Traffic Engineering

Table of Contents
1. Traffic Engineering
2. Road-User Characteristics
3. Vehicular Characteristics
4. Traffic Studies - Purpose and Types
5. Traffic Flow Characteristics and Manoeuvres
6. Traffic Capacity Studies
7. Passenger Car Unit (PCU)
8. Parking Studies
9. Accident Studies
10. Relationship among Flow, Speed and Density
11. Traffic Operations (Chapter 3 - Part 2)
12. Traffic Control Devices
13. Road Markings, Delineators and Islands
14. Intersections
15. Lighting Layouts
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Traffic Engineering

Traffic engineering is the branch of civil engineering concerned with the improvement of traffic performance of road networks and terminals. This is achieved by systematic traffic studies, scientific analysis and engineering application to ensure safe, efficient and economical movement of people and goods.

1. Road-User Characteristics

Road-user behaviour influences design, operation and control of roadways. Characteristics may be grouped as physical, mental, psychological and environmental.

Physical characteristics

• Vision, hearing and strength: sensory and physical capabilities determine perception-reaction time, detection of hazards and control of the vehicle.
• Temporary conditions: fatigue, alcohol or drug influence, illness and recent exertion reduce alertness, increase reaction time and impair judgement.
• Anthropometry: driver seat height and stature affect visibility and requirements for overhead clearances.

Mental characteristics

• Knowledge, skill and experience: familiarity with traffic rules, vehicle handling and road geometry affects drivers' responses.
• Decision making: intelligence and training influence gap acceptance and manoeuvre choices.

Psychological factors

• Attention, fear, anger, impatience, superstition and general attitude to regulations affect compliance and risk taking.

Environmental factors

• Traffic stream characteristics, roadside facilities, weather and the locality (urban/rural) influence driver behaviour and capacities.

2. Vehicular Characteristics

Vehicle features affecting traffic flow and geometric design are grouped as static and dynamic characteristics.

(a) Static characteristics

• Dimensions: maximum dimensions commonly used for design in India are:
  • Maximum width = 2.5 m
  • Maximum height: single-decked vehicle = 3.80 m; double-decked vehicle = 4.75 m
  • Maximum lengths:
    • Single unit truck with two or more axles = 11.00 m
    • Single unit bus with two or more axles = 12.00 m
    • Semi-trailer tractor combinations = 16.00 m
    • Tractor-trailer combinations = 18.00 m
• No combination is allowed to exceed two units and no combination, laden or unladen, should have overall length exceeding 18 m.
• Weight of loaded vehicles: governs pavement design (thickness, structural capacity) and limits on gradients (heavier vehicles need easier grades).
• Power of vehicle: affects achievable speeds on grades and acceptable limiting gradients.
• Driver seat height: affects visibility distances and required overhead clearance under structures.
• Vehicle length and wheelbase: influence capacity, overtaking distance, minimum turning radius and manoeuvrability at slow speeds.

(b) Dynamic characteristics

• Speed and acceleration: depend on engine power, gradient, vehicle mass and resistances (rolling, aerodynamic).
• Stability: lateral stability and safe cornering depend on track width (distance between wheel centres), wheelbase and height of the centre of gravity.
• Braking tests: are carried out to measure skid resistance of pavement surfaces. Typical measurements needed during braking tests are:
  1. Braking distance, L (m)
  2. Initial speed, u (m/s)
  3. Actual duration of brake application, t (s)

Traffic Studies - Purpose and Types

Traffic studies or surveys analyse traffic characteristics to inform geometric design, pavement design, traffic control and planning of new facilities. They provide input for capacity analysis, signal timings, intersection design and safety measures.

1. Traffic Volume Study

• Definition: Traffic volume is the number of vehicles crossing a section of road per unit time; commonly expressed as vehicles/day or vehicles/hour.
• Traffic volume indicates the relative importance of roads and helps prioritise improvement and expansion.
• Uses: planning, traffic operation and control, trend analysis, structural design of pavements and computation of roadway capacity.
• Classified volume study: records vehicle classes (cars, buses, trucks, two-wheelers, cycles) and is useful for pavement design, geometric design and capacity computation.
• Turning movement study: records directional flows at intersections; used for intersection design, signal timing and channelisation.
• Pedestrian volume study: used for designing sidewalks, crosswalks, subways and pedestrian signals.
• Counting methods: Mechanical counters (pneumatic hose, magnetic detectors, radar detectors) and manual counts. Mechanical counters can operate continuously day and night but generally do not give vehicle class or turning movement details.
• Manual counts: field teams record traffic on prescribed record sheets to collect classified counts, turning movements and other details.

Presentation of traffic volume data

• Annual Average Daily Traffic (AADT or ADT): AADT is the average daily traffic over a year and helps decide route importance and phasing of road development programmes.

• Trend charts: show volume trends over years to aid planning.
• Variation charts: show hourly, daily and seasonal variations and help decide facilities and regulations needed during peak periods.
• Traffic flow maps and volume-flow diagrams: show directions and magnitudes of flows, useful for intersection design.

• Thirtieth highest hourly volume: the hourly volume exceeded only 29 times per year; commonly used as a design hourly volume to allow for peak conditions without designing for extreme peaks.

2. Speed Studies

• Travel time: reciprocal of average speed over a route; a simple measure of network performance.
• Spot speed: instantaneous speed of a vehicle at a specified point.
• Average speed: arithmetic mean of spot speeds at a point.
• Space mean speed: average speed of vehicles over a specified length of road at an instant; obtained from vehicle travel times over the length.

Notation used in space mean speed calculations: V_s = space mean speed (km/h); d = length of road (m); n = number of vehicle observations; t_i = travel time (s) for the ith vehicle over distance d.

• The average travel time of vehicles over the study length is the reciprocal of the space mean speed.
• Time mean speed: average of instantaneous speeds of vehicles observed at a point; typically V_s < V_t under free flow conditions on rural highways.

• Running speed: average speed maintained by a vehicle while in motion over a stretch.
• Overall (or travel) speed: effective speed for a trip between terminals; total distance divided by total time including delays and stoppages.

Two common types of speed studies are:

• Spot speed study
• Speed and delay study

2(a). Spot Speed Study - Uses and Methods

Spot speed studies are useful for:

• Planning traffic control and regulations, geometric design, accident studies, capacity studies and to determine speed trends and differences between driver and vehicle types.

Factors affecting spot speeds include pavement width, curvature, sight distance, gradient, roadside development, enforcement, traffic conditions, driver characteristics, vehicle characteristics and trip purpose.

Methods of obtaining spot speed:

• Enoscope (optical timing), radar speedometer, graphic recorder, electronic meters, photo-electric meters, speedometers and photographic methods. Radar speed meters are efficient as they can record instantaneous speeds automatically, but they are relatively costly.

Presentation of spot speed data

• Average speed: prepare frequency distribution tables by grouping speeds; compute arithmetic mean to report average spot speed.
• Cumulative speed distribution: plot cumulative percent of vehicles at or below each speed on the Y-axis and speed groups on the X-axis to read percentiles such as the 85th percentile speed.

85th percentile speed: the speed at or below which 85% of vehicles pass; commonly used when setting safe speed limits.

• 98th percentile speed: used in geometric design (to represent nearly free-flow speeds).
• 15th percentile speed: used to identify slow vehicle limits or when prohibiting slow vehicles from a facility to reduce delay.

Modal (Modal) Speed

• Modal speed: the peak value of the frequency distribution curve of spot speeds; the most commonly observed speed on the section.

2(b). Speed and Delay Study

• These studies yield running speeds, overall speeds, speed fluctuations and delays between two stations far apart.
• They identify amount, location, duration, frequency and causes of delay; useful in benefit-cost analyses and detecting congestion spots.
• Fixed delays occur at signals and level crossings; operational delays are caused by turning vehicles, parking, pedestrians and internal friction due to high volume or insufficient capacity.

Common methods to carry out speed and delay studies:

• Floating car (rating check) method: a test vehicle is driven over the course at approximately the average speed of the stream; detailed information on location, duration and causes of delay is obtained.
• License-plate (vehicle number) method: records plates at two or more stations to compute journey times; does not give cause of delays.
• Interview technique, elevated observation and photographic techniques are also used where suitable.

Notation for floating car method commonly used (illustrative): q = flow in vehicles per minute in one direction; n_a = average number of vehicles counted while the test vehicle travels against the stream; n_y = average number of vehicles overtaking the test vehicle when running with the stream minus number overtaken when running against the stream; t_ω = average journey time (min) with the stream; t_a = average journey time (min) against the stream. These relationships permit estimation of mean flow and travel time for the stream.

3. Origin and Destination (O & D) Studies

Purpose: determine numbers of trips between zones (origins and destinations) to plan road networks, public transport routes and schedules, terminals and major facilities such as bridges and bypasses.

• Applications:
  • Judge adequacy of existing routes and plan new links
  • Plan transport systems and mass transit routes and schedules
  • Locate expressways or major routes along desire lines
  • Establish preferential or bypass routes for certain vehicle classes
  • Locate terminals and design terminal facilities
  • Decide locations for new bridges and standardise design standards along routes
• O & D studies provide basic data for desire lines (preferred directions of travel) and forecast trip demand.
• Collection methods include roadside interviews, license-plate tracing, return post card surveys, tag-on-car method, home interviews and work-spot interviews.

Common O & D collection methods (short notes)

• Roadside interview: quick, simple field organisation but requires stopping vehicles and may cause user resentment.
• License plate method: fieldwork is quick; office tracing across stations is labour intensive; suited for small study areas.
• Return post card: suitable where traffic is heavy and participants can return information by mail.
• Tag on car: a pre-coded card is attached to vehicles entering the area and collected on exit; useful for continuous, heavy flow areas.
• Home interview: avoids stopping vehicles and can collect socio-economic information useful for forecasting travel demand.
• Work-spot interviews: collect O & D data at workplaces, educational institutions and factories to plan work-trip transportation.

Presentation of O & D data

• O & D tables: matrix showing trips between each origin and destination zone
• Desire lines: straight lines connecting origin and destination points; line widths are proportional to trip numbers and reveal corridors where new links or bypasses may be warranted
• Pie charts: show distribution of trips by purpose, mode or direction; circle diameters may be proportional to total trips from zones
• Contour (isoline) maps: indicate trip densities and general traffic needs of areas

4. Traffic Flow Characteristics and Manoeuvres

Traffic stream properties and vehicle interactions determine lane-change, merging, diverging, weaving and overtaking behaviour. These are important for capacity and safety analysis.

Headway and gaps

• Time headway: time interval between the passage of successive vehicles measured from the front of one vehicle to the front of the next as they pass a point.
• Space headway: distance between successive vehicles measured from the front of one to the front of the next at an instant.
• Minimum time headway corresponds to maximum flow (capacity) for a given stream.

• The number of headways per unit time is directly proportional to traffic flow (volume).
• With increase in stream speed, minimum space headway increases, while minimum time headway first decreases to an optimum and then increases.

5. Traffic Capacity Studies

Key stream variables are flow (q), density (k) and space or speed (u). Their relationships form the basis of capacity analysis.

Definitions

• Traffic volume (flow), q: number of vehicles passing a point per unit time; units vehicles/hour or vehicles/day.
• Traffic density, k: number of vehicles occupying a unit length of lane at an instant; units vehicles/km.
• Traffic speed, u: average speed of vehicles (km/h).
• Relation: q = k · u
• Jam density: maximum density when vehicles are almost stationary (q ≈ 0).

Traffic capacity

• Traffic capacity: maximum hourly rate at which vehicles can pass a point on a lane or roadway under prevailing conditions; expressed as vehicles/hour/lane.
• Basic capacity: theoretical maximum under ideal roadway and traffic conditions, depending only on geometric and roadway features.
• Possible capacity: maximum achievable under prevailing conditions (usually lower than basic capacity).
• Practical (design) capacity: maximum flow that can be allowed without causing unreasonable delays, hazards or excessive restriction on manoeuvrability; used in design.

For analysis, basic and practical capacities are used to assess demand and plan improvements.

Determination of theoretical maximum capacity

Where C = basic capacity of a single lane (vehicles/hour), V = speed (km/h), and S = average centre-to-centre spacing of vehicles in m where S = S_g + L. The minimum space gap S_g may be related to speed and driver reaction time.

• The space gap allowed by a following driver depends on speeds of leading and following vehicles, vehicle types, driver behaviour, level of service, road geometry, environment and the vehicle class composition of the stream.
• If H_t is the minimum time headway (s), theoretical maximum lane capacity C may be computed as C = 3600 / H_t.

Factors affecting practical capacity

• Lane width: narrower lanes reduce capacity.
• Lateral clearance: restricted clearance reduces comfort and capacity; a desirable minimum lateral clearance from pavement edge to obstruction is about 1.85 m.
• Shoulder width: narrow shoulders reduce effective lane width and capacity.
• Vehicle mix: presence of heavy commercial vehicles reduces flow and effective capacity.
• Alignment and sight distance: horizontal and vertical restrictions reduce capacity.
• Intersections and access points: presence of grade intersections, turning movements and parking access reduces capacity.

Passenger Car Unit (PCU)

Traffic streams contain a variety of vehicle types. To analyse capacity and convert heterogeneous traffic into a uniform unit, the Passenger Car Unit (PCU) is commonly used. One PCU is the equivalent impact of a passenger car under specified conditions.

• PCU is a measure of relative space and time occupancy compared to a passenger car.
• Mathematically, PCU value of a vehicle class = (capacity of roadway with passenger cars only) / (capacity when only that vehicle class is present).

PCU values depend on vehicle characteristics, transverse and longitudinal gaps, traffic stream characteristics, roadway characteristics, traffic regulation and control, and climatic/environmental conditions. IRC suggests different PCU values for urban mid-block sections, signalised intersections and kerb parking situations.

Tentative equivalency factors suggested by the IRC

Practical capacity values (IRC suggestions for rural roads)

Parking Studies

Parking studies assess supply and demand of off-street and on-street parking and inform parking management, design of parking areas and traffic control around major trip attractors.

• Parking demand estimation:
  • Cordon counts: measure accumulation within an area by tracking vehicles entering and leaving.
  • Counting parked vehicles at various times of day to measure peak accumulation.
  • Interview techniques where parking demand is high.
• Parking characteristics: turnover rates, average duration, space occupancy, peak periods and vehicle class distribution.
• Parking space inventory: location, size, type (parallel, perpendicular, angled), accessibility and restrictions.

Accident Studies

Accident studies aim to identify causes, evaluate safety performance and recommend remedial measures.

• Objectives:
  • Study causes and suggest corrective treatments at hazardous locations
  • Evaluate existing designs and support proposals
  • Carry out before-and-after studies to demonstrate improvements
  • Provide economic justification for safety works
• Four basic elements in traffic accidents:
  1. Road users
  2. Vehicles
  3. Road and its condition
  4. Environmental factors (traffic, weather, lighting)

Accident records and formats

• Location files: track accident locations and identify high incidence points.
• Spot maps: show accident locations by symbols on maps.
• Condition diagram: scale drawing showing roadway limits, kerb lines, bridges, obstructions, signs, sight restrictions and other physical features at the site.
• Collision diagram: schematic showing approximate vehicle and pedestrian paths involved in accidents; useful for before-and-after comparisons.

Measures for reduction of accidents

• Measures fall into three categories commonly termed the 3-Es:
  • Engineering - design and layout, traffic control devices, geometric improvements
  • Enforcement - laws, policing, licensing and vehicle checks
  • Education - public awareness, driver training and school education

Relationship among Flow, Speed and Density

• At zero speed (standstill), density is maximum (jam density) while flow (volume) is essentially zero.
• As speed increases from zero, density decreases and flow increases up to a maximum (capacity) at some optimal combination of speed and density.
• Beyond that optimal point, further increases in speed (with falling density) cause flow to decline.

Notation: V_st = free flow mean speed (maximum speed at zero density); K_i = jam density (maximum density at zero speed). The maximum flow q_max occurs at a speed where the derivative of q with respect to speed is zero.

Traffic Operations (Chapter 3 - Part 2)

Traffic operations include regulation, control and management of vehicular movement on roads to ensure safety and efficient flow.

Traffic regulations

Traffic laws and regulations cover:

• Driver controls: licences, driving tests, financial responsibility and civil liability.
• Vehicle controls: registration, mandatory equipment and accessories, restrictions on dimensions and weight, inspection requirements.
• Flow regulations: direction of flow, turning and overtaking rules, signing and signalling.
• General controls: accident reporting, recording and prosecution of traffic violations.

Potential conflict points at intersections

Right-angled intersections on two-lane roads have multiple potential conflict points depending on the traffic directionality and number of lanes.

Traffic Control Devices

Common devices: traffic signs, signals, pavement markings and islands. Proper design and placement improve safety and operation.

1. Traffic signs - general

• On kerbed roads, the edge of a sign should be at least 0.6 m from the kerb; on unkerbed roads the nearest edge may be 2.0-3.0 m from the carriageway edge.
• Sign posts are typically painted with alternating black and white bands of 25 cm.
• Signs are grouped as: (a) Regulatory, (b) Warning (c) Informatory (guidance) signs.

(a) Regulatory signs

• Inform road users of laws, prohibitions and mandatory actions; violation is an offence.
• Examples and shapes:
  • Stop sign: octagonal, red with white border; intended to stop vehicles.
  • Give way (yield) sign: inverted triangle, white with red border; assigns right of way to other traffic.
  • Prohibitory signs: circular, white with red border; prohibit certain movements, entry or horn use.
  • No parking / no stopping: circular with blue background and red border; an oblique red bar indicates prohibition of parking; two oblique red bars indicate no stopping/standing.
  • Speed limit signs: circular, white background, red border and black numerals indicating limit.
  • Restriction ends: circular with white background and diagonal black band to signify end of prohibitions.

(vi) Compulsory direction control signs

• Indicate obliged directions (by arrows); circular with blue background and white arrows.

(b) Warning signs

• Warn of hazardous road conditions; equilateral triangle with apex upwards, white background, red border and black symbols.
• Should be located sufficiently in advance of the hazard; recommended advance distances:
  • NH/SH: 120 m
  • MDR: 90 m
  • ODR: 60 m
  • VR: 40 m
  • Urban roads: distances depend on local speeds and visibility

(c) Informatory (guidance) signs

• Guide road users about routes, destinations, distances and provide facility information.
• Subtypes:
  • Direction and place identification: rectangular, white background, black border, black arrows and letters.
  • Facility information: rectangular, blue background with white/black letters (telephone, petrol pump, hospital, etc.).
  • Parking signs: square, blue background with white 'P'.
  • Flood gauge signs: should be installed at causeways and low crossings.

2. Traffic signals

Signals regulate intersection flow and have advantages and disadvantages.

• Advantages:
  • Orderly movement and efficient handling of many intersections.
  • Reduction of certain accident types (e.g. right-angled collisions).
  • When coordinated, maintain progressive movement on a major road.
  • Allow periodic crossing opportunities for minor road traffic.
• Disadvantages:
  • Potential increase in rear-end collisions.
  • Poor design or location may lead to violations and inefficiency.
  • Failures (power loss) cause confusion unless backup measures exist.

Key terms

• Cycle: time for one complete sequence of signal indications.
• Phase: part of the cycle allocated to a movement or group of movements.
• Interval: any division of the cycle during which indications remain unchanged.

Types of traffic signals

• Fixed-time (pre-timed) signals: cycle and phase durations are fixed based on studies.
• Manually operated signals: controlled by an operator (traffic police or signal operator).
• Traffic actuated (automatic) signals: timings vary with detected traffic demand.
• Pedestrian signals and special signals (for trams, buses etc.).

Signal operation and design

• At each approach, signals typically use green (go), amber (prepare to stop/clearance) and red (stop).
• Stop (red) time: equals go and clearance intervals for the cross flow in a two-phase system.
• Red-amber: short combined indication near the end of red to prepare drivers; vehicles should not cross the stop line during this period.
• Clearance time (amber): provided after green to allow vehicles to clear the intersection and for safely stopping approaching vehicles; typically 2-4 s.
• Green time: is usually based on approach peak hour volumes; IRC recommends a minimum green time (see guidelines below).

Design procedures - IRC guidelines (high level)

• Pedestrian green time can be calculated assuming a walking speed (example: 1.2 m/s often used; note input shows 12 m/sec which is a typographical error - commonly 1.2 m/s is used; for the purpose of this text assume typical walking speed adjusted by local practice) and an initial walk interval for pedestrian clearance.
• Allow amber times (for example 2 s each) when calculating cycle time.
• Minimum green time for vehicular approaches is often limited; IRC suggests a minimum of 16 s for vehicle green where required.

Signal systems

• Simultaneous system: all signals show same indication at same time; seldom efficient.
• Alternate system: groups of signals alternate indications; better than simultaneous.
• Simple progressive: coordinated timings permit platoons to travel along a corridor with minimal stops.
• Flexible progressive: computerised control varies cycles and splits automatically to match demand; most efficient where implemented.

Flashing beacons

• Used as warning devices. Flashing red requires stop before the crosswalk or stop line; flashing yellow warns drivers to proceed with caution.

Road Markings, Delineators and Islands

Road markings

• Made with lines, words, symbols or reflectors on pavements, kerbs, islands or fixed objects.
• White paint is commonly used for lane markings; yellow is used for parking restrictions and centre lines where separation is required.
• Pedestrian crossing width normally ranges from 2.0 to 4.0 m depending on local requirements.

Kerb markings

• Kerbs are often painted in alternating black and white bands for visibility.

Reflector units and road delineators

• Reflectors mark hazards and guide drivers at night. Hazard markers reflecting yellow light should be visible from about 150 m.
• Delineators outline the roadway and alignment, especially useful at night or in poor visibility. Types include guide posts (0.08-1.0 m high, black and white strips), hazard markers (panels with red reflectors or black and yellow strips) and object markers (circular red reflectors on panels).

Traffic islands

• Raised or marked areas within the roadway to channelise traffic. Classified by function:
  • Divisional islands - separate opposing flows on multi-lane highways to eliminate head-on collisions.
  • Channelising islands - guide turning movements through intersections.
  • Pedestrian loading and refuge islands - protect boarding/alighting passengers and pedestrians crossing wide carriageways.
  • Rotary central islands - large central islands in roundabouts/rotaries.

Intersections

Intersections where two or more roads meet are either at grade or grade separated (interchanges). Intersection design must minimise conflict area, reduce relative approach speeds and provide adequate approach distance and sight distance.

1. Intersections at grade

• Include unchannelised, channelised and rotary intersections; involve crossing, merging and diverging manoeuvres.
• Basic requirements:
  • Keep area of conflict as small as possible.
  • Reduce relative speeds and approach angles.
  • Provide adequate approach widths and sight distances.
  • Avoid sudden changes in path.

(a) Unchannelised intersections

• Intersection area is paved without any physical restriction - vehicles may use any part of the intersection.
• Plain intersection: no additional pavement width provided for turning movements.
• Flared intersection: pavement is widened at the intersection; conflict area is large and turning vehicles are not guided.

(b) Channelised intersections

• Introduce islands to guide traffic into definite paths and reduce conflict area.

(c) Rotary intersection

• Large central island forces converging traffic to circulate in one direction (typically clockwise in countries with left-hand traffic) before exiting to their desired legs; crossing manoeuvres are replaced by weaving, merging and diverging.

Design factors for rotaries

• Design speeds: typically around 40 km/h for rural rotaries and 30 km/h for urban rotaries.
• Shape of central island: circular shape suits symmetric four-leg intersections; elongated or turbine shapes may suit skewed or multiple leg layouts but excessive elongation is undesirable.

• Turbine shapes reduce entry speeds but may create headlight glare problems at night.

Radius and geometric parameters

Design radius depends on entry speed V, coefficient of friction f and safety allowances. IRC suggests entry curve radii of 20-35 m for 40 km/h and 15-25 m for 30 km/h design speeds. Minimum central island radius is recommended as about 1.33 times the radius of entry curves.

• Weaving (wearing) angle and weaving distance: the angle between entering and exiting paths should be small but not less than about 15°; weaving length should be at least four times the weaving section width. Recommended weaving lengths: 45-90 m for 40 km/h and 30-60 m for 30 km/h.
• Width and radius at entry/exit: minimum carriageway width at entrance and exit about 5.0 m. Exit curves should have larger radii than entry curves (1.5-2 times) to allow acceleration to downstream speeds.
• Sight distance: minimum sight distances recommended are 45 m for 40 km/h and 30 m for 30 km/h.

When a rotary is justified

• Lowest limit where a rotary may be justified: about 500 vehicles per hour total on all legs.
• Upper practical limit beyond which a rotary may be inefficient: about 5,000 vehicles per hour.
• IRC suggests a practical maximum capacity of about 3,000 vehicles per hour entering from all legs for a rotary to operate efficiently.
• Rotary is suitable where motor traffic comprises about 50% or more of total traffic or where right turns are at least about 30% of total traffic.

Advantages and limitations of rotaries

• Advantages:
  • Converts crossing manoeuvres into lower-risk weaving/merging operations.
  • Lower vehicle operating costs compared to stop-and-go at signals.
  • Fewer and less severe accidents due to lower relative speeds.
  • Suitable for intersections of 4-7 roads; highest capacity among at-grade intersections when well designed.
• Limitations:
  • Require larger land area - costly in built-up regions.
  • Do not control pedestrian flows well when pedestrian volumes are high; may need supplementing with police or pedestrian crossings.
  • Unsuitable for very acute angles or more than about seven intersecting roads.

2. Grade-separated intersections (interchanges)

• Intersecting roads are separated vertically to eliminate crossing manoeuvres. Interchange ramps provide connections between grades.
• Interchange ramp types:
  • Direct ramps: diverge and merge on the right side (in left-hand traffic countries this is more complex and risky).
  • Semi-direct ramps: diverge to one side and merge from the other; may reduce some hazards.
  • Indirect ramps: convert right turn movement into left turn diverge and left merge (simpler and less hazardous), but require longer travel distances.

Advantages of grade separation

• Provides uninterrupted movement for major traffic streams and reduces stopping and collisions.
• Improves safety for turning traffic; indirect ramps allow safe conversion of right turns.
• Capacities of intersecting roads can approach free-flow conditions.
• Applicable to various angles and layouts of intersecting roads.

Disadvantages of grade separation

• High construction costs; requires large right of way.
• Site constraints (urban areas, unfavourable topography) can make construction difficult and undesirable.
• In flat terrain, grade separation may introduce undesirable vertical alignment changes (crests and sags).

Grade separation structures - overpass vs underpass

• Overpass: raising the major highway over another road; advantages include simpler drainage and often lower bridge cost for longer secondary span; allows future lateral expansion.
• Underpass: depressing the major highway under another road; may be advantageous where the main route can remain at existing grade; disadvantages include drainage difficulty and potential restriction of vertical sight distance.

Advantages and disadvantages - summary

• Overpass advantages: reduced drainage problems, possible lower cost for wide highways taken over narrow spans, aesthetic preference for through traffic, scope for future expansion.
• Overpass disadvantages: may create rolling grades and sight restrictions on steep approaches.
• Underpass advantages: visible approach warning, favourable acceleration/deceleration in some layouts, possible avoidance of altering main highway grade.
• Underpass disadvantages: drainage problems, feeling of confinement, limited option for staged construction of bridge structure.

Lighting Layouts

• Single-side lighting is economical and suitable for narrow roads; wider roads (three or more lanes) may require staggered or central lighting systems.
• Lights are installed at closer spacing on curves than on straights and are located on the outer side of curves for better visibility.
• At summit curves lights should be closer to improve visibility near the crest.
• For simple urban intersections, illumination should be at least equal to the sum of illumination requirements of the two roads forming the intersection.

• Average maintenance factor for lighting design may be assumed as 0.8 unless local data indicate otherwise.
• Spacing between lighting units should consider carriageway width, lamp-lumens, mounting height and desired average illuminance.