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chapter3_notes_ - Chapter 3 Geometric Design of Highways...

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Chapter 3 Geometric Design of Highways Objective to design highways that are safe for: ± Wide variety of vehicle performance ± Wide Variety of Human Performance SAFTEY: ± Define stopping sight distance (SSD) as the addition of reaction time distance (2.5s) and braking distance (assuming 0.35G). ± Design roads to provide sufficient stopping-sight distance so that a person can safety stop from the “design speed” which is the assumed safe speed of the road and usually a few miles per hour higher than the speed limit. EXAMPLE: ± Stopping sight distance with 2.5s reaction time and 0.35g deceleration ± Called simply sight distance and given the term S (S=SSD for considering stopping) EXAMPLE: Drive above the design speed V? Actual S at design speed with good brakes and quicker reaction time Drive faster than design speed to get same S as 2.5s reaction time S = SSD
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Problem with exceeding design Speed: It extends the sight distance envelope – the distance in which you cannot stop to avoid an object One reason why interstates are safer: Lower likelihood of an object popping up in your sight-distance envelope (the region in which you cannot stop in time)…because of: o Full access control (do not have driveways and other access points that cause turning vehicles). o Fences reduce the probability of animals entering the sight-distance envelope Non-interstates: Driving faster can significantly increase the likelihood that an object will enter your sight-distance envelope. 3.2 PRINCIPLES OF HIGHWAY ALIGNMENT ± From Surveying 101 – Principle is alignment converts a 3-D problem into two 2- D problems Figure 3.1 Highway alignment in three-dimensions. Actual S at design speed with good brakes and quicker reaction time Drive faster than design speed to get same S as 2.5s reaction time
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Figure 3.2 Highway alignment in two-dimensional views. Highway positioning defined by distance along a horizontal plane ± Stationing - with a station being 100ft or 1km ± If a point is 4250ft (1295.3m) from origin, it is station: ± 42 + 50 ± 1 + 295.300 3.3 VERTICAL ALIGNMENT ± Equal tangent vertical curves Figure 3.3 Types of vertical curves. G 1 = initial roadway grade in percent or ft/ft (m/m) (this grade is also referred to as the initial tangent grade, viewing Fig. 3.3 from left to right), G 2 = final roadway (tangent) grade in percent or ft/ft (m/m), A = absolute value of the difference in grades (initial minus final, usually expressed in percent), PVC = point of the vertical curve (the initial point of the curve), PVI = point of vertical intersection (intersection of initial and final grades), PVT = point of vertical tangent, which is the final point of the vertical curve (the point where the curve returns to the final grade or, equivalently, the final tangent), and L = length of the curve in stations or ft (m) measured in a constant-elevation horizontal plane.
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chapter3_notes_ - Chapter 3 Geometric Design of Highways...

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