Highway Engineering:

Highway engineering is a branch of civil engineering that focuses on the design, construction, operation, and maintenance of roads, highways, and other transportation infrastructure. Highway engineers work to ensure the safe and efficient movement of people and goods on roadways. They consider factors such as traffic flow, safety, pavement materials, drainage, and environmental impact when designing and managing highways.

Note:- This Notes is Provided From Nepal Road Standard 2070, Traffic Rule Regulation of Nepal and Design Criteria of Highway of other country may be different.

ROAD CLASSIFICATIOtN

Roads in Nepal are classified as follows:

A.  Administrative Classification

Administrative classification of roads is intended for assigning national importance and level of government responsible for overall management and methods of financing. According to this classification roads are classified into:

• National Highways
• Feeder Roads
• District Roads
• Urban Roads

National Highways

National Highways are main roads connecting East to West and North to South of the Nation. These serve directly the greater portion of the longer distance travel, provide consistently higher level of service in terms of travel speeds, and bear the inter-community mobility. These roads shall be the main arterial routes passing through the length and breadth of the country as a whole. They are designated by letter ‘H’ followed by a two-digit number.

Feeder Roads

Feeder roads are important roads of localized nature. These serve the community’s wide interest and connect District Headquarters, Major economic centres, Tourism centres to National Highways or other feeder roads. They are designated by letter ‘F’ followed by 3-digit number.

District Roads

District Roads are important roads within a district serving areas of production and markets, and connecting with each other or with the main highways.

Urban Roads

Urban Roads are the roads serving within the urban municipalities.

In Nepal the overall management of National Highways and Feeder Roads comes within the responsibility of the Department of Roads (DOR).

B. Technical/ Functional Classification

For assigning various geometric and technical parameters for design, roads are categorized into classes as follows:

Class –I

Class I roads are the highest standard roads with divided carriageway and access control (Expressways) with ADT of 20,000 PCU or more in 20 yrs perspective period.

Design speed adopted for design of this class of roads in plain terrain is 120 km/h.

Class II

Class II roads are those with ADT of 5000-20000 PCU in 20 yrs perspective period. Design speed adopted for design of this class of roads in plain terrain is 100 km/h.

Class III

Class III roads are those with ADT of 2000-5000 PCU in 20 yrs perspective period. Design speed adopted for design of this class of roads in plain terrain is 80 km/h.

Class IV

Class IV roads are those with ADT of less than 2000 PCU in 20 yrs perspective period.

TRAFFIC CHARACTERISTICS

Vehicle Dimensions

• Maximum Width, m                       2.50
• Maximum Height, m                     4.75
• Maximum Length, m                  18.00
• Maximum single axle load, kN     100

Equivalency Factors

1. It is not feasible to improve the standard of a road by very small increments and it is a standard practice to design and construct new roads and improvement works to withstand the estimated traffic at some future date.
2. In Nepal this forward period (perspective period) shall be 20 years, i.e. roads shall be designed with a capacity sufficient to cater for the estimated traffic volume 20 years after the date of completion of the works.
3. Different types of vehicles take up differing amounts of road space and have different speeds(For geometric design) and impose differing loads on the road structure (For structural design).
4. It is, therefore, necessary to adopt a standard traffic unit to which other types of vehicles may be related.
5. For geometric design of roads this standard is the ‘Passenger Car Unit (PCU)’ which is that of a normal car (passenger car), light van or pick-up. Other types of vehicles are taken into account by multiplying by the following equivalency factors.

CAPACITY AND LEVEL OF SERVICE

a. Among six Levels of Services (LOS) viz. ‘A’ to ‘F’ it is recommended to adopt a LOS ‘B’ for the design capacity of roads.
b. Under this condition, traffic will experience congestion and inconvenience during some of the peak hours, which may be acceptable.
c. Design capacity governs the number of lanes required for the design volume of traffic.
d. At the level of service B, volume of traffic will be around 45 percent of maximum capacity under mixed traffic condition.

C. TERRAIN CLASSIFICATION

Geometric design of roads depends significantly on the terrain conditions. Economy in the design usually dictates to change standards to suit the terrain.

DESIGN SPEED

• Overall geometric design of a road is a function of design speed. Design speed is decided based on the importance of the road (road class) and the type of terrain.

Design speed should be the guiding criterion for geometric design of the road. But in very difficult terrains and unavoidable circumstances design speed can be reduced to 75% of the values given.

SIGHT DISTANCE

Sight distance refers to the length of road ahead that is visible to a driver. It is a critical factor in highway design and traffic safety, as it allows drivers to see and react to potential hazards such as other vehicles, pedestrians, or obstacles in the road.

• Stopping Distance
• Stopping Distance

Stopping Distance

Stopping distance is the distance ahead needed by a driver to bring his vehicle to a complete stop before meeting a stationary object in his path.

Overtaking Distance

Overtaking distance is the minimum distance that should be available to the driver to overtake another vehicle safely.

Application of sight distance standards

1. Normally attempts should be made to provide a sight distance equal to the overtaking distance in as much length of the road as possible. Where this is not feasible a sight distance equal to twice the stopping distance should be made available.
2. In no case should the visibility of the road ahead be less than stopping distance for multi lane roads (>=2 lanes) and twice the stopping distance for single lane roads.
3. It is always recommended to provide visibility of road ahead to as much distance as possible.
4. For calculating the visibility of the road the driver’s eye is assumed to be located at 1.2m above the road surface and any object lying on the roads surface to be 0.15m high.

Overtaking Zones

1. In stretches of roads where sufficient overtaking sight distance cannot be provided or on single lane roads where overtaking or crossing opportunity is not available, overtaking or passing zones shall be provided.
2. The width of the overtaking zone shall be the same as that of a minimum two lane road.
3. Length of the overtaking zone shall be at least 3 times the overtaking distance on two and more lane roads.
4. On single lane roads length of passing zones shall be at least 2 times the overtaking sight distance.
5. On single lane roads overtaking/passing lanes should be provided at not more than 1km interval.
6. The start and end of overtaking zone shall be well informed by placing appropriate signs at least stopping distance before the start and end of the zone.

HORIZONTAL ALIGNMENT

• Horizontal Curves
• Transition Curves or Spirals
• Hair pin Bends
• Extrawidening
• Set-Back Distance at Horizontal Curves

Radius of Horizontal Curves

Minimum recommended values of radius of horizontal curves for various design speeds are given below:

Transition Curves or Spirals

1. Transition curves are necessary to allow a vehicle smoothly enter the circular curve from straight section and vice versa.
2. All horizontal curves with radius less than 1000m should be provided with transition curves.
3. When circular curves of very large radius (>1000m) are provided the effect of transition from straight section to circular section becomes negligible and no transition curves are provided.
4. Clothoid curves (Euler’s spiral) with curvature changing linearly with the length are used for transition curves.

Minimum length of transition curves should be as shown in Table

Hair pin Bends

Hairpin bends, also known as hairpin turns or switchbacks, are sharp, U-shaped curves in a road, typically seen in mountainous terrain. These bends are used to help a road climb or descend a steep slope by reversing the direction of the road. Hairpin bends are designed to allow vehicles to navigate the sharp turn safely and efficiently.

1. A minimum distance of 60m should be provided between successive bends of consecutive hair pin bends.
2. At hair pin bends it is preferable to pave the road to the full width of the roadway.

Extrawidening

1. When a vehicle negotiates a horizontal curve the rear wheels do not exactly follow the path of the front wheels. Their path is shifted towards the centre of the curve in relation to the front wheels path.
2. In curves the drivers of the vehicles have a tendency to keep a greater clearance between them as compared to the straight sections of the road.
3. For the reasons mentioned above the width of carriageway of roads at the curves is made wider than on the straight sections.

Value of extrawidening is adopted as shown below:

Set-Back Distance at Horizontal Curves

Adequate sight distance should be available across the inside of horizontal curves. Distance from the road centre line within which the obstructions should be cleared to ensure the needed visibility.

VERTICAL ALIGNMENT

The vertical alignment of the road should provide for a smooth longitudinal profile without any kinks and visual discontinuities in the profile. Grade changes in vertical alignments should be as less frequent as possible.

• Gradients
• Climbing Lanes
• Emergency escape ramps
• Vertical Curves

Maximum gradients

1. Vehicle operation cost is directly related with the longitudinal gradients, and so it is recommended to adopt their values as small as possible.
2. Right from the early stage of alignment fixing, it should be born in mind that it becomes very difficult to flatten the gradient at later stage.
3. Maximum gradient depends on the dynamic characteristics of commercial trucks, design speed and maximum allowable reduction in speed during climbing up the gradient.
4. Minimum longitudinal gradients for longitudinal drainage purpose is 0.5%.
5. Considering these factors (weight to power ratio of trucks-120kg/kW, with a maximum reduction of speed by 25 kmph below the design speed) maximum gradients for various design speeds shall be as follows:

Grade Compensations

1. Maximum value of longitudinal gradient shall be eased by 0.5% for each rise of 500m above mean sea level.
2. Due to loss of tractive efforts of the vehicle on curves it is recommended to ease the gradients.
3. It is not necessary to compensate grades below 4%.

Maximum (critical) Length of Grade

Climbing Lanes

1. Climbing lanes are provided on road upgrades for slow moving heavy vehicles to allow drivers of light vehicles to move without reducing speed when they encounter slow moving heavy vehicles.
2. Climbing lanes are to be provided if the length of the grade is such that a speed reduction of more than 25kmph of fast moving vehicle occurs.

• Climbing lanes are provided if the upgrade traffic flow is greater than 200 veh/hr and the upgrade truck flow is higher than 20 veh/hr(in addition to the critical length requirements of above(b)).
• Climbing lanes are generally not necessary on low traffic multilane highways.
• Width of climbing lanes should be minimum 3.5m. Length should be such that these lanes start at least 50m before the upgrade starts and should continue at least 100 m beyond it.

Emergency escape ramps

• Emergency escape ramps are to be provided on long downgrade of a highway for use by trucks that have lost control and cannot slow down. They are more effective if there is a horizontal curve on long downgrade stretch.
• The escape ramps should be made of sandpiles or loose aggregates with upwards gradients.
• Width of the ramp should be 3.5 m minimum.
• The alignment of the escape ramp should be tangent or on very flat curvature to minimize the driver’s difficulty in controlling the vehicle.

Vertical Curves

1. When two straight sections of a road in longitudinal profile meet at a point, vertical curves are provided for smooth travel along the road.
2. The type of vertical curves is selected in such a way that the rate of change of grade throughout the curve is uniform.
3. A quadratic parabola satisfies the above condition and should be used for vertical curves design.
4. If the convexity of the curve is upwards it is called a summit curve otherwise a valley curve.

Summit Curves

1. Minimum length of summit curve L is to be found from the consideration of providing a sight distance(S) throughout the curve equal to stopping distance or overtaking distance( whichever gives the higher value.
2. From the consideration of providing sight distance equal to stopping distance the height of driver’s eye and the object are taken as 1.2m and 0.15m above pavement surface respectively.
3. From the consideration of providing sight distance equal to the overtaking distance or twice the stopping distance for single lane road (whichever is higher) with the height of driver’s eye 1.2 m above pavement surface.

Valley Curves

1. The length (L) and K-value of vertical valley curve should be selected based on the required night visibility by the headlight of the vehicle of at least stopping distance as given on Table 8-1 or based on the riding comfort of the passengers and overloading on the suspension system of the automobile.
2. Minimum length of valley curve (L) from the consideration of night visibility of road surface by the illumination by the head light is to be found as follows(taking 0.75m as height of mounting of head light above pavement surface, and 2^o as the angle of illumination of the headlight).

ROAD CROSS SECTION ELEMENTS

• Carriageway
• Shoulder
• Medians
• Formation or Roadway Width
• Camber
• Superelevation
• Side slopes
• Typical Cross Sections
• Right of Way and Clearances

Carriageway:

• A carriageway refers to the part of a road designed for the movement of vehicles, excluding the shoulders and any other designated areas. It is the portion of the road where vehicles travel, and it is typically divided into lanes for traffic. Carriageways can vary in width depending on the road classification and traffic volume.
• carriageways are designed based on factors such as the expected traffic volume, vehicle types, speed limits, and safety considerations. The design of a carriageway includes determining the number of lanes, lane widths, road markings, and signage. Carriageways are designed to provide a safe and efficient flow of traffic while ensuring the comfort and convenience of road users.
• In case of single lane roads it is recommended to have two treated shoulders on either side to make a total width of 5.5m of treated surface.

Shoulder

• The width of shoulders on either side of the carriageway shall be at least 0.75m.
• For protection of pavement from water percolating under it from shoulder it is recommended to treat at least a 0.50-0.75m wide strip of shoulder near the edge of the pavement with impervious to water surfacing.
• If a small gap(<1m) of untreated shoulder is formed between the edge of the pavement and edge of the side drain in hill roads it is recommended to treat this gap with appropriate surface treatment.
• For mountainous and steep terrains the above values can be reduced to a minimum value for a lower class of the road but not less than 0.75m.
• It is desirable that the color and texture of shoulders be different from those of the carriageway.
• This contrast serves to clearly define the carriage way at all times, particularly at night and during inclement weather, while discouraging the use of shoulders as additional through lanes.
• Very wide shoulders (more than 3.75m wide) are also not desirable due to tendency of vehicles misusing it as a carriageway.

• Shoulders are the areas bordering the carriageway, designed to accommodate stopped vehicles, emergency use, and provide a buffer zone between the travel lanes and roadside obstacles. They are an important part of road design and serve several purposes.

Medians

1. For roads with 4 or more lanes, it is recommended to provide medians or traffic separators. Medians should be as wide as possible.
2. A minimum median width of 5m is recommended. But a width of 3m can be adopted in areas where land is restricted.
3. In mountainous and steep terrains maximum possible width of median dictated by the topography should be provided. In such situations simple barriers may be provided to function as a median or individual carriageways could be designed at different levels.
4. On long bridges and viaducts the width of the median may be reduced to 1.5m, but in no case this should be less than 1.2m.
5. The median should be of uniform width in a particular section of the highway. However, where changes are unavoidable, a transition of 1 in 20 must be provided.

Formation or Roadway Width

Formation width shall be a total of widths of carriageways, medians and shoulders.

Camber

• Camber, also known as cross slope or crossfall, refers to the slope or incline provided to the surface of a road or pavement to ensure proper drainage of water. It is a critical element of road design and is used to prevent water accumulation on the road surface, which can lead to hydroplaning and reduce traction.

• On roads with undivided carriageways the camber shall be on both directions from the centre line of the road. On roads with divided carriageways unidirectional camber can be provided.
• However on some sections of hill roads with undivided carriageway a unidirectional camber can be adopted. In this case the adverse effect of negative camber on movement of vehicles on curves should be properly checked.
• On straight sections of roads, shoulders should have a higher crossfall than that of the carriageway by 0.5%.

Superelevation

• Superelevation, also known as banking or cant, is the transverse slope provided to the surface of a road or a railway track on a curve. It is designed to counteract the effect of centrifugal force on vehicles or trains as they negotiate a curve, helping them to maintain traction and stability.
• Superelevation is provided on horizontal curves.
• Maximum superelevation to be provided is limited to:

1. In plain and rolling terrain                    7%
2. In snow bound areas                            7%
3. In hilly areas not bound by snows      10%

• Minimum value of superelevation should be equal to the rate of camber of the pavement.
• The rate of introduction of superelevation (i.e. longitudinal grade developed at the pavement edge compared to through grade along the centre line) should be such as not to cause discomfort to travelers or to make the road unsightly.
• Rate of change of the outer edge of the pavement should not be steeper than 1 in 150 in plain and rolling terrain and 1 in 60 in mountainous and steep terrain in comparison with the grade of the centre line.

Side slopes

• Side slopes of embankment and cuttings depend on the type of fill/cut materials and height/depth of filling/cutting.
• Recommended side slopes for embankments are given below. But wherever possible flatter slopes are recommended for aesthetic reason and traffic safety.
• If natural cross slope of the ground is more than 1:5 then the ground should be cut with more than 2m wide horizontal steps.

Right of Way and Clearances

Lateral clearances

1. For a single carriageway road that goes through an underpass, whole width of the roadway (carriageway plus shoulder widths) should be cleared in lateral direction.
2. If footpaths are provided minimum lateral clearance should be width of footpath plus 1.0 m.
3. On roads with divided carriageway, left hand side lateral clearance should be as given on (a.) and (b.) above.
4. Right hand side clearance should be 2.0 m (desirable) with 1.5m minimum.

Vertical clearances

TRAFFIC SIGNS AND SAFETY

A “Traffic Sign” means any object, device, line or mark on the road whose object is to convey to road users, or any specified class of road user, restrictions, prohibitions, warnings or information, of any description. The term Traffic Sign therefore includes not only signs on posts, but also road markings, delineators, road studs, traffic light signals and other traffic control devices. This includes,

• Regulatory signs
• Warning signs
• Information signs (including direction signs)
• Supplementary plates
• Traffic signals
• Road markings
• Sign sizes and construction

Signs are only effective if:

a) They are visible,
b) They are legible,
c) They are understandable,
d) The road users knows what they mean, and
e) The road user is motivated to behave correctly