Traffic Engineering - 2

Chapter 3 (Part 2)

TRAFFIC OPERATIONS

Traffic Regulations

Traffic regulations and laws cover four principal phases that together ensure safe and orderly movement of road users:

• Driver controls: licensing of drivers, driver testing, financial responsibility and civil liability.
• Vehicle controls: vehicle registration, statutory requirements for vehicles, equipment and accessories, limits on vehicle dimensions and weight, and periodic inspection of vehicles.
• Flow regulations: rules governing directions, turning, overtaking, and the use of traffic signs and signals.
• General controls: reporting of accidents and procedures for recording and disposing of traffic violation cases.

Potential Conflict Points at Intersections

Total potential conflict points at right-angled intersections on two-lane roads vary with traffic regulation and the directionality of the approaching roads. The following table summarises typical values:

The potential conflicts for two-way operations with varying number of lanes are given in the table below:

Traffic Control Devices

The most common traffic control devices are:

• Traffic signs
• Traffic signals
• Road markings
• Traffic islands and channelising devices

1. Traffic Signs

Positioning and mounting rules:

• On kerbed roads, the edge of the sign nearest the carriageway should be at least 0.6 m from the kerb edge.
• On roads without kerbs, the nearest edge of the sign should generally be 2.0 m to 3.0 m from the carriageway edge.
• Sign posts are normally painted with alternate 25 cm bands of black and white for conspicuity.

Traffic signs are commonly divided into three categories:

• Regulatory signs
• Warning signs
• Informatory (informational) signs

(a) Regulatory Signs

Regulatory or mandatory signs inform road users of legal requirements and prohibitions; disobeying them is an offence. Regulatory signs are further classified as follows.

(i) Stop and Give Way Signs

• The Stop sign requires vehicles to stop before the stop line; it is octagonal and red with a white border.
• The Give Way sign assigns right of way to traffic on another road; it is triangular with apex downwards, white background and red border.

(ii) Prohibitory Signs

• Prohibitory signs forbid certain movements, horn use, or entry by specific vehicle classes.
• They are circular with a white background and a red border (symbols inside indicate the prohibition).

(iii) No Parking and No Stopping Signs

• No parking prohibits parking at a specified place; typically circular with a blue background, red border and a single oblique red bar (45°).
• No stopping/No standing prohibits stopping at a specified place; typically circular with a blue background, red border and two oblique red bars (one at 45° and one at right angle to it).

(iv) Speed Limit and Vehicle Control Signs

• Speed limit signs restrict speed for all or specified vehicle classes; they are circular with a white background, red border and black numerals showing the speed.
• Vehicle control signs are circular with red border and black symbols specifying prohibited or permitted vehicle classes.

(v) Restriction Ends Sign

• Indicates the point at which previously notified prohibitions cease to apply.
• It is circular with a white background and a broad diagonal black band at 45°.

(vi) Compulsory Direction Control Signs

• These signs indicate by arrows the mandatory directions of travel for vehicles.
• They are circular with a blue background and white direction arrows.

(b) Warning Signs

Warning or cautionary signs warn road users of hazardous conditions on or adjacent to the roadway. Their standard characteristics and mounting distances are:

• They are equilateral triangles with the apex pointing upwards, having a white background, red border and black symbols.
• They should be located at a suitable distance in advance of the hazard. Typical advance distances are:

(c) Informatory (Informational) Signs

Informational signs guide road users along routes, give destination and distance information, and provide facility details to make travel easier and safer. Subgroups include:

• Direction and place identification signs: rectangular with white background, black border, black arrows and letters (destination signs, direction signs, route markers).
• Facility information signs: rectangular with blue background and white/black letters or symbols (e.g., public telephone, petrol pump, hospital, first aid post).
• Parking signs: square boards with blue background and a white letter 'P', mounted parallel to the road.
• Flood gauge signs: installed at causeways and low-lying crossing points to indicate water depth.

2. Traffic Signals

Properly designed traffic signals provide many advantages but also entail some disadvantages. Understanding signal components and operation is essential for intersection design.

Advantages of Traffic Signals

• Orderly movement of traffic and increased handling capacity at many at-grade intersections.
• Reduction in certain types of accidents, notably right-angled collisions.
• When coordinated, signals maintain reasonable speeds on the main road.
• They permit crossing traffic from minor roads to cross the main traffic stream at regular intervals.
• Automatic signals may be economically preferable to manual (police) control in many situations.

Disadvantages of Traffic Signals

• Rear-end collisions may increase if drivers do not anticipate stops.
• Poorly designed or improperly located signals may be widely violated or ineffective.
• Signal failure (e.g., power outage) can cause confusion and require immediate corrective action.

Important Terms Related to Traffic Signals

• Cycle: period required for one complete sequence of signal indications.
• Phase: portion of the cycle allocated to a traffic movement or combination of movements.
• Interval: any subdivision of the signal cycle during which indications remain constant.

Types of Traffic Signals

• Traffic control signals: include fixed-time, manually operated and traffic-actuated (automatic) signals.
• Pedestrian signals
• Special traffic signals

Traffic Control Signals

• Use a three-colour lamp facing each direction: red (Stop), green (Go) and amber (caution/clearance).
• The amber phase provides clearance time for vehicles already in the intersection at the end of green.

Fixed-time Signals

• Each phase and the cycle are timed in advance based on traffic studies.
• They are simple to implement but cannot respond to short-term fluctuations; a heavy stream may be forced to wait while a light stream receives green time.

Traffic-Actuated Signals

• Phase and cycle timing vary according to real-time traffic demand using detectors and control logic.

Types of Signal Systems

• Simultaneous system: all signals in a route display the same indication at the same time. This system often performs poorly because it does not consider progression.
• Alternate system: alternate groups of signals show opposite indications; more satisfactory than simultaneous operation.
• Simple progressive system: a time schedule is made to permit as continuous operation of vehicle groups along the main road at a reasonable speed.
• Flexible progressive system: cycle times and divisions can be automatically varied at each intersection by computer to improve progression.

Flashing beacons are used to warn or control. A flashing red requires drivers to stop before the crosswalk or stop line. A flashing yellow is a caution signal; drivers may proceed with care.

Design of an Isolated Fixed-Time Signal

Basic elements:

• The stop time (red phase R) for one approach equals the sum of the green and amber times for the conflicting cross flow; for a two-phase signal R = G₂ + A₂.
• Towards the end of the red phase there may be a short red-amber (initial amber) period to indicate get-set; vehicles must not cross the stop line during this period.
• Clearance time follows the green phase and is intended to allow approaching vehicles to stop safely after green changes to amber, and to clear vehicles that entered during green. Amber duration is typically 2-4 seconds.
• Green (go) time is decided from approach volumes, usually considering peak hour traffic.

Design Procedures - IRC Guidelines (summary)

• Pedestrian green time is calculated using a walking speed of 1.2 m/s and an initial walking time allowance of 7.0 s.
• Cycle time is calculated after allowing an amber time of 2 s for each phase.
• The minimum green time for vehicular traffic on any approach is recommended as 16 s.

Road Markings

Road markings are lines, patterns, words, symbols or reflectors applied to the carriageway, kerb faces, islands or fixed objects near the roadway. Types include:

• Pavement markings
• Kerb markings
• Object markings
• Reflector unit markings

Pavement Markings

• White paint is commonly used for lane lines and symbols.
• Yellow markings indicate parking restrictions and continuous centre or barrier lines.
• Pedestrian crossing widths typically range between 2.0 m and 4.0 m depending on local needs.

Kerb Markings

• Kerbs are often painted with alternate black and white bands for visibility.

Reflector Unit Markings

• Reflector markers act as hazard and guide markers to assist night driving.
• Hazard markers reflecting yellow light should be visible from about 150 m.

Road Delineators

Delineators outline the roadway and provide visual assistance to drivers about alignment, particularly at night. Three types are commonly used:

• Roadway indicators (guide posts)
• Hazard markers
• Object markers

• Guide posts: 0.08-1.0 m high, painted with black and white strips, may include reflectors to mark the carriageway edge.
• Hazard markers: plates on posts with three red reflectors or black and yellow diagonal stripes (45°) indicating side of obstruction.
• Object markers: circular red reflectors arranged on triangular or rectangular panels to warn of fixed objects.

Traffic Islands

Traffic islands are raised areas within the roadway to guide vehicular movement by physical channelisation. They are classified by function:

• Divisional islands
• Channelizing islands
• Pedestrian loading islands (refuge)
• Rotary central islands

1. Divisional Islands

• Separate opposing flows on multi-lane highways, converting the road into two one-way carriageways and reducing head-on collision risk.

2. Channelizing Islands

• Guide traffic into desired channels through intersections and reduce conflict area.

3. Pedestrian Loading Islands

• Provided at bus stops and busy crossing locations to protect passengers and provide pedestrian refuge.

4. Rotary

• The large central island in a rotary forces all converging vehicles to circulate in one direction (typically clockwise) before leaving for their desired route.

Intersection Types

Intersections are broadly classified into:

• At-grade intersections
• Grade-separated intersections

1. Intersections at Grade

• Roads meet at approximately the same level; manoeuvres include merging, diverging and crossing.
• At-grade intersections may be unchannelized, channelized or rotary.

Basic design requirements for at-grade intersections

• The area of conflict should be as small as possible.
• Relative speeds and the angle of approach of vehicles should be low.
• Adequate approach lengths should be provided for vehicles to decelerate/accelerate.
• Sudden, sharp path changes should be avoided.

(a) Unchannelized Intersections

• The intersection area is paved without physical restrictions; vehicles may use any part of it.
• A plain intersection has no additional pavement for turning; a flared intersection provides widened pavement (turn lanes).

• In unchannelized intersections the conflict area is large since turning paths are uncontrolled; typically one vehicle must yield while another proceeds.

(b) Channelized Intersections

• Channelization introduces islands to reduce conflict areas and to control turning paths through predefined channels.

(c) Rotary Intersections (Roundabouts)

• Vehicles circulate around a large central island in one direction, converting crossing manoeuvres into weaving, merging and diverging operations.

Design Factors of a Rotary

• Design speed: recommended about 40 km/h for rural rotaries and about 30 km/h for urban rotaries.
• Shape of central island: depends on the number and layout of intersecting roads-circular when approaches are symmetric, elongated when accommodating more legs or greater flows along one axis.
• Turbine shapes encourage speed reduction on entry and permit acceleration on exit; however, glare from headlamps at night can be a limitation.

• The island shape should avoid excessive elongation or long tangents which can encourage high speeds in one direction.

Radius of Rotary Roadway

One standard relation for horizontal curvature (assuming no super-elevation) is:

R = V² / (127 × f)

Where:

• R = radius of curve (m)
• V = design speed (km/h)
• f = coefficient of side friction (typical values: about 0.43 for 40 km/h and 0.47 for 30 km/h after allowance for safety factors)

• IRC recommends radii of 20-35 m (entry curve) for a 40 km/h rotary and 15-25 m for a 30 km/h rotary.
• The minimum radius of the central island is commonly taken as 1.33 times the radius of the entry curve.

Wearing (Weaving) Angle and Weaving Distance

• The weaving angle is the angle between the path of a vehicle entering the rotary and that of another leaving the rotary whose path crosses the former.
• The weaving length is the length of rotary roadway between adjacent channelizing islands where merging and diverging take place.
• For smooth flow the weaving angle should be small but not less than about 15° (otherwise the central island becomes excessively large).
• The weaving length should be at least four times the width of the weaving section.
• Recommended weaving lengths: 45-90 m for 40 km/h design speed, and 30-60 m for 30 km/h design speed.

Width and Radius of Carriageway at Entry and Exit

• Minimum carriageway width at entrance and exit is usually about 5.0 m.
• Exit curves should have larger radii than entry curves-commonly 1.5 to 2 times the entry radius-since vehicles accelerate when leaving the rotary.
• Pavement width at the entrance curve may be greater than at the exit due to the smaller radius of the entrance.

Other Design Standards

• Shape and size of channelizing islands depend on rotary radius, entrance and exit curve radii, and the layout of radial roads.
• Design is typically made assuming no super-elevation on rotary pavement.
• Minimum sight distances recommended are about 45 m for 40 km/h and 30 m for 30 km/h design speeds.

Conditions When a Traffic Rotary Is Justified

• Rotary is generally justified when the total traffic volume on all intersecting roads is at least about 500 vehicles per hour and can function efficiently up to about 3000-5000 vehicles per hour depending on layout.
• IRC suggests an operational upper limit of about 3000 vehicles per hour entering from all legs for efficient rotary functioning.
• Rotaries are particularly suitable where motor traffic on intersecting roads constitutes about 50% or more of total traffic, or where right-turning traffic is at least about 30% of total traffic.

Advantages and Limitations of Traffic Rotaries

• Crossing manoeuvres are converted into weaving, merging and diverging operations which generally reduce conflict severity.
• Variable operating costs for vehicles at rotaries are lower than at signalised intersections where frequent stopping increases fuel consumption and delay.
• Accident frequency and severity are usually lower at rotaries because of reduced relative speeds.
• Rotaries are effective where the number of intersecting roads is between four and seven; they also often provide the highest capacity among at-grade intersections.

Limitations

• Rotaries require considerable land area; in built-up urban areas land costs can make them uneconomic.
• High pedestrian volumes make rotaries less suitable unless supplemented by signal control or police direction.
• Very acute intersection angles or more than seven legs reduce rotary suitability.

Grade-Separated Intersections

• Intersecting roads are separated by vertical grade (one road passes over or under the other), eliminating crossing manoeuvres.
• Grade separation is achieved using overbridges (overpasses) or underpasses.
• Interchange ramps provide connections between grades and are classified as direct, semi-direct or indirect based on diverging and merging locations (right or left side).
• Direct ramps involve diverging and merging on the right; semi-direct ramps diverge to the left but merge on the right; indirect ramps use left-side diverging and left-side merging, often simpler and safer, but longer.

Advantages of Grade Separation

• Eliminates stopping for crossing traffic and reduces collision potential.
• Improved safety for turning traffic; indirect ramps can convert right turns into safer left turns and merges.
• Capacity at a grade-separated junction can approach the combined capacity of the crossing roads.
• Suitable for varied intersection angles and layouts.

Disadvantages of Grade Separation

• High construction and land acquisition costs.
• In constrained urban areas or difficult topography, grade separation may be impractical or uneconomic.
• In flat terrain, grade separation can introduce undesirable crests and sags into the vertical alignment.

Grade Separation Structures

• Over-pass: major highway raised on embankment or bridge to pass over another road.
• Under-pass: major highway depressed below ground level to pass under another road.
• The choice depends on topography, drainage, vertical alignment, cost, aesthetics and the relative importance of the roads involved.

Advantages of Over-pass

• Reduces drainage problems compared with depressed structures in some situations.
• When the wider road is carried over a narrow span, bridge cost may be lower.
• Perceived freedom of movement and aesthetics are often better when the major road is on an overpass.
• Future lateral expansion is easier with overpasses.

Disadvantages of Over-pass

• May introduce rolling grades in undulating terrain and increase grade resistance for heavy vehicles if embankments are steep.
• May require long vertical curves to preserve sight distance.

Advantages of Underpass

• Underpasses can be visible to approaching drivers, providing advance warning.
• Traffic from the cross road can accelerate while descending into the underpass and traffic from the major road can decelerate when ascending, which may aid turning movements.
• Underpasses may avoid modifying the existing vertical alignment of the main highway when the cross road is depressed.

Disadvantages of Underpass

• Drainage can be problematic; underpasses require reliable pumping/drainage where groundwater or runoff collects.
• Overhead structure can restrict vertical sight distance near valley curves.
• Perception of confinement and reduced side clearance can affect driver comfort and capacity.
• Stage construction for expansion is more difficult for underpass bridges.

Lighting Layouts

• Single-side lighting is economical and suitable for narrow roads. For wider roads (three or more lanes) staggered or central lighting may be adopted.
• Lights are usually closer on curves than on tangents and are located on the outer side of curves for better visibility.
• At summit curves lights should be spaced more closely near the crest to avoid dark patches at the top.
• For simple urban intersections the required illumination should be at least equal to the sum of illumination values for the two intersecting roads.

• Spacing between lighting units:
  
• Average maintenance factor may be assumed as 0.8.