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CE 436/536 – ROADWAY DESIGN January 22, 2008 Review Homework Assignment Did any of our discussion from last week make you notice anything different about some component of the transportation system that you previously took for granted?

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Roadway Engineering from a Safety Perspective What makes a roadway safe? Characteristics of a safe roadway might include: Good visibility Room for error Low congestion Good geometry How is safety measured? Number of accidents versus accident rate: Sample problem

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Roadway Engineering from a Safety Perspective Given: 1.A roadway segment is 5 miles long. 2.There are 10,245 cars per day on the segment of road. 3.An average of 0.07 accidents per day happen on this segment of road. Find: 1.How many accidents per million vehicle miles (acc/mvm)?

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Roadway Engineering from a Safety Perspective Solution: 1.10,245 veh/day X 5 miles = 51,225 miles/day 2.1,000,000 miles / 51,225 miles/day = 19.52 days 3.19.52 days X 0.07 acc/day = 1.37 acc/mvm State DOTs typically keep extensive accident records in conjunction with law enforcement. There is usually a database where detailed information about each accident is stored and then another database which keeps accident rate information for various segments of roadway and sorts it based on comparisons with other roadways of identical or similar classifications. This data is very valuable to roadway engineers in helping to understand problems and find solutions.

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Roadway Engineering from a Safety Perspective That road is unsafe, there are a lot of accidents! Is perception = reality in this statement? Usually not. Why is using an accident rate the only fair way to assess the safety of a segment of road or intersection (accidents per million vehicles entering)? A road segment that has a high volume of traffic might have a larger number of accidents but a lower accident rate than a roadway with a lower volume of traffic that has a fewer number of accidents.

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Roadway Engineering from a Safety Perspective Aside from pain and suffering for those physically involved, does an accident cost society anything? The average cost of a fatal accident to society is What costs so much? Emergency response Property damage Medical expenses Legal expenses Insurance rates Accident database management Lost production to society from those injured $1,000,000

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Roadway Engineering from a Safety Perspective What causes accidents? Weather Driver error (following too close, etc.) Intoxication Medical Vehicle/mechanical failure Obstacle on roadway Poor design Outdated design Which ones of these are potentially preventable?

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Human / Vehicle / Roadway Behavior and Interaction How many seconds does it take a car traveling 65 mph to go the length of a football field? How far does it take to stop once the brakes are applied given: 1.Initial speed = 65 mph 2.Grade = 0% 3.Deceleration rate = 11.2 ft/s 2 1.65 miles/hr X 5280 ft/mile / 3600 sec/hr = 95.3 ft/sec 2.Football field = 300 feet, so 300 ft / 95.3 ft/sec = 3.15 sec 1.d = V1 2 / (2 X a) = 95.3 2 / (2 X 11.2) = 405.45 feet

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Human / Vehicle / Roadway Behavior and Interaction So the car travels the length of a football field in about 3 seconds but could not stop over the length of a football field. What is the difference between theoretical and practical stopping distance? (page 38) We have calculated the distance to stop once the brakes are applied. What must happen before the breaks can be applied?

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Human / Vehicle / Roadway Behavior and Interaction Perception – Reaction Time It takes time for a driver to perceive and then react to a situation. Younger drivers might react faster than older ones. Impaired drivers have a slower P/R time. Many inputs go in to P/R time. AASHTO 2001 uses 2.5 seconds as a standard for P/R time. How many feet does our 65 mph vehicle travel during P/R time? d = 65 miles/hour X 5280 feet/mile / 3600 sec/hour X 2.5 sec = 238.3 feet {or we knew 65 mph = 95.3 ft/s so 2.5 sec X 95.3 ft/s = 238.3 feet} Total distance to stop including P/R and Braking = 405.5 + 238.3 = 643.6 feet More than 2 football fields to stop from 65 mph!

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Human / Vehicle / Roadway Behavior and Interaction P/R = 2.5 sec and deceleration rate of 11.2 ft/s 2 – are these conservative? Why? Quick overview of being appropriately conservative in design.

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Human / Vehicle / Roadway Behavior and Interaction How does the grade of the roadway affect stopping distance? How do different roadway surfaces affect stopping distance? Given: 1.A vehicle going 65 mph 2.Traveling down a 5% grade 3.Deceleration rate of 11.2 ft/s 2 Find: 1.How far does the car travel before stopping, including P/R time of 2.5 sec?

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Human / Vehicle / Roadway Behavior and Interaction Solution: 1.d = [V1 2 / (2g(a/g +/- G))] + V1(P/R) 2.V1 2 = 95.3 feet/sec (from previous calcs) 3.a = 11.2 ft/s 2 4.g = gravitational constant = 32.2 ft/s 2 5.G = roadway grade in ft/ft =.05 6.P/R = 2.5 seconds 7.d = [95.3 2 / (2 X 32.2)(11.2 /32.2 -.05)] + (95.3 X 2.5) = 711.77 feet A 5% down grade results in an additional 68 feet to stop versus a flat grade A 5% up grade results in a decrease of 51 feet to stop versus a flat grade

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Human / Vehicle / Roadway Behavior and Interaction We have been talking about stopping distance. What is stopping sight distance? How is stopping sight distance used in roadway design? 1.Horizontal curves 2.Vertical curves 3.Roadside obstructions 4.Line of sight in urban areas Decision sight distance and passing sight distance. Posted speed versus design speed.

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Homework Assignment #2 January 22, 2008 Homework, due January 29 th : PROBLEM #1 Given: 1.Highway in rolling terrain, rural conditions 2.Posted speed is 65 miles/hour 3.Design speed is 75 miles/hour 4.Horizontal curve with a radius of 2000 feet 5.A 3.4% down grade Find: 1.How far, in feet, for a vehicle to stop from the time a hazard is recognized. 2.Determine the minimum distance any objects located on the inside of the curve must be set back from the roadway to allow for adequate stopping sight distance.

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Homework Assignment #2 January 22, 2008 Homework, due January 29 th : PROBLEM #2 Write a one page technical memo to me discussing the complexities associated with driver behavior and roadway engineering. Also discuss the reasoning for being conservative when designing roadway systems for the broad range of users.

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