Length of Need

shootperchUrban and Civil

Nov 26, 2013 (3 years and 10 months ago)

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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


L
R

= dist for vehicle off the roadway to stop


(adjusted for speed and volume) (Table 5.8)



L
A

= Lateral extent of Hazard = dist to point on object most distant from
pavement OR clear zone distance (L
c
), whichever is less


L
s
=

shy line distance, distance from travel way at which object is not viewed
as an obstacle ( See Table 5.5)


L
1

= Designer selected distance upstream from object ( minimum of 25 feet,
Iowa practice is to use 37.5 feet, the length of 3 guardrail sections.



L
2

= Lateral distance of barrier to edge of pavement



a/b = flare rate used (Table 5.7 recommendations for rigid and semi
-
rigid
systems)







Length and Offset Equations

X = [ L
A

+ (b/a ) * L
1

-

L
2
] / [ (b/a) + L
A

/ L
R

]



Where X = distance parallel to traveled way from front of object to point of
barrier start, exclusive of the terminal end treatment


Y = L
A

-

[L
A

/ L
R
] * X



Where Y = offset distance from the edge of traveled way to the point the
barrier begins, exclusive of terminal end treatment

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.

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.

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