# Length of Need

Urban and Civil

Nov 26, 2013 (4 years and 5 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