Highway Planning

Highway planning

Introduction

The process of transportation planning entails developing a transportation plan for an urban region. It is an ongoing process that seeks to address the transport needs of the inhabitants of the area, and with the aid of a process of consultation with all relevant groups, strives to identify and implement an appropriate plan to meet these needs.

The process takes place at a number of levels. At an administrative/political level, a transportation policy is formulated and politicians must decide on the general location of the transport corridors/networks to be prioritised for development, on the level of funding to be allocated to the different schemes and on the mode or modes of transport to be used within them.

Below this level, professional planners and engineers undertake a process to define in some detail the corridors/networks that comprise each of the given systems selected for development at the higher political level. This is the level at which what is commonly termed a ‘transportation study’ takes place. It defines the links and networks and involves forecasting future population and economic growth, predicting the level of potential movement within the area and describing both the physical nature and modal mix of the system required to cope with the region’s transport needs, be they road, rail, cycling or pedestrian-based. The methodologies for estimating the distribution of traffic over a transport network.

At the lowest planning level, each project within a given system is defined in detail in terms of its physical extent and layout. In the case of road schemes,these functions are the remit of the design engineer, usually employed by the roads authority within which the project is located. This area of highway engineering.

The remainder of this chapter concentrates on systems planning process, in particular the travel data required to initiate the process, the future planning strategy assumed for the region which will dictate the nature and extent of the network derived, a general outline of the content of the transportation study itself and a description of the decision procedure which guides the transport planners through the systems process.

Travel data

The planning process commences with the collection of historical traffic data covering the geographical area of interest. Growth levels in past years act as a strong indicator regarding the volumes one can expect over the chosen future time, be it 15, 20 or 30 years. If these figures indicate the need for new/upgraded transportation facilities, the process then begins of considering what type of transportation scheme or suite of schemes is most appropriate, together with the scale and location of the scheme or group of schemes in question.

The demand for highway schemes stems from the requirements of people to travel from one location to another in order to perform the activities that make up their everyday lives. The level of this demand for travel depends on a number of factors:

The location of people’s work, shopping and leisure facilities relative to their homes

The type of transport available to those making the journey

The demographic and socio-economic characteristics of the population in question.

Characteristics such as population size and structure, number of cars owned per household and income of the main economic earner within each household tend to be the demographic/socio-economic characteristics having the most direct effect on traffic demand. These act together in a complex manner to influence the demand for highway space.

As an example of the relationship between these characteristics and the change in traffic demand, let us examine Dublin City’s measured growth in peak travel demand over the past ten years together with the levels predicted for the next ten, using figures supplied by the Dublin Transport Office (DTO) in 2000. shows that between 1991 and 1999 peak hour demand grew by 65%.

It has been predicted by DTO that between 1999 and 2016 a further 72.4% of growth will take place.

The cause of these substantial increases can be seen when one examines the main factors influencing traffic growth – population, number of cars per household and economic growth. Between 1991 and 1999, the population within the area increased by just over 8%, and car ownership by 38.5%, with gross domestic product increasing to 179% of its 1991 value. DTO have predicted that,between 1999 and 2016, population will increase by 20% and car ownership by 40%, with gross domestic product increasing to 260% of its 1991 value.

The significant growth indicated in consistent with the past recorded and future predicted traffic demand figures given in. High levels of residential and employment growth will inevitably result in increased traffic demand as more people link up to greater employment opportunities, with the higher levels of prosperity being reflected in higher levels of car ownership. Increasing numbers of jobs, homes, shopping facilities and schools will inevitably increase the demand for traffic movement both within and between centres of population.

On the assumption that a road scheme is selected to cater for this increased future demand, the design process requires that the traffic volumes for some year in the future, termed the design year, can be estimated. (The design year is generally taken as 10–15 years after the highway has commenced operation.)

The basic building block of this process is the current level of traffic using the section of highway at present. Onto this figure must be added an estimate for the normal traffic growth, i.e. that which is due to the year-on-year annual increases in the number of vehicles using the highway between now and the design year. shows the increase in vehicle trips predicted within the Dublin Region for the first 16 years of the new millennium. Onto these two constituents of traffic volume must be added generated traffic – those extra trips brought about directly from the construction of the new road. Computation of these three components enables the design-year volume of traffic to be estimated for the proposed highway. Within the design process, the design volume will determine directly the width of the travelled pavement required to deal with the estimated traffic levels efficiently and effectively.

Highway planning strategies

When the highway planning process takes place within a large urban area and other transport options such as rail and cycling may be under consideration alongside car based ones, the procedure can become quite complex and the workload involved in data collection can become immense. In such circumstances, before a comprehensive study can be undertaken, one of a number of broad strategy options must be chosen:

The land use transportation approach

The demand management approach

The car-centred approach

The public transport-centred approach.

Land use transportation approach

Within this method, the management of land use planning is seen as the solution to controlling the demand for transport. The growing trend where many commuters live in suburbs of a major conurbation or in small satellite towns while working within or near the city centre has resulted in many using their private car for their journey to work. This has led to congestion on the roads and the need for both increased road space and the introduction of major public transport improvements. Land use strategies such as the location of employment opportunities close to large residential areas and actively limiting urban sprawl which tends to increase the dependency of commuters on the private car, are all viable land use control mechanisms.

The demand management approach

The demand management approach entails planning for the future by managing demand more effectively on the existing road network rather than constructing new road links. Demand management measures include the tolling of heavily trafficked sections of highway, possibly at peak times only, and car pooling, where high occupancy rates within the cars of commuters is achieved voluntarily either by the commuters themselves, in order to save money, or by employers in order to meet some target stipulated by the planning authority.Use of car pooling can be promoted by allowing private cars with multiple occupants to use bus-lanes during peak hour travel or by allowing them reduced parking charges at their destination.

The car-centred approach

The car-centered approach has been favored by a number of large cities within the US, most notably Los Angeles. It seeks to cater for future increases in traffic demand through the construction of bigger and better roads, be they inter-urban or intra-urban links. Such an approach usually involves prioritizing the development of road linkages both within and between the major urban centers.

Measures such as in-car information for drivers regarding points of congestion along their intended route and the installation of state-of-the-art traffic control technology at all junctions, help maximize usage along the available road space.

The public transport-centered approach

In the public transport-centered approach the strategy will emphasize the importance of bus and rail-based improvements as the preferred way of coping with increased transport demand. Supporters of this approach point to the environmental and social advantages of such a strategy, reducing noise and air pollution and increasing efficiency in the use of fossil fuels while also making transport available to those who cannot afford to run a car. However, the success of such a strategy depends on the ability of transport planners to induce increasing numbers of private car users to change their mode of travel during peak hours to public transport. This will minimise highway congestion as the number of peak hour journeys increase over the years. Such a result will only be achieved if the public transport service provided is clean, comfortable, regular and affordable.

Transportation studies

Whatever the nature of the proposed highway system under consideration, be it a new motorway to link two cities or a network of highway improvements within an urban centre, and whatever planning strategy the decision-makers are adopting (assuming that the strategy involves, to some extent, the construction of new/upgraded roadways), a study must be carried out to determine the necessity or appropriateness of the proposal. This process will tend to be divided into two subsections:

A transportation survey to establish trip-making patterns

The production and use of mathematical models both to predict future transport requirements and to evaluate alternative highway proposals.

Transportation survey

Initially, the responsible transport planners decide on the physical boundary within which the study will take place. Most transport surveys have at their basis the land-use activities within the study area and involve making an inventory of the existing pattern of trip making, together with consideration of the socioeconomic factors that affect travel patterns. Travel patterns are determined by compiling a profile of the origin and destination (OD) of all journeys made within the study area, together with the mode of travel and the purpose of each journey. For those journeys originating within the study area, household surveys are used to obtain the OD information. These can be done with or without an interviewer assisting. In the case of the former, termed a personal interview survey, an interviewer records answers provided by the respondent. With the latter, termed a self-completion survey, the respondent completes a questionnaire without the assistance of an interviewer, with the usual format involving the questionnaire being delivered/mailed out to the respondent who then mails it back/has it collected when all questions have been answered.

For those trips originating outside the study area, traversing its external ‘cordon’ and ending within the study area, the OD information is obtained by interviewing trip makers as they pass through the ‘cordon’ at the boundary of the study area. These are termed intercept surveys where people are intercepted in the course of their journey and asked where their trip started and where it will finish.

A transportation survey should also gather information on the adequacy of existing infrastructure, the land use activities within the study area and details on the socio-economic classification of its inhabitants. Traffic volumes along the existing road network together with journey speeds, the percentage of heavy goods vehicles using it and estimates of vehicle occupancy rates are usually required. For each designated zone within the study area, office and factory floor areas and employment figures will indicate existing levels of industrial/commercial activity, while census information and recommendations on housing densities will indicate population size. Some form of personal household-based survey will be required within each zone to determine household incomes and their effect on the frequency of trips made and the mode of travel used.

Production and use of mathematical models

At this point, having gathered all the necessary information, models are developed to translate the information on existing travel patterns and land-use profiles into a profile of future transport requirements for the study area. The four stages in constructing a transportation model are trip generation, trip distribution, modal split and traffic assignment. The first stage estimates the number of trips generated by each zone based on the nature and level of land-use activity within it. The second distributes these trips among all possible destinations, thus establishing a pattern of trip making between each of the zones. The mode of travel used by each trip maker to complete their journey is then determined and finally the actual route within the network taken by the trip maker in each case.

Each of these four stages is described in detail in the next chapter. Together they form the process of transportation demand analysis which plays a central role within highway engineering. It attempts to describe and explain both existing and future travel behaviour in an attempt to predict demand for both car-based and other forms of transportation modes.

 

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