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7 February, 20:30

An engineering student claims that a country road can be safely negotiated at 65 mi/h in rainy weather. Because of the winding nature of the road, one stretch of level pavement has a sight distance of only 510 ft. Assuming practical stopping distance, comment on the student

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  1. 7 February, 22:53
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    Negotiated speed should be lower. Perception/reaction time is too less than design values.

    Explanation:

    Given:

    - The claimed safe speed V_1 = 65 mi/h

    - Sight distance D = 510 ft

    - The practical deceleration a = 11.2 ft/s ... according to standards

    Find:

    Assuming practical stopping distance, comment on the student whether the claim is correct or not

    Solution:

    - Calculate the practical stopping distance:

    d = V_1^2 / 2*a

    d = (65 * 1.46) ^2 / 2*11.2 = 402.054 ft

    - Solve for reaction distance d_r is as follows:

    d_r = D - d = 510 - 402.054 = 107.945 ft

    - The perception/time reaction is:

    t_r = d_r/V_1 = 107.945 / 94.9

    t_r = 1.17 sec

    Answer: The perception/reaction time t_r = 1.17 s is well below the t = 2.3 s.

    Hence, the safe speed should be lower.
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