# Appurtenances HDM Ch. 10.

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Appurtenances HDM Ch. 10

Kinetic Energy (g-forces)
A car weighs 3000 pounds At 20 mph KE=40,000 ft-lb (0.5mv2) At 40 mph KE=160,000 ft-lb If the car above stops in 50 ft Average deceleration in g’s is 1.07 for a car starting at 40 mph

Equations Kinetic Energy = 0.5*Mass*V2
Avg Deceleration (in g’s) for a car at some initial speed decelerating to a stopping condition over some length L: V2/(2*g*L)

Appurtenances When vehicles leave the roadway how can you reduce the # and severity of accidents?

Clear Zone Area free of hazardous objects and gently graded to permit reasonably safe re-entry to the highway or provide adequate distance for stopping References: AASHTO Green Book AASHTO Roadside Design Guide

New & Reconstructed Provide satisfactory clear zones when practical and provide barriers if not Clear zone includes: -Shoulder -Recoverable slope (or traversable slope and clear runout width)

Clear Zone Based on: Design Speed Traffic Volume Roadside Slope

Basic Recovery Width (BRW)
Basic width of recovery area that should be provided (see Table 10-1) BRW does not consider curvature, non-recoverable slopes or accident history

Non-Recoverable Slope
Slope at which it is unlikely a driver will be able to regain control of a vehicle and return to the roadway (vehicle will continue to the bottom of the slope) Embankment slopes steeper than 1:4 are considered non-recoverable Traversable, non-recoverable slopes can be present in the clear zone, but do not count towards the BRW

Curve-Corrected Recovery Width (CCRW)
Takes into account effects of horizontal curvature Obtain by multiplying BRW by the horizontal curve correction factor found in Table 10-2 Apply factor when long tangents are followed by a curve rated 15 km/hr less than the operating tangent speed

Clear Runout Width (CRW)
Width provided at the toe of a traversable, non-recoverable fill slope Minimum width should be 2.5 m (why that #?)

Desired Minimum Clear Zone Width
Larger of: BRW CCRW Sum of CCRW plus the width from the traveled way to the toe of the traversable but non-recoverable slope

Design Clear Zone Width
Should be at least the minimum and preferably greater than the minimum

Point of Need Fixed object use 15-deg divergence angle
use 10-deg on freeways/interstates

Deflection Distance Distance that the outside face line of a barrier will deflect when struck by a vehicle See Table 10-3 (based on 100km/hr; 2000 kg vehicle, 25 deg angle) Deflection distance behind barriers must be kept free of FO’s

Barrier Types Cable Guide Rail Corrugated Metal (W-beam) (1.5*cable)
Box Beam (3*cable) Concrete (10*cable)

Median Barriers Designed to withstand impact from either side
Corrugated metal beam Box Beam Concrete

Selecting Guide Rail Choose barrier w/ largest acceptable deflection
Deflection must be less than distance from barrier line to nearest hazard that can’t be removed or relocated Maintain area behind guide railing (tree dia. > 100 mm is considered a hazard) Deflections must stay within ROW

Potential Hazards Potential fatalities: Fixed Objects: Cliff
Deep body of water Flammable liquids tank Fixed Objects: Bridge piers/abutments Trees (>100mm) Utility Poles Buildings Retaining Walls Overhead sign structures

Potential Hazards (continued)
Roadside Obstacles Rock cuts Longitudinal retaining walls Ditches Cliffs Dropoffs Bodies of Water Projectiles: Mailboxes Fence Rails

Treatment Options: Remove from clear zone Relocate Modify Shield
Replace concrete headwall w/ flared end Pour a smooth concrete wall against a rock cut Place grates across drain pipe end sections Replace posts w/ break-away posts Shield Guide railing Impact attenuators Delineate

Impact Attenuators Used when fixed hazards can’t be removed or protected by railing (gore areas) Inertial Systems-Transfers kinetic energy to series of yielding masses (sand barrels)

Impact Attenuators Compression Systems-Absorbs energy by progressive deformation or crushing of the system elements (GREAT-Guardrail Energy Absorbing Terminal)

Guiderail is a Type 3 Box with a 'WYBET' terminal section
Guiderail is a Type 3 Box with a 'WYBET' terminal section. Picture taken in June 2011 Location: Interstate 81 south in the town of Castle Creek, NY---from SUNYIT student who works for NYSDOT

Vaulting When vehicle vaults over a barrier Major Cause (Curbs)
Free to hit a FO May roll over Major Cause (Curbs) Don’t use curbs w/ concrete barriers or cable Minimize use of mountable curbs Don’t use non-mountable curbing when operating speeds are >80 km/hr Place curbs <1’ or more than 10’ from guide railing

Other Fencing and cattle passes keep livestock and wild animals from entering the traveled way

Innovative Median Barriers:
Single Slope Concrete Median Barrier Moveable Concrete Barrier Truck Barrier Cable Median Barrier