Project 1 - Geometric Design Parameter- Prerequisites

Answer 1

Highway elements required for Road Geometric Design are as follows

• Cross – sectional element of road
• Pavement Surface characteristics
• Road Margin
• Typical Cross section

Cross – sectional Elements of Road – The cross sectional elements will be one of the major input before we design the geometry of road. The cross sectional elements of roads consist of following elements such as

• Camber
• Width of carriageway
• Krebs
• Pavement surface characteristics
• Road margins
• Width of formation

Camber – Camber is a transverse slope provided with respect to horizontal. Camber is expressed in terms of percentage. Road are flat but are designed and constructed with drainage for water runoff known as camber.

Surface water on a road surface can cause aquaplaning where vehicles wheels are lifted from contact with the asphalt while in motion.

Width of Carriageway – Width of a road depend upon many factors like traffic density, the present capacity of traffic, level of service and the future growth. The width of carriageway also depend upon the economy as in today’s scenario the procurement of land is costly affair which can cause the project cost escalation therefore while designing the road we should always consider the cost of land. The minimum width of the vehicle depends upon the width of vehicle and the clearance.

Kerb – Kerb indicates the edges of the pavement. Kerb is provided not only to identify the edges but also for many safety reason, Kerb is provided on the median and footpath side so that vehicle driver can identify the edges of the road so that they can restrict their vehicle to enter in the wrong lane and also provide safety to the footpaths user. Kerb also facilitate in maintaining the drainage of the road kerb are used to channelize the rain water to the drain.

Pavement surface characteristics

Friction – Friction between wheel and the road surface is an important factor in deciding the safe operational speed and also the curve design. The force that develops between the wheel and road is the load acting multiplied by a factor called coefficient of friction. IRC suggest the friction to be 0.35 to 0.4 N depending upon speed and coefficient of lateral friction as 0.15.

There are two types of friction

Longitudinal friction – This type of fiction is important to design the sight distance value while road designing.

Lateral Friction – It is important to design the curve values.

When a vehicle negotiate an horizontal curve the lateral friction develops counteracts the centrifugal force and thus govern the safe operating speed. Also the friction force is important factor in case of acceleration and retardation ability of vehicle there is a one factor called coefficient of friction plays important role.

Light Reflection – white roads cause glare during day time but provides good visibility at night on the other hand black road have poor visibility at night but does not glare during day. Concrete roads have better visibility and fewer glares. Road surface should be necessarily be visible at night and reflection of light is the factor that answers.

Drainage – Absolutely impermeable pavement surface should be used to prevent seepage of water into the pavement layers. Further both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.

Unevenness – The road surface should be smooth to provide a better riding surface. Unevenness affects the vehicle operating cost, speed riding comfort, fuel consumption, and safety and causes wear and tear.

Road Margins – Road margins is the portion of the road beyond the carriageway on the roadway.

Single lane – 3.5m

Two lane – 7m

Intermediate Lane – 5.5m

Multilane – 3.5m x number of lane

It is consider the area other than the carriageway

Shoulder – Shoulder are provided along the edge of the pavement to accommodate the stopped vehicle and not act as a emergency lane. It also provides the lateral support to other pavement layers.

Generally there are two types of shoulders

• Earthen Shoulder
• Paved Shoulder

Median – Median is generally provided in a divided carriageway like 4 – lane, 6 – laneor multilane roads. It separates the different direction movement of vehicle. Median width varies depend upon the urban and rural area.

Advantages of median.

• Th plantation in the median avoids glare from the opposite direction traffic.
• Since it divides the direction it enhances the safety of the road.
• The bigger median width will allow for the future expansion of the road.
• In median we can also provide the drain which will allow moving water out of the carriageway pavement.

Bus Bay – Bus bay are the place for the bus to halt and pick the passengers. It allows the movement of the bus to accommodate the passenger and bus according to their convenience. The bus bay are designed as per the specific dimensions and road geometry as per the IRC standards codes. The proper designing of the bus bay ensures that it may not cause any type impact on the traffic and safety of the passengers as they are to accommodate in the existing road system.

Bus Bays are designed as per IRC SP 84

In bus bays 100 meter of tapering length is provided for entering a bus into the bus bay.

Bus bay should be located in such a way that it should be 75 meter distance apart from the existing or proposed intersection.

Truck Bay – Truck bay are provided in the highway so that the drives can halt and have break and avoid unnecessary stops in the middle of the roads as the trucks travel huge miles on the roads so the driver may fell dizzy and fatigue sometime, so they use these truck bay to take little rest to overcome any fatal situations, sometime these truck bays are use to accommodate the broke down vehicle so that they do not create any disturbance to the moving traffic. In truck bay parking area or straight length is more than 100m as compared to bus bay, but taper area is less because it is considered that Truck travelers in less speed as compared to bus.

Bus bay is designed as per IRC SP 84.

Cycle Track – now days to promote the use of cycle especially in the urban areas cycle track is provided adjacent to the footpath of the roads. This is to promote the use of cycle so that passengers can restrict the use of vehicle for small movements and to promote the use of cycle among the passengers.

Minimum width of 2m with additional 1m is provided.

Footpath/Side walk – As the name suggest these are used for the walking and crossing of the road purpose the cross fall or camber is kept less and the width is between 2.5 to 3 meter.

Embankment rail – It is necessary to provide guard rail whenever there is filling. When a filling is more than 3m than it is necessary to provide Guard rail.

Road way – It is the addition of Carriageway, median and shoulder.

Right of Way – Right of way is the complete width required for complete development of highway. Which accommodates all the elements of the road highway such as carriageway, median, shoulder, footpath, cycle track, embankment, service road, drainage and green area left for future expansion? It includes roadway width formation along with road margins therefore no construction is allowed beyond right of way line.

Necessary checklist during design of roads is as follows

• Design Speed
• Topography
• Traffic
• Design Vehicle
• Environment
• Human
• Economy
• Aesthetics

All these elements are explained previously