Highway
Standards have been developed as a guide in the design of highways and to ensure that motorist’s convenience, environmental safety, and aesthetic considerations are implemented in the most economical manner consistent with highway service considerations.
Geometric Design
In preparing the design of a new highway or a redesign of an old one, the designer must give attention to the following basic considerations aimed at functionality, homogeneity, and predictability:
• The design must be suitable for the traffic volume, both daily and at the design peak hour, for the design speed and for the character of the vehicles to use the facility.
• The design must be consistent and must avoid surprising changes in alignment, grade and sight distance.
• The design must be pleasing to the user and to those who live along it.
• The design must be complete. However, for the designer to be able to ensure the effectiveness of his design to a large degree, the necessary roadside treatment, and the provision of control devices, such as lane markers and special signs, are taken into account.
• The design shall be as simple as possible from the standpoint of the builder. Excessive changes in cross sectional design or the use of a variety of types within a project will in many cases increase the cost and difficulty of construction beyond the commensurate value of such ‘uniqueness’.
• The design should be such that the finished road can be maintained at the least cost.
• The design must be safe for driving and should ensure confidence for motorists.
General Principles for Geometric Design
__________ is not a major factor for local urban streets and collector streets because their typical grid layout and closely spaced intersections usually limit vehicular speed.
• __________ ranging from 30 to 50 kph for local streets, and 50 kph or higher for collector streets, are normally used in design, depending on available right-ofway, terrain, likely pedestrian presence, adjacent development, and other controls
• Rural arterials should be designed for speeds of 60 to 120 kph, depending on terrain, driver expectancy, and, in the case of reconstruction projects, the alignment of the existing facility.
• Urban arterials should be designed for speeds of 50 to 100 kph, with lower speeds in business districts and developed areas, and higher speeds in outlying suburban and developing areas.
• Expressways should be designed for speeds not less than 80 kph.
Design Speed
The term ‘_________’ is used to designate the unobstructed, traversable area provided beyond the edge of the travelled way for the recovery of errant vehicles.
Clear Zone
The ________ includes shoulders, bicycle lanes, and auxiliary lanes unless they function as a through lane.
Clear Zone
__________ in the road design is the distance at which a driver of a vehicle can see an object of specified height on the road ahead, assuming adequate sight and visual acuity and clear atmospheric conditions
Sight Distance
__________is the minimum distance required for a vehicle,
travelling at the design speed, to stop before reaching an object in its path.
Stopping Sight Distance
It is the distance travelled by the vehicle during the total reaction time.
Lag distance
It is the distance travelled by the vehicle after the application of brakes to the instant the vehicle comes to a dead stop. It may be obtained by equating the work done in stopping the vehicle and kinetic energy
Braking distance
Reaction time of a driver is the time which elapses between the instant the driver sees the object of danger and the instant the driver just puts his feet on brake paddle.
Reaction time
Many experiments have shown drivers
require about ________ under normal conditions.
1.5 to 2 seconds
The recognition or realization that a cue or stimulus exists and requires a response
Perception
An interpretation/identification of the stimulus
Intellection
The determination of an appropriate response to the stimulus
Emotion
The physical response resulting from the decision
Volition
Higher the speed of the vehicle higher will be its kinetic energy
and longer distance will be required to stop the vehicle. Hence as speed increases, sight
distance also increases.
The speed of the vehicle
A 100% efficient brake will stop the vehicle the moment the brakes are applied but practically it is not possible.
Braking efficiency
While going down a gradient the time to stop the vehicle will be more due to gravity and thus it will require more sight distance.
The gradient of the road
Design speeds ranging from 30 to 50 kph
local streets
Design speeds ranging from 50 kph or higher
collector streets
designed for speeds of 60 to 120 kph
Rural arterials
designed for speeds of 50 to 100 kph
Urban arterials
designed for speeds not less than 80 kph.
Expressways
____________ is the distance needed for a driver to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete complex maneuvers.
Decision Sight Distance
__________is the minimum distance required to safely make a normal passing maneuver on 2-lane highways at passing speeds common to nearly all drivers, commensurate with design speed.
Overtaking Sight Distance
the distance traveled by overtaking vehicle A during the reaction time t = t1 - t0
d1
the distance traveled by the vehicle during the actual overtaking operation T = t3-t1
d2
is the distance traveled by on-coming vehicle C during the overtaking operation (T).
d3
Formula of Overtaking Sight Distance
OSD = d1+d2+d3
Formula of Stopping Sight Distance
SSD = vt + (v^2/2gf)
vehicle A is forced to reduce its speed to vb, the speed of the slowmoving vehicle B and travels behind it during the reaction time t of the driver. So d1 is given by:
d1=Vbt
Then the vehicle A starts to accelerate, shifts the lane, overtake, and shift back to the original lane. The vehicle A maintains the spacing s before and after overtaking. The spacing s in m is given by:
s=0.7Vb+6
The distance traveled by B during the overtaking operation is
2s + vbT. Also, during this time, vehicle A accelerated from initial velocity vb and overtaking is completed while reaching final velocity v.
d2 = Vbt+1/2 (at^2)
________________ are provided when OSD cannot be provided throughout the length of the highway.
Overtaking zones
For night driving on highways without lighting, the length of visible roadway is that roadway that is directly illuminated by the headlights of the vehicle. For certain conditions the minimum stopping sight distance values used for design can exceed the length of visible roadway.
Headlight Sight Distance
the height of eye of the driver is the same, which is _______ above the road surface.
1.15m
Stopping minimum sight distance is based on the distance required to stop with safety from the instant a stationary object in the same lane becomes visible.
Height of Object
A height of an object of 0.15 m is assumed for measuring
stopping sight distance on crest profiles.
Vertical Control for Stopping
For consistency the measure of stopping sight distance is taken to be the same as that on vertical curves, i.e. from the height of eye of 1.15 m to an object on the road surface of height 0.15 m.
Horizontal Control for Stopping
Since vehicles are the objects that must be seen when passing, it is assumed that the height of object for passing sight distance is 1.40 m. Passing sight distance both on profile crest and on horizontal curves should be measured between the height of eye of 1.15 m and a height of object of 1.40 m.
Control for Passing
height of object for passing sight distance
1.40 m