Road Geometrics part 1,Highway engineering, civil engineering

Road Geometrics

Introduction

"Road geometrics" means fixing up of visible dimension of a road so that the traffic could utilize it conveniently for its safe and comfortable movements. The geometric design of highways provide these facilities.

Road Geometrics include the following features:

1. Cross-section elements such as formation width, carriage way, camber, gradient etc.
2. Sight distance considerations such as stopping and passing sight distance.
3. Horizontal and vertical alignment including transition curves and super elevation.

Although the road geometrics are greatly influenced by topography of land, locality or area and traffic requirements, but design speed is the most important controlling factor in the geomteric design of highways.

Glossary of important terms used in road geometrics

Road Geometrics: the proper planning , design and fixing up of visible dimensions of highway for providing optimum speed, comfort and maximum safety to road users at a minimum operational cost is 'Road Geometrics'.

A highway engineer should plan and design the 'Road Geometrics' only in the initial stages of road alignment and prior to road construction work because it is not easy to improve and re- design the road geometrics of the existing road.

Right -of -way

 It may be defined as the width of the road between the boundary lines of the property situated on its either side.Borrow pits are generally located outside the right-of-way on a temporary land acquired for the purpose.

The right-of-way (land width) is governed by the following factors:

1. width of formation and road margins.
2. Slopes of cutting or embankment.
3. Borrow pits and drains which also include the width required for arboriculture (Road side plantation).
4. Reserved land for future widening of roads.
5. Extra width on curves for clear sight distance considerations.
6. Parking facilities and bus terminals etc. for city roads.

This right-of-way gives the road boundary on either side of the carriage way . But a 'building line' is always established to disallow any sort of building activity beyond road boundary with sufficient setback or margin to reserve sufficient space for future road widening. In addition, a further setback (sufficient space) is established up to 'control line'to exercise proper control over nature of buildings and to prevent ribbon development.

Formation width ( Width of Roadway)

Formation width is defined as the finished top width of an embankment or bottom width of an cutting which forms the sub grade for the road structure. It is also known as width of roadway. Formation width is the sum of width of carriage way and shoulders on either side including separators if any.

Recommended formation width of roads (metres) for various classes of roads

Serial no.	Road classification	Roadway width ,m Plain and rolling terrain	Roadway width ,m mountainous and steep terrain
1.	National and State Highways Single lane Two lane	12 8.8	6.25
2.	Major District Roads Single lane Two lanes	9 9	4.75
3.	other District Roads single lane Two lanes	4.75 9
4.	village roads- single lane	7.5	4

Road Margin

Road margin is defined as portion of right of way (land width) which lies beyond shoulders on either side of formation width. Road margin includes

• Parking Lanes : Parking lanes are generally provided on urban area roads to allow parking of vehicles. Parallel parking is always preferred to angular parking for providing safety to moving vehicles as well as to gather more clear distance (sufficient width) between the parked vehicle and the edge of adjacent lane.
• Drive Ways :The small paths or ways connecting the highways with commercial establishments and public places like petrol pumps, temples , schools, service centres , etc. are called drive ways. These should be properly designed and located at some distance away from the intersections for freely access to these points without any hindrance.
• Cycle tracks: Cycle tracks are the lanes provided in urban areas for the safe movement of cyclists. A minimum width of 2 m is generally provided for the cycle track , which may be increased by 1 m for each additional cycle lane.
• Foot Path: Footpath is the passage provided on either side of road generally in urban areas for the safe movement of pedestrian traffic. it is also called side walk. The minimum width of footpath is 1.5 m.
• Guard rails: Guard rails are barriers which are provided as a protective measure at the edge of the shoulder, when the road is constructed on an earth fill. Guard rails prevent the vehicle moving at a high speed to run off from the embankment . These are provided when the height of the fill exceeds 3 m. 
• Frontage roads: The approach roads provided to give access to properties situated on highways are called frontage roads. These roads run parallel to highway and are generally isolated from the main road by a separator providing preferably a grade separation(at different levels). 
• Bus bays : Bus bays are the terminals provided on the road for the stoppage of bus for the convenience of traffic . To avoid any conflict with the general moving traffic, bus bays are provided by recessing the kerb for the side movement of the bus.
• Embankment slopes: The slopes of an earth fill known as embankment slopes should be as flat as possible for safe movement of vehicular traffic as well as for aesthetic sense to improve road side landscaping.

Road shoulder

The part of formation width on either side of carriage way or actual metalled road is termed as Road shoulder. Shoulders are provided to serve as an additional lane for servicing and parking the vehicle in emergencies, when the vehicle is compelled to be taken out of the carriage way in account of accident or repairing work. In case, shoulders are not provided, roadway capacity  is reduced and chances of accidents are increased . The road shoulders also in overtaking vehicles on a single lane road.

Characteristics of road shoulder:

• It should be hard and dustless.
• The minimum shoulder width as recommended by IRC is 2.5 m.
• The width of shoulder should be sufficient enough to accommodate safely the parked vehicle from the moving vehicles on the adjacent carriage way or lane.
• The width of shoulder should be increased depending upon the class and type of highway for providing space to erect road side signals, sign boards and other traffic markings.

 Carriage way

A carriage way or pavement width is defined is that portion of a road which is meant for movement of vehicular traffic.

Width of the carriage way= Formation width -2 * shoulder width

Carriage way may have one lane, two lanes or three lanes of traffic. So, the width of the carriage way depends upon the width of the traffic lane and number of lanes . The width of road surface actually provided depends on the peak intensity of traffic. Greater the width of the road surface, greater will be its capacity to accommodate the traffic. the maximum width of vehicle as per IRC specification is 2.44 m . Taking into account minimum side clearance for the safety of vehicles, the width of carriage way recommended for a single lane road is 3.75 m. The width of carriage way for two lane pavement is 7 m. The width of carriage way for various classes of roads as recommended by IRC is given in table:

Class of road	Width of carriageway  m
single lane	3.75
two lanes, without raised kerbs	7
two lanes , with raised kerbs	7.5
intermediate carriageway (except on important roads)	5.5
multi lane pavements	3.5 m per lane

 

The number of lanes to be provided on a carriage way= Peak volume of traffic in vehicles per hour/Capacity of single lane of traffic.

In general, road capacity is defined as the maximum number of vehicles that can pass a given point on a carriage way during one hour under prevailing roadway and traffic conditions without any unreasonable delay. 

Width of carriage way on curves

It must be remembered that a greater carriage way width has to be provided on curves for the following reasons:

1. A greater pavement width is required on curves by a moving vehicle since the rear wheels do not tread the same path as traversed by front wheel.
2. While traversing around the curves , the drivers have a tendency to keep vehicle away from the edge of carriage way.
3. The minimum clearance between vehicles crossing each other on curves is always greater than that on a straight road.

Side Slopes

The slopes provided to earthen embankments for road construction so that the soil mass of usual height stands safely are termed as side slopes.In filling, side slopes of 1:1.5 is sufficient for stability of soil mass whereas in cutting a side slope of 1:1 generally serves the purpose. Flatter slopes are now a days  preferred to steep slopes in embankments as they result in safe operation of vehicles and the maintenance cost is also reduced.

IRC has recommended the following side slopes in road embankments:

• Upto 0.60 m (60 cm) height -1 in 4.
• over 0.60 m (60 cm) height -1 in 2 or natural slope whichever is flatter.

In cutting , side slopes should not be steeper than 1 in 2 except in solid rocks.

Medians

Medians or separators are provided between the various traffic lanes on a road to separate the road traffic coming in opposite directions to avoid head-on-collision. Medians also help in channelising the traffic into their respective lanes near crossings and turn abouts . Medians are provided by:

• Lane markings
• Raised platform with tree plantation
• Raised platforms without planting trees.
• Mechanical separators embedded in road like railing etc.

The width of median should not be less than 1.2 m.

Kerbs

Kerbs are provided to indicate the limits of roadway.

Kerb may be defined as the boundary between the edge of the carriage way (pavement) and the footpath or sometimes between the edge of carriage way and the median or island etc.

Road kerbs are always essential to be provided on urban roads.The top of the kerb may be about 10-20 cm above  the edge of road pavement. The kerb and the road surface near the edge together form a side channel which carries away the water that comes to it from surface of the road. The top of footpath near the kerb shall flush with the top of the kerb. Footpath is also given a small slope towards the kerb so that the water from the surface of footpath easily flows into the side channel.

 Low kerbs are provided between edge of carriage way and the shoulders to indicate the boundary  limits. The height of kerb should be such that the driver finds no difficulty in crossing these kerbs to use road shoulders in case of emergency . High kerbs are provided in urban areas on city roads and the vehicle drivers are not allowed to cross these kerbs . They infact act as protective barrier between the vehicular traffic and the pedestrians moving on a footpath.

                                          

Different types of kerbs.

Formation Level

The top of the finished level of the road or formation either in embankment or in cutting is termed as formation level.

The side slopes of the formation level should be such that the soil mass in stable and the whole material  of formation can stand safely without any failure.  

The formation level of the road is based on the following factors:

• General topography of the area
• Gradient to be provided to the road.
• Thickness of road.
• Top level of nearest road or roads of higher category in the vicinity.

The longitudinal section of the area along the alignment of road is plotted either with a theodolite or dumpy level by taking levels at suitable intervals . The line joining these levels or points is termed as Natural Surface Levels (NSL or nsl). The level of formation of the road is then decided by fixing the gradient and thickness of road. If NSL is above the formation level , cutting of soil is required , whereas is formation level passes above the NSL, filling of earth is required. When filling is required or the road is on embankment , the earth is taken from pits excavated on the rad side, which are of regular and specified dimensions. These pits are called borrow pits. If after forming the bank work, there is any surplus earth, it should be dumped in the form of a bank on the road side  itself but parallel to the alignment of road. Such banks are called spoil banks. The location of borrow pits and spoil banks should be within the temporary land acquired for the construction of road.

We will discuss road geometrics further in our next blog.