PRESENTATION GROUP 2

TOPIC:

ROAD JUNCTIONS TYPES AND ROUNABOUT DESIGN

CONTENT

I. Road junctions

1.Introduction

2.Types of Road junctions

3.Design of Road Junctions

4.Roles of Road Junctions

II.Roundabouts

1.Introduction

2.Types of Roundabouts

3.Design of Roundabouts

4.Roles of Roundabouts

I. Road junctions 1.Introduction

• In engineering road design we usually need road junctions.

• A Road junction is a place where two or more roads or railroad lines meet.

• It is also a location where vehicular traffic going in different directions can proceed in a controlled manner.

• In some cases, vehicles can change between different routes or directions of travel

2.Types of Road junctions

Two types of road junction are:

1) Road interchange

2) Road intersection

Interchange vs IntersectionInterchange Intersection

1) Road interchange• In the field of road transport, an interchange is a

road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream.

• Interchanges are junctions where roads pass above or below one another, preventing a single point of conflict by utilising grade separation And slip roads.

2) Road intersection

• These intersections do not use grade separation (they are at-grade) and roads cross directly.

• Forms of these junction types include:priority junctions, junctions controlled by traffic lights or

signals. Traffic circles or Roundabouts

Classification of road junctions based on shapes

A) 3-way juntion (3-way intersection) A1) T-junction

• T-junction has three arms of which one arm is a major road whereas the other one is usually a minor road connecting the main road.

• The roads of a T-junction meet at right angles but does not cross it.

A2) Y-junctions

• A 3-way junction (or 3-way intersection) is a type of road intersection with three arms.

• A Y junction (or Y intersection) generally has 3 arms of equal size.

• It is generally seen at places of heavy traffic. It is pretty useful in the distribution of traffic.

B) Acute Angle junction

• Acute Angle Junctions are not preferably used. They create lot of chaos in heavy traffic. The turning for heavy and bigger vehicles becomes a problem. This creates chaos and traffic.

• Acute Angle Junctions can be used at places with very low density and very less space for a Junction.

C) Staggered Junction

A staggered junction is a place where several roads meet a main road at a slight distance apart thus they do not all come together at the same point.

• Major single-point traffic-light controlled junctions of more than two roads are often reconfigured into staggered junctions in order to increase traffic flow through them.

d) Multiple Junction

When multiple roads meet at a same point, the junction is termed as “Multiple Junction”

Classification of roads junctions based on priority junctions

• A priority intersection occurs between two roads, one termed the ‘major’ road and the other the ‘minor’ road.

• The major road is the one assigned a permanent priority of traffic movement over that of the minor road. The minor road mustgive priority to the major road with traffic from it only entering the major road when appropriate gaps appear.

Three forms of priority junctions

• A simple T-junctions,• staggered junc-tions • crossroads. Though the last form should be avoided

where possible as drivers exiting the minor road can misunderstand the traffic priorities. This may lead to increased accidents

junctions controlled by traffic lights or signals.

3.Design of Road Junctions

Road Junctions is a crucial subject.Understanding the nature of traffic, the kind ofarea, density of population etc is very importantso as to propose a suitable road Junction design.

4.Roles of Road Junctions

• Road Junctions are designed at places where vehicular traffic can move in different directions in a systematic way.

• Road Junctions decrease the probability of accidents.

• The traffic at Road junctions is controlled to proceed further in their respective directions with the help of traffic signals.

II. ROUNDABOUTS

1.Introduction

A roundabout is a type of circular intersection  or junction in which road traffic is slowed and flows almost continuously in one direction around a central island to several exits onto the various intersecting roads.

MOVEMENT OF VEHICLES IN ROUNDABOUT

2.Types of Roundabouts

There are two basic types of roundabouts:

Conventional roundaboutA one-way circular roadway around a curbed central island for circulating traffic, usually with flared approaches to allow multiple vehicle entry.Mini-roundaboutA one-way circular roadway around a flush or slightly raised central island of up to 13 feet in diameter, usually without flared entries.

ROUNDABOUT

• A place where three or more roads join and traffic must go around a circular area in the middle, rather than a straight across.

ROUNDABOUT DESIGN

Roundabouts are intersections which have a single way circulatory carriageway around the circular area in the technical term called (central island).Vehicles on the circulatory carriageway are very important over those when reaching the roundabout. In addition, there are some types of roundabouts such as; Mini,Compact, Normal, Grade Separated, Signalized and Double Roundabouts.

Cont,

Therefore, roundabouts and its improvements are needed in country and all over the world to minimize the vehicles delay, reduce the probability of accidents afterwards keeping in exisistance the safe passage of all road users through the intersection(junction).

TYPES OF ROUNDABOUT1. Mini roundabout: Are roundabout that exist at the

smaller junctions to avoid the use of signals, stop signs, or

the necessity to give way in favor of one road of traffic and

which characterized by a small diameter and traversable

Mini-roundabouts can be a painted circle or a low dome

but must be fully traversable. Painted roundabouts and low

domes can easily be driven over by most vehicles, which

many motorists will do when there is no other traffic, but

the practice is dangerous if other cars are present and once

the practice is established it may be difficult to discourage.

• Mini-roundabouts work in the same way as larger roundabouts in terms of right of way, but can give different performance with regard to driver behaviour. Mini-roundabouts are sometimes grouped in pairs (a double mini-roundabout) or in "chains", making navigation of otherwise awkward junctions easier. In some countries there are different road signs used to distinguish mini-roundabouts from larger ones.

PICTURES OF JUNCTIONS

2.Compact roundabout

• Like mini-roundabouts, urban compact roundabouts are intended to be pedestrianans bicyclist-friendly because their perpendicular approach legs require very low vehicle speeds to make a distinct right turn into and out of the circulatory roadway.

2.Compact roundabout

• All legs have single-lane entries. However, the urban compact treatment meets all the design requirements of effective roundabouts. The principal objective of this design is to enable pedestrians to have safe and effective use of the intersection.

2.Compact roundabout

• Capacity should not be a critical issue for this type of roundabout to be considered.

• The geometric design includes raised splitter islands that incorporate at-grade pedestrian storage areas, and a nonmountable central island. There is usually an apron surrounding the nonmountable part of the compact central island to accommodate large vehicles

2.Compact roundabout

• The recommended design of these roundabouts is similar to those in Germany and other northern European countries. Exhibit 1-9 provides an example of a typical urban compact roundabout

Examples of roundabouts

Examples of roundabouts

3.Urban single lane roundabout

• This type of roundabout is characterized as having a single lane entry at all legs and one circulatory lane. Exhibit 1-10 provides an example of a typical urban single-lane roundabout.

• They are distinguished from urban compact roundabouts by their larger inscribed circle diameters and more tangential entries and exits, resulting in higher capacities.

Their design allows slightly higher speeds at the entry, on the circulatory roadway, and at the exit. Notwithstanding the larger inscribed circle diameters than compact roundabouts, the speed ranges recommended in this guide are somewhat lower than those used in other countries, in order to enhance safety for bicycles and pedestrians. The roundabout design is focused on achieving consistent entering and circulating vehicle speeds.

• The geometric design includes raised splitter islands, a no mountable central island, and preferably, no apron.

4.Urban double lane roundabout • Urban double-lane roundabouts include all

roundabouts in urban areas that have at least one entry with two lanes. They include roundabouts with entries on one or more approaches that flare from one to two lanes. These require wider circulatory roadways to accommodate more than one vehicle traveling side by Exhibit 1-11 side. provides an example of a typical urban multilane roundabout. The speeds at the entry, on the circulatory roadway, and at the exit are similar to those for the urban single-lane roundabouts. Again, it is important that the vehicular speeds be consistent throughout the roundabout.

4.Urban double lane roundabout

• The geometric design will include raised splitter islands, no truck apron, a nonmountable central island, and appropriate horizontal deflection.

• Alternate routes may be provided for bicyclists who choose to bypass the roundabout.

• Bicycle and pedestrian pathways must be clearly delineated with sidewalk construction and landscaping to direct users to the appropriate crossing locations and alignment.

• Urban double-lane roundabouts located in areas with high pedestrian or bicycle volumes may have special design recommendations such as those provided in Chapters 6 and 7.

Pictures

5.Rural single lane roundabout

• Rural single-lane roundabouts generally have high average approach speeds in the range of 80 to 100 km/h (50 to 60 mph). They require supplementary geometric and traffic control device treatments on approaches to encourage drivers to slow to an appropriate speed before entering the roundabout.

• Rural roundabouts may have larger diameters than urban roundabouts to allow slightly higher speeds at the entries, on the circulatory roadway, and at the exits. This is possible if few pedestrians are expected at these intersections, currently and in future. There is preferably no apron because their larger diameters should accommodate larger vehicles.

• Supplemental geometric design elements include extended and raised splitter islands, a no mountable central island, and adequate horizontal deflection. The design of these roundabouts is based primarily on the methods used by Australia,France, and the United Kingdom. Exhibit 1-12 provides an example of a typical rural single-lane roundabout.

• Rural roundabouts that may one day become part of an urbanized area should be designed as urban roundabouts, with slower speeds and pedestrian treatments. However, in the interim, they should be designed with supplementary approach and entry features to achieve safe speed reduction.

Pictures

3.Design of Roundabouts

4.Roles of Roundabouts