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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 50 - 70 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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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. 1+07.84 to Sta. 6+75.26 is 567.41 feet. Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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. Project Lead The Way, Inc. Copyright 2010

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 Project Lead The Way, Inc. Copyright 2010

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 0+90.00 P.C to the left Curve Data R = 125’ Δ=59º L = 128.72 D=45.83º 90.0’ Project Lead The Way, Inc. Copyright 2010

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 0+90.00 P.C to the left Curve Data R = 125’ Δ=59º L = 128.72 D=45.83º STA 2+18.72 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 (301.95-218.72 = 83.23’). Project Lead The Way, Inc. Copyright 2010

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 0+90.00 P.C to the left Curve Data R = 125’ Δ=59º L = 128.72 D=45.83º STA 2+18.72 P.T. STA 3+01.95 P.C. to the right R = 200’ Δ=42º L = 146.61 D = 26.65 4+48.56 5+19.79 P.I. at Big Road Project Lead The Way, Inc. Copyright 2010

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 http://www.fhwa.dot.gov/environment/flex/ch03.htm Project Lead The Way, Inc. Copyright 2010