At all intersections on undivided urban roads whe. Design Considerations The turning path -of a semi-trailer should be used for the. The vehicle executing the. Figure illustrates the esBential design features of right-turn lanes. The minimum length of a right turn lane shall be equal to the deceleration length for the particular approach speed. Where storage is required, the length should be increased according to the expected queue.
Storage length can be estimated as follows The following ' formula 'can be applied: L. Averag number of right. A right-t,urn lane shorter. As traffic will not maintain its highest volume at all times, even the! In this case, shortage in the length should be adjusted in the taper length with the storage length maintained as long as possible.
However, less than half the reccmmended lengths should not be used. The taper is normdly formed by a S-curve ccmposed two circle arcs. Where the right turn lane is obscured by a crest, it will be necessary t. For n""' intersect :.
Accuracy of the estimation cannot be satisfactory in most cases. New intersections, thnrefore, should be examined after openig? If two or more lanes are provided to cope with heavy right turnhJg traf::ic, storage length will be shortened to an ordinary diBtance divided by the number of the lanes. Width of Right Turn Lanes Right turn lanes uhall desirably shall not be less than 3. Seagull Island A seagull island is a triangular island used to separate right turning traffic frcm through traffic n the same carriageway as shown in Figure Adequate storage lnngth is required in.
Opposed Right-Turns. When two opposing single-lane right turns are expected to run simultaneously the turning radii and the tangent points should be such that. Central islands may ways Ghost island should be used where the island is of the width of or less tran the turn lane. It should also be used in rural intersect ions where there is no street lighting. Kerbed islands sf. Medians should also be kerbed on both sides from the start of the U.
These factors dete1mine the radius of the kerb and the width of the left-turn lane. There ate two types of treatment ifor'[,;. At urban intersections the radius of the kerb for the left-turn should bf, a minimum of 6m. While radii large1: than 10m increase the speed of turning movements they reduce the safety of pedestrian ' crossings and crE! For simple left turns in urban areas, such , radii should only be used after careful consideration of the above. Radii larger than 15m should not. Separate Left-Turn Lanes Where the volume o.
Radius for Se. Figure ,;i ves the relationship between these factors. The values of R 1 from the formula. The 'superelevation of curves on separate turning lanes ai: int.
The values of f given i. For R withi. A compound curve with successive radii 1. SR ,, R and 3R satifies this 1 1 requirement. For R more 1. In these cases the front wheels of the occasional semi-trailer can be steered wide enough to prevent the back wheels running over.
When a corner is land is to be introduced to create a separate left-turn lane and a th:cee-centred curve is justified, the combination of radius. Figure indicates the combination of radius and angle of turn which provides these minimum island areas. Single lane Uoo with provision for passing a stalied vehicle Width w. This width is to be 3 adopted where traffic volumes require two lanes and parking is prohibited.
Width W is carried for the whole length of the lefi turn lane. Design conditions which define the lane width of left-turn lane should be found in Table according However where traffic volumes are high greater lengths may be provided. Care must he exercised in designing diverging tapers to eruiure that through traffic is not led into an auxi:. Care must bl! Sufficient lengths of straight, horizontal.
Diverging and merging tapers should be designed to encourage low relative speed manoeuvres. Taper Le. Left-turn deceler. The canbined length sMuld be equal to the. Lengths of deceleration lanes are as shOW:l in Figure The ratio from F:lgure multipli. In, urban areas, :lt is desirable that traffic using the left-turn should flcm continuously. Figure illustrates these principles.
Accele ratj. When the volume of merging traffic :ls lao, or where traffic signals. Where the volume oJf merging traffic is high and signals are not provided, a driver reacl]. He shou. Ld therefore be able to continue on a route parallel til the throogh traffic until a merging opportunity occur!! In such cases, a length of parallel acceleration lane togetller with the merging taper Tm should be considered.
The combined length should be Bqual to the distance reqUired for a vehicle to accelerate fran the design speed of the left-turn to. Lengths of acceleration lane are"as'shown: in Figure If necessary a correction for, grade as shown in 'Fi';. Width of Au xi l i at:'r Lanes Widths of auxili. NOTE: Where! Ratio from this. Ve-hicle acceleratHI'I as required Ii. E Traffic islands are divi'sibnal or channelisation islands.
Visibility to approaching traffic, both day and night, is an essential factor in any iSland location. Only traffic islands will be ,considered here.
The following design aspects should'be considered shape; location and size of islands:. They should be located and designed so that the proper line of travel is obvious and any changes in direction are gradual and smooth. The approach end of any island should be offset from. This approach offset should be a miniinum of l.
The sides of islands should. For roads with design speeds exceeding f. Where pedestrian refuge is being provided, barrier kerb should be used. In urban areas, raised islands should 2e of an. Islands in rural areas should:. Where an island has to provide for stop lines, traffic signals and pedestrian crossings,' the side of the i.
Medians are used to separate opposing traffic streams, provide refuge for pedestrians and reduce the numbet; of points of eros sing conflict along a road, The fa! Sm ! Unless stopp:lng sight distance is available at its approach end, a median. Medians should not also begin on the arc or a horizontal curve but at or before the first tangent point or 30m or more beyond the second tangent point. A length of painted median should precede the approach end of the median so that the approaching driver will notice the obstruction ahead.
On high speed roads, any short length of kerbed median should be offset from the delineated through traffic lane by approximately O. Sm See F::gure Where the island is less than 1. Where a median would alter the number of lanes, the treatment to be adopted should follow that as sho. Median Openings Where openings an: provided in medians, the treatment of the median and ends should be in accordance with those shown in.! Jfllre depending on the width of the median.
Td Td. Tny : ''! Outer Separators are used to separate the through traffic lanes from service roads. They should be as wide as possible with a desirable width of S. The same applies where median is required.. The length of the uidening formula: L. The outer edges of the carriageway shall be widened over the same length as the central widening even if the requ. The widening of both inner and outer edges shall be carr:j. S or Reverse curves composing of two cl. S-curve may when the road is on a curve produce adverse curvatures, in which case the length of widening should be increased or an alternative curvature selected.
Figure shows aspects of widening of the major road by s-curves. Types of Treatment" Treatments on minor road for better traffic control benefits not only the minor road but also the major road. S of treatments description of which is given in Section 1. The type of the minor road treatments should be selected according to the class of the road and that of the major road to uhich i t is connected, as shown in Table Guide islands in the centre should be flared or higher treatments.
Guide Islands Guide islands are placed at the centre of the minor road at interseetions to define the movements of turning traffic and to control the speed of turning and crossing vehicles. They also provide space for traffic 'oncrol devices and refuge for pedestrians. Guide island shall be designed to the following:i. G cutting the nearczr edgt of the through lana and th. Ual to! R1 I rouOilad to wholcz m l hngrnt to offsido: through lan Draw circlcz..
Exfttn:l line! T'l'le curve should be perptndicular to the Kge: of the nearer througn lane. Establish line Q :erpendiculo. Draw circle Q with rlldius R1 [ roundec: to whole ml tangent to line or. Q 11nd the nt:arer edge of the: offside through lane. Draw circle with r! The kerbed part of ihe isl! Widening of the Hinor Road The width of the carriage;,ay shall remain unchanged up to the corners of 1:he intersection if no guide islands are present.
Where guide island,; are present, the entry lane shall have a minimum Nidth of 3,5m and the exit lane a minimum width of iSm past the island. Provision of ri ghttu rn lanes and examinatfon of the number needed are lSually emphasized on the major road. However, increasing the number of right turning lane on minor crossroad especially at signalised intersections also profits the m.
Right turning vehicles. The green time alloted to the crossroad can alllo be cut down. In this case, h wever, green time assigned to the crossroad should n t be shorter than 15 seconds and be sufficient for pedestrians who cross the major road in the same phase ,. An auxiliary lane reserved for left turning traffic may be added to the ap? The design of the left turn lanes guidelines set out in Section 3.
Shoulders Shoulder widths srtall in general remain unchanged in the intersection area but may be reduced to 2. In general kerbs should not be used along the outer carriageway edges,. In general, the erossfall of the through lanes of the major road sha:. Crossfalls on auxtliary lanes may follow the crossfall of the adjoining through lane or fall to the opposite side as is required by drainage or side friction criteria.
The a:. The crossfall of the minor road shall, towards the edge of the through lane be the same as the gradient of the through lane. Where the major road is on a steep grade, this may create an adverse camber for turning vehicles. In such a situation, diverging lanes should be considered. Superelevation oJ' corner lanes in connection with triangular islands on the minor road shall in general not exceed 6 percEmt.
Generally, it is the major intersections, signalised or not, which determine the overall capacity and performance of the road network. Significant volurr. The capad ty of intersections are very important and to achieve balance, the intersection design should take into account the capacity of the approach roads. The level of service concept is used in the capacity analysis of intersections. The required level of service to te used for intersections along the various categories of roads are as shown in Table General Capacity analysis are seldom required for rural intersections since their volumes are rarely sufficient to make capacity a design consideration.
Safety is normally the major consideration in rural situations, which may necessitate the provision of separate lanes for left or right turning vehicles. The method of capacity analysis as detailed below is therefor-e mor-e pertinent for urban intersections.
It is based on the Highway Capacity. The designer is advised to refer to the above publication for a better understanding of the subject. Determine the "conflicting traffic" through which each minor road movement, and the major road right turn, must cross. In urban aree. Regulatory' signs channeli sation.
Flared Intersection; A flared intersec:ion is a simple unchannelised intersect ion with addi tiona! Speed change lanes. Right turn lanes permit thrrugh vehicles to pass on the left side of another vehicle waiting to canplete a right turn at an intersect ion.
Raised traffic islands, raised markers and -painted matkings can be used for channelisation. At grade intersections present. Traffic An. The needs of commerdal vehicles 'should be considered. Consideration should also be given to operating speeds and turning path requirements at the. Human Factors In an intersection design, driver characteristics should be considered, i.
Variation just:. Points of Conflict The number of conflict points can be reduced prohibiting certain traffic movements and eliminating some roads from the intersection. Conflict po:l. Area of Conflict Where roads cross at an acute angle or the opposing legs of an intersection are offset, excessive intersection area results. In general, large areas of uncontrolled pavement invite dangerous vehicle manoeuvres and should be eliminated. Channelisation and realignment can both reduce conflict. Major Movements Preference 'Should be given..
Minor movements shcould be subordinated to majoror high speed''movements. Control of Speed The operat. Traffic Control ane. Geometric Design In intersection de:sign, the possible use of control devices and other road furniture should be considered. Most of the criteria. The design of an interEection to be controlled by signals can differ significantly from one requiring only channeli sat ion and signs.
Left turn lanes' at a sign ali s:ed intersect ion requires additional consideration, as queueing: vehicles on the most left lane waiting for the green signal would block the entrance to the le. This is much less significant in unsignalised intersections. Capacity The design must provide adequate traffic capacity thrrughout the expect. This may involve the design of separate construction stages before the ultimate development of the intersection is. Location of Intersect ion The efficiency of major roads, in.
Intersections should not be located at sharp horizontal curves, steep grades or at the top of crest vertical curves or at the. Future co-ordination of traffic signals should also be carefully considered in detennining intersection spacing. Table gives the desirable. The layout for a particular site depends on the traffic pattern; traffic volume; the area which is econcmically available for improvement; topography;. As well as separating channeli sat ion is used.. Excessive channeli sation Care should be taken to install only the minimum number of island as excessive channelisation can All intersecti.
In T-junctions and. Traffic The capacities of rrl. Detailed traffic forecasts for such intersections must be carried out in order to provide the necessary data for capac:i ty calculations. A detailed traff:lc forecast shall provide. A staged construc. Howiaver, the land requirenents must be sufficient for the full desi.
I1 the absence of any data, a value of o. Design Sr. Vehicles on the minor road can be assumed to approach the intersection at the design speed of the road and drivers should be able to perceive the intersection fran a di3tance not less than the stopping sight distance as. Design Vehicles The design of the '7ariouz intersection layouts should be. To Des [gn Of Roads". Table s haws a general scheme to :;elect the design ve.
P design This design is used at intersections where absolute minimum turns are stipulated such as at local street intersections, intersection of. For major highways with important turning movements which involves a large percentage of trucks, larger radii and speed change lanes should. WB design This design should be used where truck combinations will make turning novements repeatedly. Where designs for such vehicle are warranted, the simpler symmetrical arrangements of three-centred compound curves are preferred if smaller vehicles make uP.
It is also desirable to provide for channelisation to reduce the paved area. Select ion of Intersect ion! Where the. Hign Vthitles, The Uigher lltsi! Jl Ymide May Be Used. The fundamental factor. Table A shows the general acheme teo select the intersect ion type according to the traffic volume.
Other factors such as class of road, lanE! Roundabouts Roundabruts may bE! Moreover, they require large1: land space and capacity according to the dem,md of each approach cannot be realiably assigned, llhen tht!
As such, roundabouts cater well only for situation where the approaches have similar level of traffic flew. Roundabouts are not encruraged and should only be provided where theJ:e is problem in power supply to traffic signals, or where the number and layout of approach legs are not suitable for signal control. Signal controlled intersect ions are applicable to very high traffic volu! Traffic sig11als require reliable electricity supply for their operation, hence limiting their use only to developed areas.
Sign ali sed lnterse:t ions can handle heavy traffic with adequate number of approach lanes. This, however, requires longer clearance time for vehicles to cross the wide road, lE! They must be provided for all full access controlled roads and should be considered for road with design speeds excet! Grade separation is also reccmmended i. The design of interchanges is cove red in a separlte Arahan Teknik. Ccmbi nation and Coordination In Successive Intersect ions Minor roads at elose proximity creates They intersections on the major road.
Local serviC! Local streets should not be linked to the major road near najor intersections. If this is unavoidable, only left-turning movements should be allowed. Right-turns from the major road and from the croe sroad should be physically prevented with continucus kerbed median and remodeling the entrance to t. When a nf! Relocation of existing roads and systematj c traffic control may be required.
It is recognised, however,. In such cases the best possible sight distances and proper traffic control devi::es should be provided. Where roads intersect at angles less than 70 the alignment of the minor road should be modified. Staggered T-Junctions A four-way intersect ion has considerably more traffic conflict points than two three-way junctions and allows higher operating speeds on the minor road. Signalised four-way intersections especially in rural areas should generally be avoided or eliminated.
Two staggered T-junctions can take the place of one four-way intersection. However, where large volumes or crossing traf. Staggered T-junctions may either have a left-right or right-left configuration. The minimum desirable distances between staggered T-junctions are given in Figure 3-! Vertical Alignment It is desirable to avoid substantial grade changes at intersec. At all intersections where there are GIVE WAY signs, STOP signs or traf fie signals, the gradients of the intersecting highways should be as flat as practicable so that these sections can be used as storage space for vehicles stopping at the intersection.
Where conditions make such. These should be treated as s'pecial cases. A general principle is that the horizontal and vertical alignment of the major road as well as its superelevation or crossfall is unchanged thrcugh the intersection and that the carriageway of the minor road and of the additional lanes are designed to fit that of the major road. The grade shall also in general be connected taJllentially with or without a vertical curve to the cross-section of the major road.
General The operator of a vehicle approaching an intersection at grade should have an unol:s tructed viaY of the whole intersection and a length of the intersecting road sufficient: to pe tmi t control of the vehicle to avoid collision.
When traffic at the intersection is controlled by signals or signs, the unol:structed view may be confined to the area of control. As for the sight distance of the driver of a vehicle passing through ar.
There must be a sufficient unol:structed vi5'1 to recognize the traffic signs or traffic signals at the ictersect i. And there must also be a sufficient sight distance to make a safe departure after the vehicle. All intersections alsc must be either stop or sign:ai" controlled.
Sight Triar. ThiE: is known as the sight triangle and is shown en Figure Any object within the sight triangle high enough above the elevation of the adjacent roadways to consitute a sight obstruction should be removed or lowered. Such objects include cut slopes, trees, bushes and other erected objects.
This also requires the elimination of parking within the sight triangle. Dangerous conditions may arise if, despite the provision of sight triangle.
No Stop or Signal Control at Intersection For this set of conditions it is assumed that the oper-ator of a vehicle on either road must be able to see the intersection in sufficient time to stop his vehicle if necessary before reaching the intersection. The safe stopping distances for intersection design are the same as those used for the design of. Where an obstruction which cannot be removed, except at prohibitive cost fixes the vertices of the sight triangle at points that are less than the safe stopping distances from the intersection, signs showing the safe speed should be so located that the driver can slow down to a speed appropriate to the available sight distance.
The critical speed v of Vehicle B can then be 1 evaluated in terms of these known factors. Distance da is the minimum stopping distance for Vehicle A. When vehicle A is at a distance da from the intersection and the drivers of Vehicles A and B first sight each.
By sj. The signs on road B showing. For this casE! If the minor road SE! Signalised Intersection The sight distance is the sum of a di'stance travelled during the total reaction time, which is the tnterval between the instant that tht! The total reaction t'ime can further be dlvided into the time required to make decision whE! Hufficient data is not available on the total reaction time. For urban areas, however, shorter total reaction time is used. This is because, with a lot of inte1:sections in urban areas, drivers are always ope1:ating their vehicles with an anticipHion of possible encoonters of intersect iomo.
An acceleration of 0. Stop controlled Intersection In this case, time for decision making as in signalised i'tersection is not necessary because every driver must stop. On the major road, drivers can operate their vehicles without worrying about intersections. Stopping signt distance defined for open road is sufficient.
From the discussion above, the criteria shon in Table is obtained. See Figure The required sight distance along the major highway can be expressed as The term.
J represents the time necessary for the vehicle operator to look in both directions and to shift gears, if necessary, preparatory to starting. A value of 2 seconds is assumed. The time t required to cover a given distance during accelefation depends upon the vehicle acceleration. The value of t can be read directly from Figure for nearSy level conditions for a given distance S in feet. ALAN I In testing whether the sight distance along a major road is adequate a: an intersection the distance should be measured from a:l eye level of l.
For divided highways. Along a major road, the longer distance of the two: the sight distance de;cribed here. The former will exceed the latter at higher nnges of the des. The safe speed may be computed' for known sight distance and the w:i:dth of pavement on the path of the ,. Where the major road has dual carriageways with a central median width enough to shelter turning vehicles 4.
Effect of Skew When two roads intersect at an angle considerably less than. The difficulty in looking for approaching trafLcmakes it undesirable to treat the intersection based on the assumptions of no control intersections even where traffic on both roads is light. Treatment by controlled intersection or safe departure whichevHr is the larger should be used at skew intersections, In case of departure the distance S is larger for oblique than for right angle intersecti. The width of pavement on the path of the crossing.
Effect of Grades The differences :ln stopping distances on various grades at interseo! Grades on an intersection leg should be limited to l percent. In case of departure derivation of.
Normally the grade across. The effect of grade on acceleration can bo' expressed as a multiplic and to be used with the t:lme t as determined for level conditions for a glven dfstance as shown in Figure 3.
General Right turn lanes inprove capacity and safety and should be considered in t'1e following cases: a. At all intersections on undivided urban roads whe. Design Considerations The turning path -of a semi-trailer should be used for the. The vehicle executing the. Figure illustrates the esBential design features of right-turn lanes. The minimum length of a right turn lane shall be equal to the deceleration length for the particular approach speed.
Where storage is required, the length should be increased according to the expected queue. Storage length can be estimated as follows The following ' formula 'can be applied: L. Averag number of right. A right-t,urn lane shorter. As traffic will not maintain its highest volume at all times, even the! In this case, shortage in the length should be adjusted in the taper length with the storage length maintained as long as possible.
In this case, shortage in the length should be adjusted in the taper length with the storage :Length maintained as long as possible. However, less than half the recommended lengths should not be used. The taper is normally formed by a S-curve composed of two circle ,arcs. Where the! For new intersections, right turning traffic must be estimated by utilizing the information on land development projects and location of traffic generating facilities along the roads crossing.
Accuracy of the estimation cannot be satisfactory in most cases. New intersections therefore, should be examined after opening and the design should be refined for actual operating Conditions, as the storage length is most difficult to predict, at the time of original construction, it should be prepared for future refinament.
If two or more lanes are provided to cope with heavy right turning traffic, storage length will be shortened to an ordinary distance divided by the number of the lanes. A seagull island is a triangular island used to separate right turning traffic from through traffic n the same carriageway as shown in Figure Adequate storage length is required in approach to the island and a merging taper appropriate to the speed of the through carriageway must be provided on the departure side.
When two opposf. The minimum centra. Figure Central islands may be made in one of the following ways: a painted as crass hatched areas on the pavement ghost islands. Ghost island should be used where the island is of the width of orb less than the turn lane. It should also be used in rural intersections where there is no street lighting. Kerbed islands shall be used where the islands are wide. Medians should also be kerbed on both sides from the start of the taper of the right turning lane, or if no turning is present, then from the start of the larger of the two rounding curves at the central area of the intersection.
These factors determine the radius of the kerb and the width of the left-turn lane. These are usually provided where traffic volumes are low and where land acquisition costs prevent more extensive treatment or the angle of turn prohibits the installation of an island. At urban intersections the radius of the kerb for the left-turn should be a minimum of 6m. This allows most commercial vehicles to negotiate the turn at low speeds without encroaching either on the footway with the rear wheels or on the opposite side of the road's centre line with the front wheels.
While radii larger than 10m increase the speed of turning movements they reduce the safety of pedestrian crossings and create problems in locating signal pedestals and.
STOP lines. For simple left turns in urban areas, such radii should only be used after careful consideration of the above.
At rural intersections where provision for pedestrian is not a consideration, larger. Radii larger than 15m should not be used without left-turn island as they create large areas of uncontrolled pavement. Where the volume of left-turning traffic is high or the skew favours such a layout, a corner island can be introduced. Figure gives the relationship between these factors.
A desirable maximum value in rural areas is 0. In urban areas this should not exceed 0. The values of f given in. Figure are greater than those used for open highway design as drivers turnirig or1 curves of small radius at intersections accept a lower level of comfort.
For R1 within the range of m the turn should be designed to provide for tracking of the design vehicle. A compound curve with successive radii 1. For radii R1 between m the vehicle tracking can be accomodated by using a compound curve with successive radii 2R1, R1 and 2R1.
Figure illustrates the combination of: radii and widths required for the tracking of: the design vehicle. For R1 more than 45m the off-tracking is negligible and a single radius R1 is acceptable. Method of attainment of superelevation runoff for open road should basically be followed in the design of intersection. Compound curves are also unnecessary where there is a painted island or an island is either not required, or cannot be provided.
In these cases the front wheels of the occasional semi-trailer can be steered wide enough to prevent the back wheels running over the kerb or running onto the shoulder.
When a corner island is to be introduced to create a separate left-turn lane and a three-centred curve is justified, the combination of radius and angle of turn should provide minimum island area as follows: i. In urban areas, 8m2 for adequate definition of the island, shelter for pedestrians as well the posssible installation of traffic signals. In rural areas, 50 m2 for adequate definition of the island. Figure indicates the combination of radius and angle of turn which provides these minimum island ,areas.
There are three design conditions: 1 Single lane flow width W1. This is the normal application and is used in rural or semiurban locations where there is a shoulder on the inner edge of pavement. It may also be applied in urban areas where the inner edge of the lane is kerbed but the corner is small. This width is diserable for urban locations where parking is prohibited and the corner island has an inner edge longer than approximately 20m.
Width W3 is carried for the whole length of the left-turn lane. Design conditions which define the lane width of left-turn lane should be found in Table according to the class of road. The table in figure gives the required widths for various radii and design conditions. The are the general design principles on pavement tapers: a Pavement tapers for diverging movements should provide for a rate of lateral movement of 0. However where traffic volumes are high greater lenghts may be provided.
The minimum lengths of pavement taper for diverging and merging movements can be computed by the formula V x Yd Left-turn deceleration movements should be separated from the through traffic stream.
This may be done by providing in the left-turn approach a length of parallel by a diverge taper Td. The combined length should be.
Lengths of deceleration lanes are as shown in Figure The ratio from Figure multiplied by the length from Figure gives the length of deceleration lane on grade. In urban, areas, it is desirable that traffic using the left-turn should flow continuously. If calculation indicate that a queue would form at the STOP line, a length of parallel lane long; enough for the left-turn vehicles to by-pass the end of the queue should be provided.
Figure illustrates these principles. In urban areas where the through and left-turn movements are expected to flow concurrently, there should be art area which enables the two streams of traffic to merge at a small angle. When the volume of merging traffic it, low, or where traffic signals are installed, this may be provided by a merging taper of lengt h Tm at the exit of the left-turn. Where the volume of merging traffic is high and signals are not provided, a driver reaching the exit to the left-turn lane may not find any gap immediately available in the through traffic stream to permit merging.
He should therefore be able to continue on a route parallel to the through traffic until a merging opportunity occurs or until he adjusts his speed to create an opportunity to merge. In such cases, a length of parallel acceleration lane together with the merging taper Tin should be considered. The combined length should be equal to the distance required for a vehicle to accelerate from the design speed of the left-turn to the design speed of the through road.
Lengths of acceleration lane are as shown in Figure If necessary a correction for grade as shown in Figure Pedestrian island provide refuge for people waiting for public transport or crossing wide streets.
Traffic islands art: di visic. Visibility to approaching traffic, both day and night, is an esseritial factor in any island location. Only traffic islands will be considered here.
Where the length of deceleration lane shown is less than the standard taper Td , Td should not he reduced. Figure 3. Traffic islands are used to : a b c d e f g h separate opposing streams of traffic; guide traffic away from and past fixed obstructions and other hazardous points; reduce the Area of conflicts and control the angles at which conflicts occur; provide shelter for turning or crossing vehicles; prohibit undesirable or unnecessary traffic movements; control speed; separate through and turning movements as well as define their respective alignments; and provide for and protect traffic control devices.
Traffic islands may be defined by pavement markings, kerbs or a combination of these. Large islands in rural areas may be constructed without kerbs or with kerbs only at the points where separate roadways converge or diverge. The following design aspects should be considered for shape, location and size of islands: a They should be located and designed so that the proper line: of travel is obvious and any changes in direction are gradual and smooth.
This approach offset should be a minimum of: 1. The sides of islands should also be offset from adjacent traffic lanes by 0. Where pedestrian refuge is being provided, barrier kerb should be used. A smaller area may be adopted where traffic signals need protection. Islands in rural areas should desirably have a minimum area of 50m2. In rural. Instead pavement markings should be used.
Medians are used to separate opposing traffic streams, provide refuge for. Pedestrians and reduce the number of points of crossing conflict along a road. The following design aspects of medians should be considered : a The approach end of each median island should be set. Medians should not also begin on the arc or a horizontal curve but at or before the first tangent point or 30m or more beyond the second tangent..
On high speed roads, any short length of kerbed median should be offset from the delineated through traffic lane by approximately 0. If median is narrower than 2m, a length of barrier line may be used in the approach See Figure instead of the painted median. Where the island is less than 1. If ii is not possible, the end of the median should be terminated at the pedestrian crossings. Where openings are provided in medians, the treatment of the median and ends should be in accordance with those shown in Figure depending on the width of the median.
Outer Separators ax-e used to separate the through traffic lanes from is;ervice roads. They should be as wide as possible with a desirable width of 5. Treatment for outer separator openings are as shown in Figure 3. Widening of the riajor road to provide space for the central island should on a straight portion be made symmetrically around the centreline of the road and on a curve portion be made to the inside of the centreline.
The same applies where widening of a median is required. The outer edges of the. Widening even if the required widening is different from the central widening due to changes in lane width. The widening of both inner and outer edges shall be carried out to a smooth continuous alignment composed of the usual.
S or Reverse curves composing of two circle arcs will in most cases provide a curvature which has acceptable dynamic and optical properties and is recommended. S-curve may when the road is on a curve produce adverse curvatures, in which case the length of widening should be increased or an alternative curvature selected. Figure shows aspects of widening of the major road by S-curves. Quick, departure of traffic from the major road and smooth merging into it helps to maintain a smooth and safe traffic flow on the major road.
There are basically 3 degrees of treatments ; description of which is given irk Section 1. The type of the minor road treatments should be selected according to the class of the road and that of the major road to which it is connected, as shown in Table Guide islands in the centre should be provided for flared or higher treatments.
They also provide space for traffic control devices and refuge for pedestrians. Guide island. Warning or information signs can be placed if they do not affect the visibility of the vehicles.
Figures A and B gives the standard design of guide islands which are to be used. Where guide islands are present, the entry lane shall have a minimum width of 3. Provision of right-turn lanes and examinati'on of the number needed are usually emphasized on the major road. However, increasing the number of right turning lane on minor crossroad especially at signalised intersections also profits the major road. Right turning vehicles departing from two lanes can clear the intersection in a shorter time.
The green time alloted to the crossroad can also be cut down. A more favourable split of green time to the major road increases its capacity. This effect is more significant if the widening of the major road is costly and the crossroad is two lane.
Because, intersections are usually bottlenecks of any stretch of road, the benefit of any increased capacity extends to the whole stretch. This is more cost effective than widening of the major road. In this case, however, green time assigned to the crossroad should not be shorter than 15 seconds and be sufficient for pedestrians who cross the major road in the same phase.
The design of the left. The purpose of delineators is to outline the edge of the roadway and to indicate the roadway alignment. Closure Taper of equipment, workers, materials, and work ers vehicles a When an improved shoulder is closed on a high-speed roadway, it should be treated as a 2: If the temporary black symbol or word message and border on operations require measures different from orange backround unless specified or show to those normally in effect,the existing perma- be otherwise in the accompanying illustrations.
With a moving operation, exist, such as uneven pavements, vehicles the buffer space is the space between the loading or unloading.
This Arahan Teknik will be continually updated from time to time and in this respect any feedback from users will be most welcomed. As hese systems must be essentially unyielding, they are almost exclusively constructed of massive sections of concrete. Whenever a flagger is on duty, the advance 4. Departmental policy on guardrail installation has been included in this issue. The obstructed space available for lateral daily traffic volume.
A short taper is used to cause traffic control two-way traffic where traffic is to slow down by giving the appearance of required to alternately use a single lane restricted alignment. Published on Oct View 1. Repair or replace any damaged or missing devices. This material is applied hot and sets on laying.
Optimum highway, the vertical axis of the barrier should barrier system performance is provided by a be inclined in order to remain perpendicular to level surface in front of the barrier.
Traffic h Provide a buffer space between traffic and volume and length of time that the closure will workers. The centre lines on a undivided 2-way street with 4 or more lanes shall be a continuous double white lines, each mm wide and separated by a space of mm. Where barriers can cause errant vehicles to vault a barrier or to are installed on superelevated sections of strike it so that kalan vehicle overturns.
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