Road junctions and intersections
A road junction is the point at which one road meets another; an intersection
is the point at which two or more roads cross еach other. Both junctions and
intersections are the worst danger spots in a road system.
The problems of reducing danger at these points are those of cost and space.
If junctions and intersections are such that all classes of traffic meet each other at
the same level, there is a danger of collision. Almost complete segregation of different classes can be achieved, and the need for users of the same class to
cross traffic streams can be avoided. The perfect example of complete segregation of different classes of traffic and of the avoidance of crossing traffic streams is the clover-leaf junction, at which no collision can occur between vehicles.
All forms of road junction can be classified into three groups: multi-level junctions, roundabouts and flyover-junctions. Multi-level junctions. The clover-leaf, the most typical of these, has already been mentioned. There is need for multi-level intersections where three conditions are fulfilled:
- only a small percentage of the traffic must turn to left or right
- the major volume of traffic is travelling on a fast through route
- the volume of traffic would otherwise be sufficient to justify the provision of
a roundabout.
Roundabouts. Unlike multi-level intersections, roundabouts do not enable
traffic to cross without dropping speed but pedestrians and cyclists cannot be
segregated unless costly over- or under-passes are constructed.
The success of a roundabout depends greatly upon the ease with which vehicles using it can “weave” or pass from one traffic lane to another. The greater the length of the road in which the weaving can be carried out and the smaller the angle of approach of converging streams of traffic, the more easily can weaving be
performed. The angle should not be greater than 30 degrees. The greater the diameter of the island, the smaller the angle of convergence.
1. What is the difference between a road junction and intersection?
2. In what case is there a danger of collision?
3. What is the perfect example of complete segregation?
4. What are the main groups of road junctions?
5. What conditions should be fulfilled for multi-level junctions?
Road signs
Traffic signs or road signs are signs erected at the side of or above roads to give instructions or provide information to road users. The earliest road signs were milestones, giving distance or direction; for example, the Romans erected stone columns throughout their empire giving the distance to Rome. In the Middle Ages, multidirectional signs at intersections became common, giving directions to cities and towns. The first modern road signs erected on a wide scale were designed for riders of high or “ordinary” bicycles in the late 1870s and early 1880s. With traffic volumes increasing since the 1930s, many countries have adopted pictorial signs or otherwise simplified and standardized their signs to overcome language barriers, and enhance traffic safety. Such pictorial signs use symbols in place of words and are usually based on international protocols. Such signs were first developed in Europe, and have been adopted by most countries to varying degrees. Pre-industrial signs were stone or wood. In the late 18th and 19th centuries painted cast iron became popular. Since 1945 most signs have been made from sheet aluminium with adhesive plastic coatings, these are normally retroreflective for night-time and low-light visibility. Before the development of reflective plastics, reflectivity was provided by glass reflectors set into the lettering and symbols.
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New generations of traffic signs based on electronic displays can also change their text (or, in some countries, symbols) to provide for “intelligent control” linked to automated traffic sensors. Traffic signs can be grouped into several types. For example, Annexe 1 of the Vienna Convention on Road Signs and Signals (1968), which on 30 June 2004 had 52 signatory countries, defines eight categories of signs:
A. Danger warning signs
B. Priority signs
C. Prohibitory or restrictive signs
D. Mandatory signs
E. Special regulation signs
F. Information, facilities, or service signs
G. Direction, position, or indication signs
H. Additional panels
Answer the following questions
1. What is a road sign?
2. What were early road signs made of?
3. When did multidirectional signs become common?
4. Why did pictorial signs appear?
Road surface markings
Road surface marking is a kind of device or
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material used on a road surface in order to convey
official information. They can also be applied to mark
parking spaces or areas for other uses.
Road surface markings provide guidance and
information to drivers and pedestrians. Uniformity of
the markings is an important factor in minimizing confusion about their meaning.
However, countries and areas categorize and specify road surface markings in
different ways.
Road surface markings can be mechanical, non-mechanical, or temporary.
Mechanical devices may be raised into the road surface. They are either
reflective or non-reflective. Most are permanent; some are movable.
Cat’s eyes equip most major routes in the British Isles. They consist of four
reflective lenses mounted in durable white
rubber housing, two facing fore and two
facing aft. The lenses are available in a
variety of different colours, mainly white,
yellow, orange, green, red and blue.
Botts’ dots are round non-reflective
raised pavement markers named after
California Department of Transportation engineer
Elbert Botts, who invented the epoxy that keeps
them glued down. Generally they are used to
mark the edges of traffic lanes often together with
reflective raised pavement markers. They are used
only in warm climates since snow plows usually
remove them along with the snow.
A rumble strip is usually either applied in the direction of travel along an
edge- or centerline to alert drivers when they drift from their lane, or in a series
across the direction of travel to warn drivers of a stop ahead or nearby danger spot.
Answer the following questions.
1. What is the function of road surface marking?
2. Why is uniformity of markings important?
3. What types of markings are distinguished?
Arch types
Arch bridge is one of the most popular types of bridges, which came into use
over 3000 years ago and remained in height of popularity until industrial
revolution and invention of advanced materials enabled architect to create other
modern bridge designs. However, even today arch bridges remain in use, and with
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the help of modern materials arches can be build on much larger scales. The basic principle of arch bridge is its curved design. Abutments carry the load of the bridge and are responsible for holding the arch in the unmoving position. Also the type of connections used at the supports and the midpoint of the arch maybe used – counting the number of hinges which allow the structure to respond to stresses and loads. Arch configuration is another method of classification. Examples of tied
arch, trussed arch and spandrel-braced arches are shown. A tied arch is commonly
constructed using curved girder sections. A trussed arch has a curved through truss
rising above the deck. A spandrel-braced arch carries the deck on top of the arch.
Arch bridges rely on vertical members to convey the load carried by the arch.
Answer the following questions.
1. When did arch bridges appear?
2. Are arch bridges used today?
3. What is the main principle of arch bridges?
4. What do hinges allow?
5. What is the function of an abutment?
Suspension types
The longest bridges in the world are suspension bridges or their cousins, the
cable-stayed bridge. The deck is hung from suspenders of wire rope, eye bars or
other materials. Materials for the other parts also vary: piers may be steel or
masonry; the deck may be made of girders or trussed. The cables that go from the top of the towers down to the ground are the backstays. The backstays are connected to big rock or concrete piers buried in the ground. The backstays keep the towers from bending in.
Suspension bridges are very light. This allows them to span very long
distances. The longest suspension bridge in the world is the Askashi Kaikyo
Bridge in Japan. In addition to the long span, this bridge was designed to resist
huge earthquakes and hurricane force winds.
Some bridges have in the past suffered from structural failure. This may be
combination of poor design and severe weather conditions.
When it was opened in 1940, the Tacoma Narrows Bridge was the third
longest suspension bridge in the world. It later became known as “Galloping
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Gertie”, due to the fact that it moved not only from side to side but up and down in
the wind. Attempts were made to stabilize the structure with cables, but they were
unsuccessful.
Eventually on November 7, 1940, only four months after it was built the
bridge collapsed in a wind of 42 mph. The bridge was designed to withstand winds
of up to 120 mph. Some experts have blamed the collapse of the bridge upon a
phenomenon called resonance.
Today all new bridges prototypes have to be tested in a wind tunnel before being constructed. The Tacoma Narrows bridge was rebuilt in 1949.
Answer the following questions.
1. What are the main materials for piers and deck?
2. What is a backstay?
3. What is the longest suspension bridge?
4. Why was the Tacoma Narrows Bridge called “Galloping Gertie”?
5. Why did the Tacoma Narrows Bridge collapse according to some experts?
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