Traffic Engineering is that phase of engineering which deals with the planning, geometric design and traffic operations of roads, streets, and highways, their networks, terminals, abutting lands and relationships with other modes of transportation for the achievement of safe, efficient, and convenient movement of persons and goods. Traffic Engineering applies engineering principles to help solve transportation problems, and brings into play knowledge of psychology and habits of users of the transportation systems.

(Arizona Department of Transportation early 1970’s)

There are many people who still to this day have queries as to why a traffic problem is so difficult to understand and resolve and that why an engineer would be needed to be called upon once a situation has occurred. Many questions have been typically answered by saying installing a traffic signal, lower or increase the speed limits or install more signs. Whenever excessive traffic controls are installs, extreme traffic conditions usually result. The issues of traffic have come down to travel costs, travel times, waiting times, benefits associated with the destination. Therefore strong economy would result in more activity which in its case will then result in more travel and the more travel there is the higher the traffic.

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The traffic growth can be related to many factors these include things such as the increase of car ownership or the increase in commuting, other things include road improvement and poor public transport.

The history has proven to us that safety goes hand-in-hand with a smoother running traffic operation and if this smooth running traffic was to be unsettled there will no doubt be more accidents. Unpredictable traffic operation may be caused by vehicles slowing down on the roadways, for example low speed limits and too many road designs. Driving at a lower speed doesn’t necessarily mean safer traffic operation in fact it can sometimes cause other road users to get angry and do more dangerous manoeuvre. Traffic control devices are all signs, signals and devices placed on streets and highways by a public body having power to guide traffic.

The traffic within this country and around the world may be predicted as in the near future it is obvious more cars are likely to be used, therefore due to increasing traffic around specific areas, mostly in the south east. The objective is to describe the characteristics of road traffic flow, growth and formulate systems for safety, traffic management and appraisal within an area in Kingston and to Model highway junctions using current standards and the computer program PICARDY. The group has been asked to focus on the junction for Portsmouth Road with Surbiton Road by designing a roundabout in order to improve the traffic flow at the junction by using AutoCAD to map out the layout of the roundabout this must be scaled.

Traffic survey diagram: Successful

Figure 1: Traffic survey of Kingston

Figure 2: Portsmouth road junction connected to Surbiton road.

Traffic management:

Within traffic management there are many measures these include capacity enhancement, accident reduction, environmental protection, the providing of assistants to pedestrians and cyclists, providing a service for those with disabilities and regulating on and off parking.

(Slinn, Guest and Matthews, 2005)

the amount of roads in the UK have risen up in recent times which shows that there is more car users and more cars in this country and according to some experts this will only rise more and more with time, let’s have no doubt this is a major problem for this country as its this problem which will bring the traffic engineer face to face with an ever increasing population and cars being more affordable, even for low income households. One of the major problems along with maintaining the infrastructure in the United Kingdom is trying to avoid congestion building up during peak hours.

Over the past few years we have seen more and more roundabouts this due to the fact that this is a more popular alternative to the junctions, roundabouts often help prevent long queues and help traffic run smoothly. The way the roundabout works is quite simple the priority always lies with people on your right hand side so way must be given to these people, On entering the roundabout, the vehicle circulates the middle of the roundabout in a clockwise manner.

These days the most common roundabout is mini roundabout as it does not require a lot of space and a 7m diameter max is needed. This allows vehicles to remain at a reasonable speed whilst entering and exiting the roundabout. Certain measures can be used to avoid accidents which include Deflection islands and nearside kerb build-outs. These speed reduction measures still enable LGV, buses and large vehicles to use the roundabout.

The Roundabout:

Having researched into many roundabouts to get a general idea of what design and features would need to be included into this design. The junction being focused on is Portsmouth road and Surbiton road as shown in Figure 4 below. For this scenario a mini roundabout must be used taking into account the measurements and signs that would need to be put up.

Figure 4: Portsmouth road and Surbiton road

A mini-roundabout is a type or form of junction control at which vehicles circulate around a white, reflectorised1, central circular road marking (central island) of between one and four metres in diameter, as shown in Figure 5

Figure 5: mini-roundabout

The central road marking is either flush or slightly raised as a dome (no more than 125mm), in order that it can be driven over by larger vehicles that are physically incapable of manoeuvring around it. The dome is also raised to discourage vehicles from driving over the central island. Three white arrows are painted on the carriageway, within the gyratory area, around the central road marking, showing the direction of circulation.

A blue mini roundabout sign as shown in figure 6 is indicated clearly before the drivers reach the junction normally at 50m .These roundabouts are used for four main reasons which are:

1. To improve the traffic of a previous junction.

2. Accident remedial measure

3. Calming down traffic

4. Provide access to a new development

Figure 6: blue sign

Mini roundabouts were developed to improve the safety at junctions but are now included in new development proposals. Mini roundabouts are normally called upon when a junction experiences problems with safety or side road delays. These roundabouts can be used to break up long sections of roads or a deviation of route. Mini roundabouts are often thought as another junction due to highway space because they are alleged to be less costly.

All this has shown this information has been sufficient enough and has shown us that this is the exact roundabout need to for three way roads at Portsmouth and Surbiton road junction, it also gives us a clear idea of how the roundabout needs to be constructed.


Data Input:

Run Title: Roundabout Non failure

Modelling parameters:

Junction type:

The junction type arms need to be defined so for Arm A: Portsmouth road S side, Arm B: Surbiton Road, Arm c: Portsmouth Road N side. The junction standard should be selected as Urban.

Principal Options:

Data for Major Arm – measured:

Traffic survey:

The traffic survey was carried out on the 13th October 2010. Students were divided into groups in which traffic surveys had to be carried out over the four different junctions shown in Figure 1.

The task was to count the number of vehicles which passed through the junction and tally them by using the five bar gate counting technique to mark each passing unit. There are two different types of car units in which cars and light good vehicles are counted as one PCU with vehicles with more than two axils and above as well as buses are noted as two PCU. The data was obtained through an hour of six ten minute time periods.

Once values were gained and noted they were then entered into PICADY and the results were analysed.

The values which we had entered mean that the road will be successful as the RCF values are below 0.85. Once above the desired 0.85 limit the junction will fail in which alteration or improvements to the road need to be considered

The values that had been entered would mean that the road will be successful as the RCF values are below 0.85.

Traffic survey failure diagram

As the aim of this assignment is to design roundabout for a failed junction values for the arms were increased by factors of two or three. The steps had been carried out the same way except the values in ‘Demand data’ had been changed to become a failed junction. This script shows that the values are above 0.85 and the stars in the RFC column shows the max number of cars that will build up at the junction at the most congested time.

Failure is shown below:






08.40 0.1

08.50 0.1

09.00 0.2

09.10 0.3

09.20 0.7 *

09.30 0.4

09.40 0.2

09.50 0.1

10.00 0.1







08.40 0.4

08.50 0.4

09.00 0.8 *

09.10 1.8 **

09.20 3.0 ***

09.30 2.9 ***

09.40 1.0 *

09.50 0.4

10.00 0.4







08.40 0.1

08.50 0.1

09.00 0.2

09.10 0.3

09.20 0.3

09.30 0.3

09.40 0.2

09.50 0.1

10.00 0.1

Analysis of Results:

Having entered the values into PICADY programme the values of the junction showed that there would be no capacity failure and that the junction was performing sufficiently. As we are asked by this project to design a mini – roundabout for the junction, it was necessary to increase the traffic flow in order to fail the junction through the PICADY analysis. This was done by increasing the PCU numbers from Portsmouth road North to South which originally had the highest flow on the junction. By altering the PCU values and obtaining a failure at the junction it will then be possible to design a suitable calming solution.

Once analysing the PICADY results it was decided that we should be able to install a mini -roundabout at this junction as it is the best option to cope with the extra flow and will be the only traffic calming system which would be able to fit in the space available. As a standard roundabout would not be appropriate as the land around is not large enough for the design specification and to fulfil the design standards due to the proximity of the river and local resident housing.

With the implementations of the design a mini-roundabout has been proposed in this report as the junction will be future-proofed to deal with any increase in traffic flow over the next 10 years. By implementing these features at the junction as it is not at full capacity yet, a designer would have to deal with pre-empting potential problems and build up of traffic which is the result as flow increases and the junction reaches full capacity. This will cause less disruption for road users during the construction process and if the work is carried out when the PCU flow is low, then it would be more suitable for everyone concerned so the best possible time to start would be as soon as it is possible to do so.


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