (Bearing N 25°3' E) to Lead The Way Boulevard (N 0°4' W)
0+00
Begin Project
78.68’ N3º37’W FROM BM to centerline of Project Road
STA
P.C to the left
Curve Data
R = 125’
Δ=59º
L =
D=45.83º
STA
P.T. BM
90.0’
83.23’
The next important point along the road is the PT. To get the station number, simply add the length of the curve to the previous station number.
How long is the next straight section of road? You may need to read the station numbers if not clear in the presentation. [click] 83.23’, which is the station number of the next PC minus the station number of the last PT ( = 83.23’).
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33. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Road Chart
Continue adding information in this manner until you arrive at the end of the road.
Station
Transit line for Project Road from Lead The Way Boulevard (N 90º W)
0+00
Begin Project
78.68’ N3º37’W FROM BM to centerline of Project Road
STA
P.C to the left
Curve Data
R = 125’
Δ=59º
L =
D=45.83º
STA
P.T.
STA
P.C. to the right
R = 200’
Δ=42º
L =
D = 26.65
P.I. at Big Road
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34. Road Design
Civil Engineering and Architecture
Unit 3 – Lesson 3.4 – Site Considerations
Resources
U.S. Department of Transportation Federal Highway Administration .(n.d.). Flexibility in Highway Design. Retrieved December 15, 2009, from
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