Overtaking sight distance - geometric design

Overtaking sight distance - geometric design

1. OVERTAKING SIGHT DISTANCE
Theminimumdistanceopentothevisionofthe
driverofavehicleintendingtoovertakeslow
vehicleaheadwithsafetyagainstthetrafficof
oppositedirectionisknownastheminimum
overtakingsightdistance(OSD)orthesafe
passingsightdistance.
TheovertakingsightdistanceorOSDisthe
distancemeasuredalongthecentreoftheroad
whichadriverwithhiseyelevel1.2mabovethe
roadsurfacecanseethetopofanobject1.2m
abovetheroadsurface.

2. Factors affecting the OSD
speedsof
overtaking vehicle
overtakenvehicle
thevehiclecoming from opposite direction,if
any.
Distancebetweentheovertaking and
overtakenvehicles.
Skill and reaction time of the driver
Rateof acceleration of overtaking vehicle
Gradient of the road

3. Analysis of OSD
Fallow the Fig. 4.14, p-96 of highway engineering by S.K.Khanna
and C.E.G. Justo
d1is the distancetraveled by overtaking vehicle
duringthereactiontimetsecofthe driver
frompositionA1toA2.
D2is the distance traveled bythevehicleAfromA2
toA3duringtheactual overtaking operation, in
timeTsec.
D3is the distance traveled by on-coming vehicleC
fromC1toC2during the over taking operation of
A, i.e.Tsec.
Bis the overtaken or slow moving vehicle.

4. Bis the overtaken or slow moving vehicle moving
with uniform speedVbm/sec orVbKmph;
Cis a vehicle coming from opposite direction at
the design speed V m/sec or Vkmph
The distance traveledby the vehicle A during this
reaction time isd1and is between
thepositionsA1 and A2. this distance will be
equal toVb.tmeter
where t is the reaction time of the driver in
second= 2 sec.

5. OSD=0.28Vb.t +0.28Vb .T+2s+0.28V.T
OSD=d1+ d2+ d3
S = SPACING OF VEHICLES = (0.2 Vb+ 6)
The minimum overtaking sight distance =d1+d2+d3for
two-way traffic.
On divide highways and on roads with one way traffic
regulation, the overtaking distance =d1+d2as no vehicle
is expected from the opposite direction.
If the speed of the overtaken vehicle is not given
Vb=(V-16)kmph, where V= speed of overtaking vehicle inkmph

6. Overtaking Zones
Itisdesirabletoconstructhighwaysinsuchawaythatthe
lengthofroadvisibleaheadateverypointissufficientfor
safeovertaking.Thisisseldompracticableandthere
maybestretcheswherethesafeovertakingdistancecan
notbeprovided.Buttheovertakingopportunityfor
vehiclesmovingatdesignspeedshouldbegivenat
frequentintervals.Thesezoneswhicharemeantfor
overtakingarecalledovertakingzones.
Theminimumlengthofovertakingzoneshouldbethree
timethesafeovertakingdistancei.e.,3(d1+d2)forone-
wayroadsand3(d1+d2+d3)fortwo-wayroads.
Desirablelengthofovertakingzonesiskeptfivetimesthe
overtakingsightdistance.i.e.,5(d1+d2)forone-wayroads
and5(d1+d2+d3)fortwo-wayroads.

8. Example-1
The speed of the overtaking and overtaken
vehicle are 70 and 40kmph, respectively on a
two way traffic road. If theacclerationof
overtaking vehicle is 0.99 m/sec²,
a)Calculate safe overtaking sight distance
b)Calculate the minimum and desirable length of overtaking
zone
c)Draw the neat-sketch of the overtaking zone and show the
position of the sign post.
Example-2
Calculate the safe overtaking sight distance for a
design speed of 96kmph, assume all other data
suitable