According to Rogers (2018), the functional effectiveness of a highway is measured in terms of its ability to assist and accommodate the flow of vehicles with both safety and efficiency. In order to measure its level of effectiveness, certain parameters associated with the highway must be measured and analysed. These properties include:
- The quantity of traffic
- The type of vehicles within the traffic stream
- The distribution of flow over a period of time (usually 24 hours), q
- The average speed of the traffic stream, u
- The density of the traffic flow, k
Among the list above, the last three parameters are usually function of the first two and they are interrelated in the expression below;
q = uk
Analysis of these parameters will directly influence the scale and layout of the proposed highway, together with the type and quantity of materials used in its construction.
Nigeria Highway Manual Part 1-Volume 1 recommends a structural design period (SDP) of not more than 20 years for flexible pavement design because of difficulty of projecting traffic over a period that long. Table 1 shows different flexible pavement categories in Nigeria and recommended structural design periods.
Estimation of Design Traffic Volumes
Estimation of deign traffic volumes begins with the knowledge of traffic flow information. The important traffic flow information are outlined below:
Hourly traffic (HT): the total number of vehicles passing a particular point on the road in one hour.
Daily traffic (DT): the total no of vehicles passing a point on the road in twenty-four (24) hours.
Average daily traffic (ADT): an average of daily traffic passing a point on the road over the duration of at least seven (7) days.
Monthly average daily traffic (MADT): an average of the number of vehicles passing a point on the road each day for a month.
Annual average daily traffic (AADT): an average of the number of vehicles passing a point on the road each day for a year.
To obtain HT, DT, ADT and AADT, traffic count must be done either manually or with automatic traffic counters or video monitoring. For design purposes, 7 days, 24 hours continuous automatic count are required to be carried out along the road at strategic points where traffic volumes change. The 7-day count usually gives the ADT. Where automatic count is not possible, manual count can be taken for 12 to 14 hours each day (depending on when the traffic peaks occur) and then associated with 24 hours counts for the 7 days to obtain design traffic.
The count should classify the traffic stream, if possible. The classification of the vehicle stream enable the road capacities and road pavement strengths to be determined accurately.
Factors that affect Road Capacity
- Road configuration e.g. two-lane two way, multi-lane divided or undivided;
- Operating speed;
- Lane and shoulder width;
- Traffic composition;
- Direction distribution of traffic flows; and
- Passing opportunities – sight distance, overtaking lanes, climbing lanes or slow vehicle turnout lanes
To determine this stream the classes of vehicles below (Table 2) should be incorporated in the count. The basic minimum classification is to determine the percentage of heavy vehicles in the ADT.
Traffic counts ideally provides the hourly flow and ADT for 7 days. The flows should be analysed to obtain the ‘hourly observed traffic volume’, ‘the 30th-highest hour’ and the ‘100th-highest hour’ which are used in the design purposes. The 30th-highest hour’ flow is the flow which is exceeded in only 29 hours of the year while the ‘100th-highest hour’ flow is the flow which is exceeded in only 99 hours of the year. If hourly flows are ordered from highest to lowest, it is ideal to use the 30th –highest hour flow for design of rural roads and 100th-highest hour for design of urban roads. This is because seasonal peaks are more common in rural areas than urban areas.
To predict hourly flows, it is necessary to know the ADT and the peaking factor, β. The parameter, β is a descriptor of the traffic flow on a given road and depends on factors such as the percentage and incidence of seasonal traffic, the relative sizes of the daily peaks etc. The peaking factor can fluctuate between -0.1 and -0.4. The value of -0.1 indicates minimal seasonal peaking while the value of -0.4 suggests very high seasonal peaks. As a general rule, the value of -0.2 is used as typical value. The value of β is ideally used in Urban designs. Thus, the flow between the highest and the 1030th highest hour can be determined with the expression below;
QN = 0.072 ADT (N/1030)β
QN = two-directional flow in N-th hour of the year (veh/h)
ADT = average daily traffic (veh/day)
N = hour of the year
β = peaking factor
Note: 30th highest hour is usually taken as the design hour. If N is 30 and β is -0.2, then the value of QN would = 0.072 ADT (30/1030)-0.2 = 0.146 x ADT for the 30th highest hour.
Rogers, M. (2003). Highway Engineering. Blackwell Publishing Ltd Editorial Offices: 9600 Garsington Road, Oxford OX4 2DQ
Highway Manual Part 1: Design; Volume 1: Geometrics. 2013. Federal Republic of Nigeria.