Traffic Control Methods

2.1 Types of Intersections

Review

• Crash study and Objectives
• Causes of road crash
• Crash data collection, Reporting and Records
• Crash study
• Crash analysis (collision of moving vehicle with parked vehicle, collision of vehicles approaching at intersection from right angle)

Road Intersection and Types20812076

• Area where two or more roads join or cross including the roadway and roadside facilities for traffic movement within it (AASHTO 2001).
• Traffic movement in different directions. About two third of all urban road crashes and one third of all rural road crashes occur within 18 m of an intersection.
• Types:
  • At Grade Intersection
  • Grade Separated Intersection

At Grade Intersection

• Intersection where all roadways join or cross at the same level

• All traffic maneuvers take place on the same plane.

Basic Requirement of At Grade Intersection208220752074

• Minimum conflict points
• Separate provision for pedestrians and cyclists if their volume is high
• Sudden change of path should be avoided
• Adequate turning radius and width of pavement should be provided
• Adequate visibility for vehicles
• Small approach speed
• Proper sign should be provided
• Good lighting shall be provided for night

Types of At Grade Intersection

Based on number of intersecting legs:

• T or Three-leg intersections with three approaches T Y
• Four-leg or cross intersections: with four approaches
• Multi-leg intersections: with five or more approaches. Scissor Cross Staggered T Staggered and Skewed Multiway

2.2 Un-Channelized and Channelized Intersections

Unchannelized Intersection

• At grade intersection without channelizing island for directing traffic into definite path.
• Traffic from all approaches intermixes and proceed to their desired path
• Lowest class of intersection, frequent road crashes
• Two types:
  • Plain (Simple) intersection,
  • Flared intersection

Plain (Simple) Intersection

• No additional lanes are provided for turning movements at intersection approaches.

Flared Intersection

• Approach roads are widened near the intersection by adding one or more lanes to facilitate right/left turning traffic.

Channelized Intersection20762075

• Vehicles approaching an intersection are directed to definite paths by islands, marking etc.
• Channelization can be partial or complete.
• Islands are introduced into the intersectional area.
• It reduces the number of possible conflicts by reducing the area of conflicts available in the carriageway.
• Channelized intersection provides more safety and efficiency.

Advantages: Disadvantages:

• Vehicles are confined to definite path
• Decrease conflict area
• Control angle of conflict
• Speed control of entering vehicle
• Refuse island for pedestrian
• Place for installation of traffic signs
• Require large area
• Difficult to design
• One of crossing vehicle will have to stop while other crossing

Conflict Points at Intersections

• Conflicts occur when traffic streams moving in different directions interfere with each other.
• Four types of conflicts: Diverging, Merging, Crossing, Weaving
• The number of possible conflict points at any intersection depends on:
  • number of approaches
  • turning movements
  • type of traffic control

Types of Conflicts

Conflict Points

2.3 Grade Separated Intersections (Overpass, Underpass and Interchanges)

Grade Separated Intersection20762074

• Intersection where cross flow of traffic are made at different levels so that the movements of traffic in different direction do not conflict with each other
• Grade separation is achieved by separation of intersecting highway in vertical level by means of bridge or underpass.
• Grade separation structures: Overpass, Underpass
• Types of Grade Separated Intersection
  • Grade Separated Intersections without Interchange
  • Grade Separated Intersections with Interchange

Advantages: Disadvantages:

• No crossing conflicts
• Increased safety
• Better travel time, capacity almost full capacity
• Increased operational efficiency, comfort and convenient
• Possible for any angle of intersection and any number of intersecting roads
• Stage construction of ramps possible
• Requires large area
• Costly infrastructures
• Unnecessary rise and sags are introduced in vertical alignment

Grade Separated Intersection without Interchange

• Simply introduction of an overhead bridge or underpass which enable the traffic stream on the intersecting roads to cross over each other without any conflicts.
• Ramps (Connecting roads) are not provided to connect roads at different levels.

Grade Separation Structures (Overpass)

• Major highway is taken above by raising its profile above the general ground level by embankment and over bridge across another highway

Advantages: Disadvantages:

• Drainage problems are avoided
• Future and lateral expansion is possible
• Potential steep gradient at the entry and exit
• Restriction on sight distance if long vertical curves are not provided

Grade Separation Structures (Underpass)

• A highway is taken below by depressing it below ground level to cross another road/rail road etc.

Advantages: Disadvantages:

• Vehicles on major road are not changing its vertical profile if minor road is taken underground
• No restricting on sight distance
• High cost of construction than overpass
• Drainage problem

Grade Separated Intersection with Interchange

• Intersection in which vehicles moving in one direction of flow may transfer to another direction by the use of connecting roads (Ramps)
• Interchange Facilities
  • Direct
  • Semi Direct
  • Indirect

Types of Ramp Facilities

Semi Direct

Grade Separated Intersection with Interchange

Advantages: Disadvantages:

• Avoids necessity of stopping
• Increasing safety and easiness for turning traffic
• Save travel time and VOC
• Stage construction is possible
• Costly
• Difficult, costly and unfavorable when the topography is not favorable
• Undesirable crests and sags

Types of Interchange

• Trumpet Interchange (Three leg interchange)
• Diamond Interchange
• Cloverleaf Interchange
• Partial Cloverleaf Interchange
• Directional Interchange
• Bridged Rotary (Rotary Interchange)

Trumpet Interchanges

• Trumpet interchanges have been used where one highway terminates at another highway at almost 90o.
• The principal advantages are low construction cost

Directional Y

Diamond Interchange

• A diamond interchange is a common type of road junction, used where a freeway crosses a minor road.
• The diamond interchange uses less space therefore suitable for area with narrow ROW major road
• Disadvantage is that a possibility of illegal turnings.

Cloverleaf Interchange

• A cloverleaf interchange is a two-level interchange in which left turns (right turns in left hand driving) are handled by loop ramps.
• Suitable when two high volume and high-speed roads intersect

Advantages: Disadvantages:

• Traffic not impeded
• Direct left turn
• Simple to use
• Require large area
• Right turn require extra distance
• Carriageway larger

Partial Cloverleaf

• Partial clover leaf is a modification that combines some elements of a diamond interchange with one or more loops of a cloverleaf to eliminate only the more critical turning conflicts.
• It provides more acceleration and deceleration space on the freeway.

Directional Interchange

• A directional interchange provides direct paths for right turns.

• These interchanges contain ramps for one or more direct or semi direct left turning movements.

2.4 Rotary Intersection

Rotary Interchange

• The major highway is taken above or below rotary intersection to separate major through flow
• Advantages:

• Relatively less area than other interchange
  • Carriageway area is also less
  • U-turns are easy
• Disadvantages:
  • Capacity is limited by capacity of rotary itself
  • Weaving maneuvers

Rotary Intersection20812068

• A specialized form of at grade intersection laid out for movement of traffic at one direction round a central island before proceeding to respective direction.
• Shapes: Circular, squarish with round edge, elliptical, elongated, oval, rectangular, complex

• Advantages:
  • Crossing conflicts are converted into weaving
  • Reduced severity of crash
  • Equal opportunity for all vehicles, self functional
  • Continuous and fairly uniform so reduced operational cost
• Disadvantages:
  • Requires large land area
  • Requires favorable and flat topographical condition
  • Right turning traffic has to travel extra distance
  • Complex in mixed traffic condition
  • Not suitable when pedestrian traffic is high

Guideline for Selecting Rotary

• Right turning traffic is more than 30% of all approaching traffic
• Traffic volumes of intersecting legs are approximately equal
• Highest traffic volume of 3000 PCU/hr and lowest traffic volume is 500 PCU/hr.
• Intersecting legs are more than four but not more than seven

Design Guidelines (IRC: 65-1976)207820762075

1. Design Speed

• 30kmph (urban area) to 40kmph (rural area)

2. Shape of Central Island

• Depends on the number and layout of the intersecting roads
  • Circular, elliptical, complex etc.

Rotary Intersection

Example: 350 630 405 At an urban intersection, two highways of 15m carriageway intersect at right angle with the traffic flow in PCU/hr for design year as mentioned in figure. Design rotary intersection making suitable assumption. 540 350 410

Tasks

• Review road intersection and types
• Review types of at grade intersection
• Review unchannelized and channelized intersections
• Review conflicts in intersection
• Review grade separation structures
• Review grade separated intersections with and without interchanges
• Review rotary intersection and design

2.5 Traffic Control Devices: Traffic Sign, Marking and Islands

Review

• Road intersection and types
• At grade intersections: Configurations, channelized, unchannelized
• Grade separation structures
• Grade separated intersections with and without interchanges
• Rotary intersection and design

Traffic Control Devices2072

• Various devices used to control, regulate and guide traffic.
• Requirement: Attention, meaning, time of response, respect of road users
• Types:
  • Traffic Signs
  • Road markings
  • Traffic Signals
  • Traffic Island

Traffic Signs2082

According to Traffic Sign Manual 1997 DoR “Traffic sign means any object, device, line or mark on the road whose object is to convey to road users, restrictions, prohibitions, warnings or information of any description.”

Three main types:

• Regulatory signs
• Warning signs
• Informatory signs

Importance of Traffic Sign

• Give timely warning of hazardous situations
• Regulating traffic by imparting messages to the drivers about the need to stop, give way and limit their speed.
• Give information on highway route, directions and points of intersect

Principle of Traffic Signs

• Should fulfill a need
• A message be presented in simple way
• Sign should be easily noticed by driver
• Symbol and legend on signs must be easy to read;
• Command respect from road users;
• Give adequate time for proper response;
• Long lasting

Basic Requirements of Traffic Signs

• High visibility during both night and day time
• Lettering or symbols of adequate size
• Proper location
  • 0.6m away from the edge of the kerb
  • 2.0 ~ 3.0m from the edge of the carriageway (Roads without kerb)

Limitation of Traffic Sign

• Easily damaged due to impact or vandalism
• Visual quality degrade over time due to dirt and deterioration
• Require continuous maintenance

Regulatory Signs

• These signs give orders on what they must do and they must not do.
• DoR’s Traffic Sign Manual 1997 have 33 types of Regulatory Signs (A1-A33)
• Regulatory signs are either Mandatory or Prohibitory.

Regulatory Signs (Mandatory)

• The mandatory signs give instructions to drivers about what they must do.
• Most of them are circular with a white symbol and border on a blue background.
• Stop sign and Given Way sign have their own distinct individual shapes.

Regulatory Signs: Mandatory

STOP

Give Way

Ahead only

Restriction

Ends

Turn Left Ahead

Keep Left

Turn Left

Regulatory Signs (Prohibitory)

• The prohibitory signs give instructions to drivers about what they must not do.
• E.g. Max. Speed Limit sign, No Parking sign, No Entry sign
• Most are circular and have a red border.

No Vehicles Over

Height Shown

No Entry

No Trucks

No Vehicles Over Maximum

Gross Weight Shown

Axle Weight Limit

Warning Signs

• Alert driver to danger or potential danger ahead
• Also called danger or cautionary signs
• Require motorists/cyclists to be careful and may need a reduction of speed for their own safety.

Informatory Signs

• These are used to guide the road users along routes about information of destination and distance and provide information to make travel easier, safe and pleasant
• Shape: square or rectangular
• White background with black letters or green background with while letters

Informatory Signs (Direction and Place Identification Sign)

Advance Direction Sign

Route Confirmation Sign

After Junctions

Informatory Signs (Facility Information)

Overnight

Accommodation

Hospital

Refreshments

Informatory Signs (Other Information)

Telephone

Parking

Place

Bridge Name Plate

Supplementary Signs

• Gives additional information to clarify the message given by the main sign
• Mounted 75 mm below main sign
• Usually used with regulatory sign
• E.g. distance to hazard , time

Road Marking or Traffic Marking2076

Road markings are lines, patterns, symbols, words, colors or reflectors on the surface of the pavement or curb or fixed objects near highway to regulate, warn, or guide traffic.

Functions:

• Guide driver in correct positioning of the vehicle in lane
• Guide driver for turning movements at junction
• Give continuous message to driver

Limitations:

• May be hidden by dirt, snow or other vehicles directly over them
• May be worn by sand or gravel.
• Wear due to traffic and the environment and must be maintained or replaced.
• Removal of markings from the pavement is a difficult task

Road Marking: Design

• Light Reflecting paints (when light occurs, it reflects) or reflectorized by adding glass beads (called ballotini) should be used to easily attract the attention of the road user.
• Raise pavement markers to 10-25 mm high
• Colors: white, black, yellow, red

Road Marking: Types

• Pavement Markings
• Kerb (Curb) Markings
• Object Marking
• Reflector Units

Pavement Markings: Longitudinal Markings

Lane Line

Hazard Line

Warning

Barrier Line Do

Not Cross

Edge of Carriageway No Parking

Multi-Lane Road

Pavement Markings: Transverse Markings

• Diagonal lines – in traffic islands

Pavement Markings: Parking Space Markings

• Promotes efficient use of the parking spaces and prevent encroachment on bus stop, roadway etc.

• 100-150 mm wide

Pavement Markings: Lane Arrows

• Guide drivers which lane they should take when approaching a junction
• First one 15 m from junction and second one 45 m from junction

Pavement Markings

Pavement Markings: Words

• Only used in support of standard signs
• Limited to as few words as possible, never > 3 words
• White in color and elongated
• 2.4 m high and 1.8 m at low speed roads

Pavement Markings: Symbol

Kerb Marking

• Alternate black and white line painted or chequed black and white on the kerb

• 500 mm width
• Indicates pavement limit and increase visibility from a long distance

Object Marking

• Piers and abutments, monuments, traffic islands, median sign post, culvert headwalls, trees, poles etch should be marked
• Alternative black and yellow stripes with downwards slope of 45o towards the side of the obstruction
• Width 100 mm

Reflector Unit

• A road marking unit comprising a housing with a light- transmitting window, which automatically flash light in dark or bad weather
• Also known as cat’s eye
• Used in sharp curved roads, intersections, hilly areas, foggy weather and fly-overs.

Traffic Island

• Raised areas constructed within carriageway to establish physical channel to guide traffic
• Provided to avoid or minimize major or minor traffic conflict

Types of Traffic Island

• Divisional Island (Median)
• Channelizing Island
• Rotary Island
• Pedestrian refuse

Divisional Island

• Separate opposing flow of traffic
• Head on collision are reduced
• Usually installed at road with 4 or more lanes
• To eliminate the head light glare during night driving, the width of this island should be quite large
• Also provides a refuge for pedestrians

Channelizing Island

• To guide traffic into proper channel at intersection.
• To control and direct traffic movement, usually turning traffic

• Should be located with respect to approach that their visibility is clear and the intended path is automatically selected.
• Serve as location for other traffic control devices, refuge islands

Rotary Island

• Rotary island is much larger than the central island of channelized intersection
• All approaching vehicles forced to move around a large central island before weave out into the desired direction
• Crossing maneuver is converted to weaving

Pedestrian Refuse Island

• Located on the cross walks to provide refuse for the pedestrians on wide roads where traffic flow is heavy.
• Serves as safety zones for protection of pedestrian to cross road
• Particularly useful at intersections in urban areas

Tasks

• Review traffic signs, importance, limitations, principles, basic requirement
• Review types of traffic signs (regulatory, warning, informatory, supplementary)
• Review road marking, function, limitations, design
• Review types of road marking (pavement marking, kerb marking, object marking, reflector units)
• Review traffic islands (divisional, channelizing, rotary, pedestrian refugee)

2.6 Traffic Signal2072

Why signals are used

At an intersection the conflicting streams cannot be separated in space, so a traffic signal separates them in time: each stream is given exclusive use of the conflict area for part of a repeating cycle. It is the cheapest form of at-grade control that can handle heavy crossing flows, because it needs only simple geometric design.

A signal installation must:

• draw the attention of the road user;
• convey a meaning that is understood at a glance;
• give enough time to respond; and
• impose the minimum waste of time consistent with safety.

Definitions and notation

The whole of signal design rests on a small vocabulary, so it is worth stating it precisely:

Classification by purpose

1. Traffic control signals — the familiar three-colour system: red = stop, green = go, amber = clearance time.
2. Pedestrian signals — installed so pedestrians can cross safely; they show WALK and DON'T WALK indications.
3. Special traffic signals — installed at particular locations to warn traffic of a specific situation (flashing beacons, lane-use signals and the like).

Classification of traffic control signals by mode of operation

1. Manually operated — worked by a traffic police officer according to the demand seen on the ground; still common at many Nepalese intersections.
2. Fixed-time (pre-timed) automatic — the same set of phases and the same cycle time repeat continuously; the settings can only be changed at the controller.
3. Traffic-actuated (automatic) — detectors vary the phases and the cycle with the demand. Much more efficient, but also much more costly.

Advantages and disadvantages

Sections 2.6–2.8 are written from P. M. Parajuli, Complete Manual on Transportation Engineering II, Ch.1 §1.3.2 (book pp.39–51) — the reference cited by the worked solutions below. Lecture 12 of the course deck, which covers this topic, was not available.

2.7 Warrants for Traffic Signalization

Warrants for installation of a traffic control signal

A signal is not automatically the right answer at a busy junction: it introduces delay to the major stream and can increase rear-end collisions. A signal should therefore not be installed unless at least one of the following warrants is met, and the data supporting it must come from proper traffic engineering studies.

(a) Minimum vehicular volume warrant

• Average volume for eight hours of the day on both approaches of the major street ≥ 650 veh/h (single lane) or 800 veh/h (two or more lanes); and
• on the minor street, in one direction only, ≥ 200 veh/h (single lane) or 250 veh/h (two or more lanes).

If the average approach speed or the 85th-percentile speed on the major street exceeds 60 km/h, or the intersection lies inside a built-up area, these figures may be reduced to 70% of the above.

(b) Interruption of continuous traffic flow on the major road

When the major street carries 1000–1200 veh/h and the minor road, with only 100–150 veh/h in one direction, suffers undue delay or hazard during any eight hours of an average day.

(c) Minimum pedestrian volume warrant

When 150 or more pedestrians per hour cross a major street carrying more than 600 veh/h on both approaches — or 1000 veh/h where the main street has a raised median. The 70% relaxation for high approach speed applies here too.

(d) Accident experience warrant

When the accident record shows that other remedial measures have failed and five or more accidents involving injury or property damage have occurred within a 12-month period. Even then the signal must not seriously disrupt the traffic flow.

(e) Combination of warrants

When no single warrant is satisfied on its own, but two or more of warrants (a), (b) and (c) are each satisfied to the extent of 80% of their stated values.

Sections 2.6–2.8 are written from P. M. Parajuli, Complete Manual on Transportation Engineering II, Ch.1 §1.3.2 (book pp.39–51) — the reference cited by the worked solutions below. Lecture 12 of the course deck, which covers this topic, was not available.

2.8 Traffic Signal Design by Webster Method2082208120802076207520732071

General principles behind the timings

1. The red phase of one road is the sum of the green and amber of the other road. Pedestrian crossing time is accommodated inside it when turning movements are barred.
2. Near the end of red, amber may be shown together with red — the red–amber or initial amber — meaning "get set". Vehicles must not cross the stop line during it.
3. Clearance amber is provided after green, before red, to satisfy two requirements:
   • a vehicle approaching the stop line can stop at it, and
   • a vehicle already too close to stop at the legal speed can clear the intersection area.
   Usually 2–4 s, longer for higher approach speeds.
4. Green time follows from the peak-hour approach volume, and must clear the queue in most cycles.

Typical design data

• Cycle length for a two-phase signal: mostly 40–60 s.
• Amber: 3–5 s — 3 s up to 50 km/h, 4 s for 50–65 km/h, 5 s for 65–80 km/h.
• Green: of the order of 20 s; red slightly less than the green of the other road.
• Pedestrian clearance is computed at a walking speed of 1.2 m/s.

Method A — trial cycle method

The crudest of the three methods; useful only as a first estimate.

1. Count traffic on roads A and B simultaneously for 15 minutes; call the counts N₁ and N₂.
2. Assume a trial cycle length C seconds.
3. Number of cycles in the 15-minute period = 900/C.
4. Taking a headway of 2.5 s per vehicle, the green times are

5. Cycle time C = G_A + G_B + Y_A + Y_B; iterate until the assumed and computed C agree.

Method B — Webster's method (optimum cycle)

Webster's method finds the optimum cycle length C₀ — the cycle that gives the least total delay at the intersection. It is the method the examiners ask for most often. The field work is to estimate, for every approach:

• the saturation flow S (PCU/h), by timing the discharge of a compact queue during a fully used green; and
• the normal flow q (PCU/h) during the design hour.

In the absence of field data the saturation flow may be taken as 160 PCU/h per 0.3 m of approach width. Mixed traffic must be converted to PCU separately for the normal flow and for the saturation flow.

For each phase take the critical (highest) ratio of normal to saturation flow:

The total lost time per cycle is

The optimum cycle and the green split are then

Y must be less than 1, otherwise the intersection is over-saturated and no cycle length can clear it. The same formulation extends to any number of phases.

Method C — design on the basis of pedestrian crossing time

Used when a two-phase signal at a cross-road has to be designed together with pedestrian signals — the situation in most of the past questions.

1. Choose the amber from the approach speed: 3 s up to 50 km/h, 4 s for 50–65 km/h, 5 s for 65–80 km/h.
2. Pedestrian clearance time for a road = its carriageway width ÷ 1.2 m/s.
3. Minimum red on a road = pedestrian clearance time for that road + the initial walk interval (7 s where a pedestrian signal is installed, else 5 s).
4. Minimum green on a road = (pedestrian clearance of the cross road + initial interval) − the amber of that road. Equivalently, min green of one road = min red of the cross road − that road's amber.
5. Increase the green of the heavier road in proportion to the approach volume per lane:

6. Cycle length C = G_A + Y_A + G_B + Y_B, then round up to the next multiple of 5 s. Distribute the excess between the two greens in proportion to their volumes.
7. Red on a road = green + amber of the other road.
8. For the pedestrian signals: DON'T WALK on a road = the vehicular red of the other road, and

Timings are finally expressed as a percentage of the cycle, because controller settings are made in per cent, and checked on site during the peak hour.

Method D — signal timings as per IRC guidelines

IRC's guidance combines the pedestrian criterion with a queue-clearance check and with Webster's formula, and is the most complete of the three procedures.

1. Pedestrian walking speed 1.2 m/s and initial walk time 7 s. The pedestrian green so obtained is the minimum green for the vehicular traffic on the minor and major roads.
2. The green for the major road is increased in proportion to the traffic on the two approaches.
3. Cycle time is then computed allowing 2 s of amber for each phase.
4. Check the queue clearance green for each approach lane: the first vehicle takes 6 s and every subsequent PCU 2 s; the minimum green so found is limited to 16 s.
5. Compute the optimum cycle from Webster's formula. Saturation flow may be assumed as below; above 5.5 m take 525 PCU/h per metre of width.
6. Lost time comes from the amber, the inter-green and an initial delay of 4 s for the first vehicle on each leg.
7. Revise the cycle and the phases in the light of steps (4) and (5), and set the cycle to a convenient multiple of 5 s.

Sections 2.6–2.8 are written from P. M. Parajuli, Complete Manual on Transportation Engineering II, Ch.1 §1.3.2 (book pp.39–51) — the reference cited by the worked solutions below. Lecture 12 of the course deck, which covers this topic, was not available.

2.9 Traffic Calming Measures

Review

• Traffic Signal, advantages, disadvantages, types of operational control, warrants, design using webster’s method

Traffic Calming

• Traffic calming is a set of engineering measures and devices that reduce the negative effects of motor vehicles in built-up areas.
• The objective is to improve safety, enhance livability and protecting the environment.

Traffic Calming Measures

• Traffic calming measures generally refers to local speed control and other measures of traffic restraints to discourage unnecessary through traffic and restrict or divert heavy or commercial vehicles where appropriate but may more widely include network planning, parking policies and stimulating the alternative transport modes.
• Can be broadly classified according to the following structures:
  • Changes in vertical alignments
  • Changes in horizontal alignments
  • Other measures

Changes in Vertical Alignment

• These measures change the road surface elevation, causing drivers to reduce speed for a comfortable ride.
• Most effective and reliable for speed reduction (IRC, 2018)
• Eg: Speed humps, speed tables, rumble strips, uneven road surface, raised pedestrian crossing, raised intersections

Rumble strips

Speed hump

Uneven road surface

Raised intersection

Raised pedestrian crossings

Changes in Horizontal Alignment

• These measures alter the horizontal path of the roadway, forcing drivers to slow down by changing direction or narrowing the travel path.
• However, are less effective than vertical ones (IRC, 2018)
• Eg: Lane narrowing, curb extension, chicanes, parking facilities, pedestrian medians, roundabouts, junction area reduction

Chicane

Lane narrowing

Other measures

• Measures that help to force drivers slowing down, manage traffic flow or restrict certain vehicles without necessarily changing the road geometry.
• Eg: Gates, partial or total road closure, speed control traffic lights, driver’s feedback sign, road marking and surface treatments, road lighting

Driver’s feedback sign

Gates

2.10 Importance and Design of Street Lighting

Street Lighting

• Crash rate is higher at night.
• Street lighting is primarily intended to enable the road users to see accurately and easily the carriageway and the immediate surroundings in darkness.

Importance of Street Lighting20752072

• Enable road user to see accurately carriageway and surrounding
• Reduce the strain on night driving
• Enhance comfort in driving
• Reduces glare problem
• Reduction in crash rates
• Improves the efficiency of traffic operation so improved traffic flow
• Pedestrians get better visibility, safety and security
• Indirect benefits like reduction of crime, extension of business hours, improved aesthetic appearance

Factors Influencing Night Visibility20822076

• Amount and distribution of light flux from the lamps
• Size of object
• Brightness of object
• Brightness of the background
• Reflecting characteristics of the pavement surfaces
• Glare on the eyes of the driver
• Time available to see an object
• Weather conditions such as fog, dust, smoke, rain etc.

Requirement of Level of Illumination of Roads

• Adequate to safe driving without headlights
• On residential and non-traffic streets, the lighting be suited to the needs of pedestrian
• In ideal case the lights should be sited in such a way that the bright patches link up to cover the entire road surface.
• The average level of illumination for roads with fast moving vehicles may be 20 to 30 lux, and mixed traffic may be 4 to 8 lux
• As per NRS, level of illumination should be 30 lux on important high speed roads and 15 lux on other main roads.

• ISI (IS1944) Recommendation Classification of lighting Installation Type of Road Average level of Illumination on road (lux) Group A1 Important traffic routes carrying fast traffic Group A2 Other main roads carrying mixed traffic like main city streets, arterial roads Secondary roads with considerable traffic like principal local traffic routes, shopping streets Group B1 Group B2 Secondary routes with light traffic

Design of Lighting System20802075

• Determination of level of illumination required
• Choice of lamp
• Luminaire distribution of light
• Spacing of lighting units
• Height and overhang of mounting
• Lateral placement
• Lighting layout

Design: Determination of Level of Illumination Required

• Done by assessing the facility to be lighted
• Requirement for different roads (NRS, IS1944 Recommendation)

Design: Choice of Light Source (Lamp)

• The choice of the lamp depends on its light flux.
• The choice of lamp is also governed by initial cost, life, color, rendering quality, wattage, brightness, efficiency, operation and maintenance cost etc.

• Tungsten filament 8 – 14 lumens per watt, short life 1000 hrs, very low efficiency and cheapest

• 50 to 70 lumens per watt, life 6000 hrs, good rendering properties Sodium vapor lamp
• 100 to 120 lumens per watt, life 4 years, very energy efficient

Mercury vapor lamp

• 23 to 60 lumens per watt, life 7500 hrs, energy efficient and contains mercury Mercury Halide lamp
• 75 to 100 lumens per watt, life 6,000–15,000 hrs, energy efficient

LED lamp

• 130 lumens per watt, life 60000 to 100000 hrs, highly energy efficient, white light

Design: Luminaire Distribution of Light

• The distribution should be downward so that max. percentage of light is utilized for illuminating pavement and adjacent areas
• Light distribution should produce maximum uniformity of pavement brightness.
• Light distribution should cover the pavement, the kerbs and provide adequate lighting on adjacent area i.e. 3~5 m beyond the pavement edge.

Design: Lateral Light Distribution

Type I (Two way lateral distribution)

• Luminaire mounted at the center of area
• Up to 1 MH in front of and behind luminaire
• Lateral spread of 15 degrees on either side of roadway axis
• Suitable for narrow streets

Type II (Narrow asymmetrical lateral distribution)

• Luminaries located at or near the side of relatively narrow roadways of width 1.0 - 1.75 MH in front of luminaries
• Lateral spread of 25 degrees
• Mostly used on smaller side streets or jogging paths.

Type III (Medium asymmetrical lateral distribution)

• Lateral spread around 45 degrees
• Luminaries mounted at or near the side of roadways, where the width does not exceed 2.75 times the mounting height.
• Suitable at parking areas and other areas where a larger area of lighting is required

Type IV (Wide asymmetrical lateral distribution)

• Lateral spread around 60 degree and semicircular distribution
• Placed at side of road, where the roadway width < 3.7 times the mounting height.
• Suitable at wide roads and the perimeter of parking areas and businesses

Type V (Normal symmetrical distribution)

• Produces symmetric circular or square distribution that has the same intensity at all angles
• Luminaries mounting at or near center
• Suitable at center wide roadways, center islands of parkway, and intersections

Design: Longitudinal Light Distribution

Short distribution: The maximum candlepower beam strikes the roadway surface between 1.0 and 2.25 mounting heights from the luminaire.

Medium distribution: The maximum candlepower beam strikes the roadway surface between 2.25 and 3.75 mounting heights from the luminaire.

Long distribution: The maximum candlepower beam strikes the roadway surface between 3.75 and 6.0 mounting heights from the luminaire.

Design: Spacing of Lights2082

Utilization Factor:

• The fraction of the luminous flux coming from a luminaire that reaches street/road; Street Coefficient of Utilization (%)
• It depends on efficiency of luminaries, distribution of luminaries, reflectance of room, geometry of the space
• Utilization factor curves for a luminaire are developed as a function of ratio of transverse distances (width of area) to mounting height (h) House Width of Area Mounting Height Ratio

Example: Calculate the spacing between the lighting units to produce a lux equal to 7

𝜇 from the following data: Width of road 14 m Mounting height 8 m Lamp size 7000 lumen Luminare type II.

Design: Height and Overhang of Mounting

• Mounting height affects the illumination intensity, uniformity of brightness, area covered, and glare
• Higher mounting height, greater coverage, more uniformity, reduction of glare but, a lower illumination level
• Typical mounting height ranges from 6m to 15 m, consider minimum vertical clearance above pavement is 6 m

• Overhand (Mast arms) support the luminaries at a lateral dimension from the pole. OVERHANG B C
• Overhang is usually 2m, 3m, and 4 m depending on pole height, should not be more than 1.8 m for height upto 9 m. A Height of Mounting C' B' A' C' B' Length of Shadow

Design: Lateral Placement

• Depends on geometry, characteristics of roadway, environment, available maintenance, economics, and aesthetics

• Road with kerb – min. 0.3 m from edge
• Road without kerb – min. 1.5 m from the edge and min. 5 m from center line of road

Design: Lighting Layout

• Single side – suitable for narrow road upto 2 lanes, spacing 30 to 60 m
• Both side (Opposite or Staggered ) – used for wider road with 3 or more lanes
• Central - used for wider road with 3 or more lanes

Tasks

• Review importance of road lighting, factors influencing night visibility, requirements of level of illumination in roads
• Review design of the lighting system: choice of lamp, luminaire distribution of light, spacing of lighting units, height and overhang of mounting, lateral placement, lighting layout

End of Chapter 2 — Traffic Control Methods