There is a significant difference between urban and rural geometry. The idea that a car takes
x metres to stop from y km/h, regardless of where it is, is perfectly true but misses the point
completely.
What is typically found in the rural environment is relatively low volumes of vehicles moving at high speed for considerable distances whereas the urban environment is charac- terised by high volumes moving at low speeds over relatively short distances. High speeds require high values of horizontal and vertical curvature and relatively flat gradients. In consequence, cuts and fills several metres high can occur in the rural environment. The lower speeds prevailing in urban areas mean that it is possible to get the vertical alignment closer to ground level. Ideally, the urban vertical alignment is just slightly below ground level to enable draining the surrounding properties towards the road reserve. Any significant departure from ground level also creates problems with regard to access between adjacent properties and the road reserve.
Intersections in the rural environment are spaced at anything up to kilometres apart and their effect on the smooth flow of traffic is minimal. Urban intersections are very closely spaced and could be as little as 100 m apart. Their effect on traffic flow is significant. In fact, it is the intersection that dictates the efficiency of the urban network. A single vehicle turning left at a rural intersection could, in all probability, execute the turn the moment the intersection is reached and following traffic – if any – would not even have to slow down to avoid the turning vehicle. On the other hand, the same single vehicle in an urban envi- ronment could cause a considerable backup queue to generate while awaiting a gap in the opposing traffic stream.
Rural intersections are typically no more than priority-controlled bell mouths between opposing two-lane two-way roads. Their urban counterparts could be multi-lane layouts with dedicated turning lanes, median and other islands, pedestrian cross walks and with a high level of sophistication brought to bear on their signalisation.
Competing activities, that is, activities other than vehicle movement that also require a share of the road space, are very low in the rural areas. Whether in a vehicle or on foot, anybody on the road is going somewhere. In the urban area on the other hand, there is a very high proportion of competing activities and modes of transport. It is this very diversity of activity that gives the urban environment its rich character. Ignoring it in the name of mobility will almost certainly result in the production of an environment that is dull, unin- teresting and, in fact, sterile.
In the rural environment, differences between the various vehicles to be found on the roads are, very generally, not critical. Cars, intercity buses and trucks all move at about the same pace and there is little impediment to overtaking. The only time that this does not apply is during holiday seasons when, on certain rural roads, volumes become so high that traffic is brought to a complete standstill. In the urban environment, simply because vehicles are so much closer together, physical dimensions start to play a role. An articulated vehicle takes up as much space as about three passenger cars. Its ability to accelerate from a stopped condition is significantly slower than that of a passenger car. A left turning bus or truck very
14 Geometric design of roads handbook
often requires the active assistance of opposing vehicles to complete the manoeuvre. The urban designer must be more sensitive to the operational differences between vehicles than his or her rural counterpart.
Congestion simply means that there are more vehicles on the road than it can comfortably accommodate. Historically, this problem was addressed by providing more infrastructure. In terms of the supply/demand equation, the focus was on supply. It is, however, also neces- sary to consider demand. For an unaltered number of person trips, vehicle trips could be slashed by replacing them with bus travel. One bus could take the place of a queue of pas- senger cars that is three-quarters of a kilometre long. It has often been said at high politi- cal levels that 80 per cent of passenger trips should be by bus and that this goal should be achieved in the very near future.
While both the percentage and the time frame could be queried, the urban geometric designer is required to produce street networks supportive of public transport. This does not only mean the provision of bus lanes and careful consideration of the location of bus stops. It also means that pedestrians should not have to walk too far to get to the nearest stop. Obviously, this is going to impact on the layout both of the residential area and the destina- tion area. Public transport requires high population densities to be economically viable. The alternative is for a bus to wind its way for several kilometres through a low-density residen- tial suburb to acquire its full load of passengers. This is another consideration that has to be brought to bear on the design of the residential area. Using the bus as the design vehicle for a certain road would also suggest that maximum gradients and minimum curve radii are going to become critical features of its location. The location and layout of termini will require careful attention. The rural designer is not beset by this problem.
In the rural environment, storm water drainage usually involves removing excess water from the road and onto the surrounding area as soon as possible and providing culverts or bridges to accommodate cross flows that are already concentrated in watercourses. In the urban environment, the road reserve is, in fact, the conduit for storm water being trans- ported from surrounding areas. However, because of the higher speeds found in rural areas, water on the road surface is potentially more hazardous and liable to cause aquaplaning than in urban areas, where water on the road surface tends more to be just a nuisance, although wet urban roads tend to produce large numbers of tail-end accidents. The approach to storm water drainage design differs totally between the rural and the urban environment. This includes, in the urban area, drawing a distinction between major and minor storms.
A further complication in urban design is that provision must be made for numerous other facilities, specifically facilities not usually found in rural areas. Sewerage, water and power reticulation, street lighting, telephone lines and, in older city areas, gas lines are usually located in the road reserve. Sewerage and storm water drains operate on gravity and they can have a serious impact on the location of the road reserve in which they are to be accommodated.
Land acquisition costs are typically 20 per cent of the total cost of provision of a rural road with construction accounting for the other 80 per cent. In the urban environment, the proportion is reversed in spite of the fact that construction is more expensive in urban than in rural areas. This is because land acquisition typically involves developed sites up to and including multi-storey business premises in addition to the fact that urban land prices even for undeveloped properties are significantly higher than in rural areas. The cost of construction in urban areas is also higher than in rural areas. This is because material that has to be dumped to spoil or imported for the design layers has to be carted for longer dis- tances than in the rural situation. Even if the haulage distances were similar, travel times are longer in the urban area. Construction sites are more constricted in the urban area and
What a geometric designer needs to know 15
provision for bypasses can be difficult. Unit prices thus tend to be higher for urban than for rural construction. However, this is not sufficient to change the percentage differences between costs of land acquisition and construction.
1.12 CONCLUSION
There are various definitions in the literature of the goals of transportation. The following is a fairly standard definition:
Transportation is directed towards the accessible, convenient and economical move- ment of people and goods with a minimum of environmental side effects and within a concern for the safety of road users, whether within or outside vehicles, and the safety of the community as a whole.
Very simply, the objective of geometric design is to give practical expression to these higher goals.
Traditionally, road users were tacitly understood to be people in cars and design addressed their needs largely to the exclusion of all else. It has now been realised that road users include everybody within and adjacent to the road reserve, regardless of why they are there. They may be in the road reserve as pedestrians or cyclists with a set destination in mind or simply walking, jogging or riding for exercise, out on the street taking a break from the office, meeting friends at sidewalk cafes or window shopping. The reasons for people being in the road reserve are probably endless and these reasons are going to dictate their behav- iour patterns while in the road reserve. There are also a considerable number of people who live or work in close proximity to the road and whose needs differ from those of the people actually within the road reserve itself. It is necessary for the geometric designer to achieve a design that effects an acceptable trade-off between all these conflicting demands and sup- ports the creation of a sense of place, specifically of one where people would wish to live and work and play.
The person at the drawing board must thus go far beyond knowledge and application of laid down geometric standards. An attitude of caring, encompassing the road user, the com- munity and the environment in which it lives, is required. This attitude finds expression, amongst other things, in
• The concepts of human factors, consistency of design and public involvement referred in Section 1.1
• The need for design to be by a team rather than as a solo effort
• Consideration of the framework of supporting disciplines and their involvement in underpinning successful design as illustrated in Figure 1.1
This does not mean that a bleeding heart is a fundamental requirement for good design. In reverting to the introductory comment on the objectives of transportation, the point is stressed that the designer must be a realist. In addition to an attitude of caring, designers must also bring a host of technical skills to bear on problem definition and solution in their chosen field.
The intention of this book is to set readers on the path towards acquiring those skills required to give sensible expression of a sound philosophy of design.
17
Chapter 2
Policy
2.1 INtrODUCtION
There seems to be a mystique attached to policy that is totally unjustified. In fact, a policy could simply be a desired end state, for example, ‘Our policy is that, in 2015, we want the situation in this Department to be positioned to…’. Alternatively, a policy could be a state- ment of intent. ‘We do not have sufficient to gravel to maintain our shoulders. It is our policy, therefore, to provide all roads with surfaced shoulders’. Finally, a policy could simply be a statement of preferences: ‘All our roads are going to have a design speed of 120 km/h’.
The formulation of policy leads into another activity, also seemingly shrouded in mystery. This is strategy development. In its simplest form, strategy is the enunciation of a series of actions aimed at moving from a current situation towards a desired end state. ‘This where we are now and this is what we must do to achieve the desired end state’.
According to the Concise Oxford English Dictionary, a policy is defined as ‘a course or principle of action adopted or proposed by an organisation or individual’. The defini- tion offered by The Merriam-Webster Dictionary is that it is ‘a definite course or method of action selected from among alternatives and in light of given conditions to guide and determine present and future decisions’. What these dictionaries are offering as definitions of policy are, in fact, definitions of strategy. It is presumed that they see strategy as part of policy. The common denominator of these definitions of policy is that they are all aimed at providing a uniform approach to problems that may be encountered in the future.