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Length of Need

Length of Need. L R = dist for vehicle off the roadway to stop. (function of speed and friction) Lateral extent of Hazard = dist to point on object most distant from pavement OR clear zone distance, whichever is less. Length of Need Variables.

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Length of Need

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  1. Length of Need LR = dist for vehicle off the roadway to stop. (function of speed and friction) Lateral extent of Hazard = dist to point on object most distant from pavement OR clear zone distance, whichever is less

  2. Length ofNeed Variables • LR = dist for vehicle off the roadway to stop (adjusted for speed and volume) (Table 5.8) • LA = Lateral extent of Hazard = dist to point on object most distant from pavement OR clear zone distance (Lc ), whichever is less • Ls = shy line distance, distance from travel way at which object is not viewed as an obstacle ( See Table 5.5) • L1 = Designer selected distance upstream from object ( minimum of 25 feet, Iowa practice is to use 37.5 feet, the length of 3 guardrail sections. • L2 = Lateral distance of barrier to edge of pavement • a/b = flare rate used (Table 5.7 recommendations for rigid and semi-rigid systems)

  3. Length and Offset Equations X = [ LA + (b/a ) * L1 - L2 ] / [ (b/a) + LA / LR ] Where X = distance parallel to traveled way from front of object to point of barrier start, exclusive of the terminal end treatment Y = LA - [LA / LR ] * X Where Y = offset distance from the edge of traveled way to the point the barrier begins, exclusive of terminal end treatment

  4. Sample Deflections for Barrier types • 3 strand cable 11.5 feet • W beam weak post 7 feet • Thrie beam – weak post 3 feet • Blocked out thrie beam – strong post 2 feet • New Jersey (concrete barrier) 0 feet • Variations in these can occur dependent on post spacing and metal gauge. Although New Jersey barrier is listed as no deflection, evidence exists that they also are displaced.

  5. Opposite Direction of Travel Consideration • General: analysis procedure is same; distances are just measured from nearest edge of the opposite direction traveled way Case 1: Object is outside of clear zone for opposite direction, but barrier for opposite direction is in the clear zone - Need protective end treatment on both ends of barrier Case 2 Hazard is in the clear zone and extends beyond the clear zone limit (e.g. a river); shield the clear zone distance segment Case 3 Neither object or barrier for opposite direction is in clear zone - no further treatment is needed.

  6. Additional Key Elements of Barrier Placement • Top of rail height • Adequate post strength /spacing • Block out of rail from post to prevent snagging • Splices placed in correct order (upstream end on top) • Transition design in special stiffened sections • Other

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