turning roadways minimum edge-of-traveled-way designs effects of curb radii on turning paths the...

TURNING ROADWAYS

Minimum Edge-of-Traveled-Way Designs

Effects of Curb Radii on Turning Paths

The effect of curb radii on the right turning paths of various design vehicles turning through an angle of 90 degrees (on

streets without parking lanes) is shown in Exhibits 9-29 to 9-30.

Exhibit 9-29 shows the effects of a 4.5 m radius.P Ok without any encroachment on an adjacent laneSU and BUS swing wide on both streets WB-15 occupy an area wider than two lanes.

Exhibit 9-30 shows the effects of a 12-m curb radius.P, SU, and BUS ok without any encroachment on an adjacent laneW-15 needs to occupy two lanes.

TURNING ROADWAYS


Effects of Curb Radii on Turning Paths

Exhibit 9-31 shows the effects of the angle of intersection on turning paths of various design vehicles on streets without parking lanes.

With parking allowed along a curbed street, vehicles are able to turn without encroachment. At least 4.5 in advance of the right turning radius and 9 m beyond the radius on the exit should be restricted from parking.

BUS and WB-15 at least 7.5 m and parking is restrictedat the far end of the turn for at least 12 m beyond the radius.

TURNING ROADWAYS


Effects of Curb Radii on Pedestrians

Crosswalk distance,and right of way or corner setback needs increase with the curb return radius. see Exhibits 9-33 and 9-34

corner radii = f( ROW, angle of return, number of pedestrians, width and number of lanes, posted speeds)

Passenger vehiclesminor cross streets with few trucks/ 4.5 – 7.5 mmajor streets with parking lanes

minor cross streets 7.5 m

Minor cross streets with occasional trucks without too much encroachment 9 m

TURNING ROADWAYS


Effects of Curb Radii on Pedestrians

Truck combinations or buses turn frequently 12 m

TURNING ROADWAYS


Corner Radii into Local Urban Streets

Corner radii in urban areas for turning movements may be smaller than those normally used in rural areas.

Corner radii = f (types and number of turning vehiclespedestrian volumes)

1.5 m to 9 m Most often 3-4.5 m

With curb parking lanes on both of the intersecting streets and parking restricted for some distance from the corner, curb radii 3-4.5 m.

ISLANDSAn island is a defined area between traffic lanes used for control of vehicle movements. It provides an area for pedestrian refuge and traffic control devices. It could be a raised curb or a pavement area marked out by paint.

Islands often guide traffic entering intersections through a definite path. This type of intersection is termed as an channelized intersection.

Islands serve functions of channeliztion, division and refuge and have the following detailed purposes:

Separation of conflictsControl of angle of conflictReduction in excessive pavement areasRegulation of traffic and indication of proper use of intersectionArrangement to favor a predominant turning movementProtection of pedestriansProtection and storage of turning and crossing vehiclesLocation of traffic control devices.

ISLANDS

Channelizing Islands

Channelizing islands control and direct traffic movements into the proper paths for their indented use. A common form is the corner triangular shape that separates right-turning traffic from through traffic.

The radius of curved portions of the island should be equal or exceed the minimum for the turning speeds expected. A few large islands

are preferable to a greater number of smaller islands.

See Exhibit 9-35.

Curbed islands generally should not be used in rural areas and at isolated locations unless the intersection is lighted and curbs are delineated.

ISLANDS

Division Islands

Divisional islands often are used on undivided highways at intersections. They alert the drivers to the crossroad ahead and regulate traffic through the intersection. They normally control left

turns at skewed intersections and at locations where separate channels are provided for right-turning traffic.

See Exhibit 9-36.

Refuge Islands

A refuge island for pedestrian is on e at or near a crossroad or bicycle path that aids and protects pedestrians and bicyclists who cross the roadway.

Raised curb corners and center channelizing or divisional islands are refuge islands.

ISLANDS

Refuge Islands

Size and location of refuge island =

f( location and width of crosswalk, location and size of transit loading zones, and wheelchair ramps)

A minimum of 1.8 m wide for bike use.

ISLANDS

Island Size and Designation

Island sizes and shapes vary materially from one intersection to another.

The smallest curbed corner island is an area of 5 m2 for urban and 7 m2 for rural intersections. 9 m2 is preferable for both.

Corner triangular islands should not be less than about 3.5 m and preferably 4.5 m on a side after the rounding of corners.

Elongated or divisional islands should be not less than 1m wide and 6-8 m long. Curbed divisional islands introduced at isolated

intersections on high-speed highways should be 30 m or more in length.

ISLANDS

Island Delineation and Approach Treatment

Delineation of small islands is effected primarily by curbs and curb-top reflectors.

See Exhibit 9-37 and 9-38 for combining curb corners and turning roadways.

The approach corner s rounded with a radius of 0.6 m to 1.5 m.

Exhibit 9-37 shows curbed corner island adjacent to through-traffic lanes on an urban street.

See Exhibit 9-39, 9-40.

TURNING ROADWAYS W/ CORNER ISLANDS

Where the inner edges of the traveled way for right turns are designed to accommodate semitrailer combinations or where the design permits passenger cars to turn at speeds of 15t km/h or more, a corner island is needed to form a separate turning roadway between two intersection legs.

Right-Angle turns with corner islands

The principal controls for the design of turning roadways are the alignment of the traveled way edge and the turning roadway width.

A turning roadway should be designed to provide at least the minimum size island and the minimum width of roadway.

See Exhibit 9-41.

TURNING ROADWAYS W/ CORNER ISLANDS

Oblique-Angle turns with corner islands

See Exhibit 9-42 for the minimum design dimensions for oblique angle turns.

Design Classifications

A – Primarily passenger cars; permits occasional design SU to turn with restricted clearances.

B - Provides adequately for SU; permits occasional WB-15 to turn with slight encroachment on adjacent traffic lanes.

C - Provides fully for WB-15.

FREE-FLOW TURNING ROADWAYS

The design of a free-flow alignment for right turn makes the turning roadways easy and smooth in operation. Compound curves preceded by a right-turn deceleration lane is needed.

See Exhibit 9-43.

SE FOR TURNING ROADWAYS

Maximum SE rates discussed in Chapter 3 for open highways also apply to turning roadways at intersections.

The free-flow of turning roadways is often of limited radii and length.

It is difficult in attaining SE without abrupt cross-slope change at turning roadway terminals

The rate of change in cross slope in the runoff section should be based on the maximum relative gradient listed in Exhibit 3-27 and Exhibit 9-44.


Development of SE at Turning Roadway Terminals

For design of a highway, the through traffic lanes may be considered fixed in profile and cross slope. As the exit curve diverges from the through traveled way, the curved or tangent edge of the widening section can only gradually vary in elevation from the edge of through lane. see Exhibits 9-45 to 9-48.

Exhibit 9-45 shows the variation in cross slope where a turning roadway leaves a through road that is on tangent

Exhibit 9-46 shows the cross slopes for the conditions where the through lanes and the turning roadway curve in the same direction.

Exhibit 9-47shows cross slopes for through lanes and the turning roadway in opposite direction

Exhibit 9-48 shows designs with a parallel speed-change lane.


Turn-Lane Cross Slope Rollover

The design control at the crossover line is the algebraic difference in cross slope rates of the two adjacent lanes.

The algebraic difference is the sum of cross slopes of the two adjacent lanes. A desirable maximum algebraic difference is 4-5%.

see Exhibit 9-49.


SE Transition and Gradeline Control

The attainment of superelevation over the gradually widening auxiliary lane and over the whole of the turning roadway terminals

should not be abrupt.

Method 1: The roadway width in transition area is variable, cross-slope change should be developed by using the method of traveled way edge change in grade with respect to the point of rotation for a full-width auxiliary lane.

Example: The limiting curve of a turning roadway has a radius of 70 m, the design speed is 50 km/h from Exhibit 3-41. The limiting SE rate would be 11% or less.


SE Transition and Gradeline Control

Method 2: A rate of change is the function of TW width and the change in grade of the ETW with respect to the point of roadway rotation.

Example: If the ETW grade change is 0.65% and the width being rotated is 3.6 m, the rate of change in cross slope is 5.41%

per 30 m length.

0.65% = 3.6 * rate in 1 meterRate in 1 m = 0.66%/3.6 = 0.1805%rate in 30 meters = 0.1805% * 30 = 5.41%.

If the cross slope on the through roadway in Exhibit 9-45 is 1% AB = BC = 15m, CD = DE = 7m

What is the cross slope at A, B, C, D, and E?

INTERSECTION SIGHT DISTANCE

Each intersection should have enough sight distances at all locations along each highway or street legs in order to avoid from conflicts and improve operational efficiency.

The driver of a vehicle approaching an intersection should have an unobstructed view of the entire intersection, including any traffic control devices and should have sufficient lengths along the intersecting highways to permit the driver to avoid potential collisions.

Sight distance is also needed at intersections to allow the drivers of stopped vehicles a sufficient view of the intersecting highway to decided whether to enter the intersecting highways or to cross it.


Sight Triangle

Specified areas along intersection approach legs and across their included corners should be clear of obstruction that might block a driver’s view of potentially conflicting vehicles.

There are two types of sight triangles: approach sight triangles and departure sight triangles.

see Exhibit 9-50 for clear sight triangles.

Approach Sight Triangles: At each quadrant of an intersection, a triangle area has its length of the legs to enable drivers to see

potential obstructions.

Departure Sight Distance: At each quadrant of an intersection approach controlled by stop or yield signs.


Sight Triangle

Identification of sight obstructions within sight triangles:

Any objects (such as buildings, parked vehicles, highway structures, roadside hardware, hedges, trees, bushes, un-mowed grass, tall crops, walls, fences, and the terrain itself) at a height above the elevation of the adjacent roadways within the sight triangle should be removed or lowered.

The driver’s eye height is 1080 mm and the object height is also 1080 mm.


Intersection Control

The recommended dimensions of the sight triangle vary with the type of traffic control use at an intersection.

Intersection controls:

CASE A - Intersection with no controlCASE B - Intersection with stop control on the minor road Case B1 Left turn from the minor road Case B2 Right turn from the minor road Case B3 Crossing maneuver from the minor roadCASE C Intersections with yield control on the minor road Case C1 Crossing maneuver from the minor road Case C2 Left or right turn from the minor roadCASE D Intersections with traffic signal controlCASE E Intersections with all-way stop controlCASE F Left turns from the major road



CASE A - Intersection with no control

Exhibit 9-51 shows the distance traveled by an approaching vehicle during perception reaction and braking time as a function of design speed of the roadway on which the intersecting approach is located.

For approach grades greater than 3%, multiply the sight distance values in Exhibit 9-51 by the appropriate adjustment factor from Exhibit 9-53.

If the design speed is not known, use 85th percentile of the midblock running speed.

No departure sight triangle is needed at an uncontrolled intersection because such intersections typically have very low traffic volumes



CASE B - Intersection with Stop control on the minor road

Departure sight triangles for intersections with stop control on the minor road should be considered.

Case B1 Left Turn from the Minor Road

a = 4.4 m + ½ W Vehicle coming from lefta = 4.4 +1.5 W Vehicle coming from rightW = lane width

b = ISD = 0.278 Vmajor tg




Example: a passenger car turning left onto a two-lane major road is considered in intersection sight distance. Assume Design speed of the major road is 90 km/h, design the sight triangle.

a = 4.4 m + ½ W Vehicle coming from left = 4.4 + 3.6/2 = 6.2 ma = 4.4 +1.5 W Vehicle coming from right = 4.4 + 3.6 * 1.5 = 9.8 mW = lane width


= 0.278 * 90 * 7.5 = 187.7 or 190 m




Example: a passenger car turning left onto a four lane undivided major road is considered in intersection sight distance. Assume Design speed of the major road is 90 km/h, design the sight triangle.

a = 4.4 m + 1½ W Vehicle coming from left = 4.4 + 1.5 *3.6 = 9.8 ma = 4.4 +2.5 W Vehicle coming from right = 4.4 + 3.6 * 2.5 = 13.4 mW = lane width


= 0.278 * 90 * 8.0 = 200 m



CASE B - Intersection with Stop control on the minor road (Right Turn from the Minor Road)

A departure sight triangle for traffic approaching from the left should be provided for right turns from the minor road onto the major road.

The time gap can be decreased 1s for right turn maneuvers without undue interference with minor road traffic. See Exhibit 9-57



CASE B - Intersection with Stop control on the minor road (Crossing Maneuver from the Minor Road)

The departure sight triangles for left and right turns onto the major road will also provide more than adequate sight distance for minor road

vehicles to cross the major road.

Check is needed when 1) no left or right turns permitted, 2) crossing more than 6 lanes, or 3) heavy vehicles crossing the intersection.

See Exhibit 9-57 for the gap time tg



CASE C – Intersection with Yield Control on the Minor Road

Drivers approaching yield signs are permitted to enter or cross the major road without stopping. Therefore the sight distances needed by drivers on yield controlled approaches exceed those for stopped approaches.

Case C1: Crossing maneuver from the minor road

Dimension A is given in Exhibit 9-60

Dimension B is calculated using

gmajor

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278.0

167.0 min



Case C2: Left and Right Turn Maneuvers

Dimension A is 25 m

Dimension B is calculated using

Time Gap is given in Exhibit 9-63.Exhibit 9-64 for passengersExhibit 9-65 for other design vehicles



Case D: Intersections with Traffic Signal Control

At signalized intersection, the first vehicle stopped on one approach should be visible to the driver of the first vehicle stopped on each of the other approaches.

No needs for departure and approach sight triangles unless the signals are in the flash mode.



Case E: Intersections with All-Way Stop Control

At signalized intersection, the first vehicle stopped on one approach should be visible to the driver of the first vehicle stopped on each of the other approaches.

No needs for departure and approach sight triangles.



Case F: Left Turns From the Major Road

Time Gap for Case F is given in Exhibit 9-66Exhibit 9-67 for passenger carsExhibit 9-68 for other design vehicles.



Effect of Skew

When two highways intersect at an angle less than 60 degree,s, and when realignment to increase the angle of intersection is not

justified, intersection sight distance should be checked.

The sight distance criteria for Case A no b applied to oblique-angle intersections and the sight distance at least equal to those for Case b should be provided whenever practical.

w2 = w1/sin() for case C1.


Stopping Sight Distance at Intersections for Turning Roadways

Stopping sight distance is provided in Exhibit 9-70.SSD should be available at all locations along a turning roadway. It controls the vertical and horizontal alignments.

Vertical Control:

Exhibits 3-75 and 3-76 apply to the case here.

Horizontal Control:

The lateral clearance, centerline of inside lane to sight obstruction, for various radii and design speeds is

shown in Exhibit 3-57.

DISCOURAGE WRONG-WAY ENTRY

An inherent problem of interchanges is the possibility of a driver entering one of the exit terminals from the crossroad and proceeding along the major highway in the wrong direction in spite of signing.

Provision of a median as a deterrent to wrong-way movement (see Exhibit 9-72) is a very effective treatment. The median makes the left-turn movement onto the exit ramp terminal very difficult.

Additional techniques are providing for all movements to and from the freeway to reduce intentional wrong-way entry, using conventional easily recognized interchange patterns to reduce drive confusion and hence wrong-entry and narrowing the arterial highway median opening to prevent LT movements on freeway off-ramps.

INTERSECTION DESIGN

Intersection design is a process to consider the design-hour traffic volume, character or composition of traffic and design speed.

When design an intersection, LT traffic should be removed from the through lanes, whenever practical. Exhibit 9-75 shows a guide to traffic volume where LT should be considered on two-lane highways.

SPEED CHANGE LANES AT INTERSECTIONS

Drivers leaving a highway at an intersection are usually required to reduce speed before turning. Drivers entering a highway from a turning roadway accelerate until the desired open-road speed is reached.

A speed change lane is an auxiliary lane, including taper areas primarily for the acceleration or deceleration of vehicles entering or leaving the through traffic lanes.

MEDIAN OPENINGS

General Design Consideration

Elements: The median width for intersection conditions, the location and length of the opening, and the design of the median end. They are often developed in combination to fit the character and volume of through and turning traffic.

Conditions: Spacing of opening should be consistent with access management classifications or criteria.

Where the traffic pattern at an intersection shows that nearly all traffic travels through on the divided highway and the volume is well below capacity, a median opening is sufficient.

Design: based on traffic volumes, urban/rural area characteristics, and type of turning vehicles.

MEDIAN OPENINGS

Control Radii fro Minimum Turning Paths

The path of each design vehicle making a minimum left turn at 15 to 25 km/h is critical to the design of median openings.

The difference between the minimum turning radii for left turns and those for right turns are small and are insignificant in highway design.

See Exhibit 9-76 for control radii at intersections for left turns.

Turning RadiiNormally, P and occasional SU 12 m

SU and occasional WB-12 15 mWB 12 and WB15 23 m

See Exhibit 9-77, 78, and 79.

MEDIAN OPENINGS

Shape of Median End

One form of a median end at an opening is a semicircle (for narrow medians). The design control radii for the median ends (with semicircle) are 12, 15, 23, 30 m.

Another form is a bullet nose form. The bullet nose form has two symmetrical portions of control radius arcs and an assumed

small radius on the median edge .

Normally the bullet nose form is better than the semicircular form because the driver of the LT vehicle can follow the path of the inner rear wheel and result in less intersection pavement and a shorter opening length.

Medians about 1.2 m or less, no difference between the two forms. Median width is 3.0 or more, the Bullet nose form is better.

MEDIAN OPENINGS

Minimum Length of Median Opening

The width of crossroad traveled way plus shoulders for any three or four leg intersection on a divided highway.

The width of the crossroad traveled way + median

MEDIAN OPENINGS

Median Openings based on control radii for design vehicles

Exhibit 9-78 shows that the minimum median openings based on a control radii of 12 m are not well suited for two-lane crossroads because trucks cannot turn left without difficult maneuvering and encroachment in median ends or outer shoulders.

Exhibit 9-79 indicates that minimum lengths of median openings based on a control radius of 15 meters are suitable for truck

operations except that WB 15 will encroach on adjacent lanes.

Exhibit 9-82 shows the use of the control radius of 23 m or 30 meters can accommodate a WB 12 and WB 15 respectively.

MEDIAN OPENINGS

Effect of Skew

The skew angle of intersection causes the increase of the length of median openings.

Exhibit 9-84 shows the relationships between skew and median opening for different control radius.

In general, median openings longer than 25 m should be avoided regardless of skew.

see Exhibit 9-86.

MEDIAN OPENINGS

Above-Minimum Designs for Direct Left Turns

Median openings that enable vehicles to turn on minimum paths, and at 15-25 km/h are adequate for intersections where traffic for the most part proceeds straight through the intersection.

When turning speeds are above that for the minimum vehicle paths, median openings may be designed to have above-minimum control radii and bullet node median ends.

see Exhibit 9-87 for R, R1, R2

Exhibit 9-87 also shows the minimum radius for turning speeds of 30, 40, and 50 km/h.

INDIRECT LT and U Turns

When medians are narrow,

Exhibits 9-88 and 9-89 offer two options with respect to indirect LT and indirect U turn movements.

Exhibit 9-90 shows a design that provides for indirect LT to be made from the right, via separated turning roadways connected to a crossroad. This design is undesirable because the vehicles must pass through the intersection twice and create delays by reducing speed in turning right.

Using local streets to make indirect left turn or U turn. There are three options available: Around-the-Clock, median, openings for the

individual properties, and jug-handle type ramp or at grade intersecting loop.

When medians are wide,

Exhibits 9-91 shows the indirect LT through a crossover.

DESIGN OF U TURN MEDIAN OPENINGS

Median openings designed to accommodate vehicles making U-turns only are needed on some divided highways in addition to openings provided for cross and left turning movements.

These median openings should be located at:

1) locations beyond intersections to accommodate minor turning movements not otherwise provided in the intersection or interchange areas

2) Locations ahead of an intersection to accommodate U-turn movements interfering with through and other turning movements at the intersection.

3) Locations where traffic is not permitted to cross the major highways but is required to turn right.

4) Locations where at regularly spaced openings and at optimum spacing.

DESIGN OF U TURN MEDIAN OPENINGS

Design of a U-Turn median opening should provide the width of the highway including median sufficient to allow the design vehicle to turn from auxiliary LT lane in the median to the lane next to the outside shoulder or outside curb and gutter on the roadway of the opposing traffic lanes.

Median of 5.0 and 15 m or wider are needed to permit P and SU to have a U turn.

Normally U-turns should not be permitted from the through lanes. See Exhibit 9-92.

FLASH OR TRAVERSABLE MEDIANS

Paved flush or traversable-type medians 3.0 to 4.8 m wide may be the optimum type of design for LT vehicles in commercial and industrial areas.

See Exhibit 9-94

TWLT should be used only in an urban setting with low speed and no more than two through lanes in each direction.

AUXILIARY LANES

Auxiliary lanes are often used preceding median openings and at intersections preceding right-turning movements and/or after completing a RT movement to provide for acceleration, maneuvering and weaving.

Auxiliary lanes should be 3 m wide at least and desirably 3. 6 m.

It has three components: entering taper, deceleration length, and storage length.

Deceleration 70, 100, 130, 165, 205 mLength: 50, 60, 70, 80, 90 km/h on grade less than 3%

Storage Length: number of turning vehicles likely to arrive in an average 2 minutes period within the peak hour at unsignalized

intersections.1.5 –2 * number of vehicles arrived in a cycle.

AUXILIARY LANES

Taper: a Taper rate of 8:1 to 15:1 is used on high-speed highways

Short tapers are preferred for deceleration lanes at urban intersections because of slow speeds during peak

periods. normally 30 m for single-lane turn and 45 for dual turn

lane for urban streets.

see Exhibit 9-95.

Median LT Combining these three components, designers can designLanes median left turning lanes. A median LT lane is shown in

Exhibit 9-96.

Median End The curbed nose is offset from the opposing TH traffic Treatment lane 0.6 m. See Exhibit 9-97.

AUXILIARY LANES

Offset LT For median wider than 5.4 m, offset is desirable.Lanes Offset is good because of:

1) better visibility of opposing TH traffic2) decreased possibility of conflict between opposing LT

movement3) more LT vehicles served in a given period of time

see Exhibit 9-98.

Simultaneous Simultaneous LTs may be considered at an intersection of Left Turns two major highways.

INTER. DESIGN W/ FRONTAGE ROADS

Frontage roads along with major arterials or freeways intersect major highways within a limited ROW.

Outer separation decides the intersection design with frontage road. The outer separation should be 50 m or more in width at the intersection. When outer separation is 100 m it allows for overlapping left-turn lanes.

See Exhibit 9-100

DRIVEWAYS

Driveways are in fact intersections and should be designed consistent with their intended use.

Driveways should not be located within the functional area of an intersection or in the influence area of an adjacent driveway.

The impact lengths associated with motorists entering or leaving a driveway should be considered in establishing driveway separation distance. See Exhibit 9-101

RAILROAD-HIGHWAY GRADE CROSSING

A railroad-highway crossing involves either a separation of grades or a crossing at grade.

Horizontal The highway should intersect the tracks at a right Alignment angle with no nearby intersections or driveways.

crossings should not be located on either highway or railroad curves.

When highways are parallel to railroads, enough distances should be provided between the highways and railroads. If the distances cannot be provided,

Interconnect highway traffic signals with grade crossing signals

Place a “Do Not Stop on Track” sign


Vertical The intersection of highway and railroad should be as Alignment level as practical. Vertical curves should be of

sufficient length to ensure an adequate view of the crossing.

To prevent drivers of low clearance vehicles from becoming caught on the trucks, the crossing surface should be at the same plane as the top of rails for a distance of o.6 m outside of the rail.

The surface of the highway should also not be more than 75 mm higher or lower than the top of nearest rail at a point of 9 m from the rail unless track SE makes a different level. See Exhibit 9-102.


Signs, pavement markings, flashing light signals, and automatic gates are the primary traffic control devices for highway-railroad grade crossings.

Sight distance is the primary consideration at crossings without trai-activated warning devices.

Sight distances should be provided for the following two events:

1) Vehicle driver can observe approaching train in a sight line that allows the vehicle to pass thought the crossing prior to the train’s arrival at the crossing

2) Vehicle driver can observe approaching train in sight line that permits the vehicle to stop prior to encroachment in the crossing area.


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turning roadways minimum edge-of-traveled-way designs effects of curb radii on turning paths the...

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