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15 Cards in this Set
- Front
- Back
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Superelevation is chosen by considering what? |
Primarily safety but other factors are comfort and appearance |
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The superelevation of the orad should take ino account the... |
1. Speed environment 2. Design speed of the curve (85th percentile driver has to negotiate it) 3. Stability of heavily laden vehicles 4. The difference between the inner and outer formation levels (flat country) 5. The length available to produce superelevation |
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What is the maximum superelevation in normal state highway conditions? |
10% or 0.1m/m unless rare cases in mountanous terrain (12% used). In flat terrain max superelevation should be 0.06-0.07m/m |
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Why is there a maximum superelevation? |
1. To prevent stationary vehicles sliding down slopes in icy conditions 2. To prevent tall vehicles from overturning when stopped or travelling slowly 3. To reduce hazards at intersection when vehicles turn across superelevation |
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What are the 3 options when R is greater than R min? |
1. Reduce superelevation and rely on maximum friction 2. use maximum superelevation and reduce friction required 3. reduce both |
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What is Sk in the superelevation formula? |
The ratio of the maximum superelevation to the centrifugal ration Sk = (emax/(emax + fmax)) Can be found in a table |
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The length of superelevation development (Le) is dependant on what? |
1. rate of rotation of road crossfall 2. relative change of gradient between kerb and axis of rotation (usually centre line)(not where cut and fill is a problem) Formula works for two lane roads |
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What is a non-transition circular curve? |
50-70% of superelevation development occurs before circular curve |
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What is a reverse circular curve? |
Superevlevation development must be within circular curves if the tangent points are coincident |
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Available friction in practise depends on time and space and? |
1. Road surface characteristics 2. tyre characteristics 3. speed and duration of braking usually consider the "car on sealed road" situation |
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How does one calculate the margin of safety? |
Safe speed/design speed Vsafe = root(R x g (fmax + e)) |
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What are 3 important circle geometry formulas |
T = R tan I/2 E = R (sec I/2 -1) where sec=1/cos L = RI in radians |
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What are some functions of a spiral curve (Euler's clothoid)? |
1. Provide length to apply superelevation 2. Facilitate vehicle position on the road 3. Allow transition from straight to circular motion 4. Enhance appearance 5. Allow time for speed adjustments |
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When are transition curves not appropriate? |
1. The shift distance P is less than 250mm 2. In low speed areas where drivers regulate their speed from their judgement of the apparent curvature of the road |
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What must the angular rotation be restricted to? |
2.5%/sec for greater than 80km/h and 3.5%/sec for less than 70km/h |
