By Lusuga Kironde, 21st August 2010 @ 16:00, Total Comments: 2, Hits: 4886
Traffic congestion is currently at the tip of the tongue of everyone who lives in a large African city like Dar es Salaam. The never ending and growing traffic jams are providing excuses even for those who are well known to be late reporters for any appointment. Non time keepers become saints when they report at all. The general excuse is “foleni”, (traffic jam). However, there is a sense of desperation.
“When will traffic jams be brought to an end?” You find such a question in many newspapers these days. Politicians reply by promising to deliver an instant cure, usually in the form of large capital investment in highways, flyovers, railway lines, modern buses and so on, which require huge resources. Although we in Africa are currently faced with growing levels of traffic congestion, such has been a problem in developed countries as way back as the 1950s.
In the 1960s governments like that of the UK were setting up Committees to propose ways of dealing with congestion. Policy makers need to realize that the private vehicle is now with us and is here to stay, so policy undertakings need to take this fact into consideration. Thus it is sad to see new high rise developments coming up without any consideration for accommodating the car.
A good example is the new commercial property developments along Ohio Street in Dar es Salaam, where a new office block was constructed replacing a large car parking facility. The result has been cars of visitors to that block parking along a main route instantly creating congestion. There are many such construction, a number belonging to government agencies, where accommodating the car is ignored.
Future land use planning and property development undertakings must take into consideration the private car. Policy makers need to know the numerous causes of congestion and to be aware that congestion must be addressed on a multi-solution approach. The following is what has been identified by experts as what policy-makers should know about the causes of congestion.
The causes of congestion are numerous, e.g. too many vehicles for a given road’s design or intersection capacity at a particular time, dynamic changes in roadway capacity caused by lane-switching and car-following behaviour. They are also invariably linked to other indirect factors such as land-use patterns, property development patterns, employment patterns, income levels, car ownership trends, infrastructure investment, local and regional economic dynamics, and so on Generally, however, experts have identified two principal, broad categories of causal factors of congestion, that is: micro-level factors (e.g. those that relate to traffic “on the road”), and macro-level factors that relate to overall demand for road use.
In this context, congestion is “triggered” at the “micro” level (e.g. on the road), and “driven” at the “macro” level by factors that contribute to the incidence of congestion and its severity. This has important implication for policy since – while congestion takes place on the roads, it is not only, nor necessarily primarily, a traffic engineering problem. It is also useful to look at category as being either recurrent or non-recurrent.
Recurrent congestion is generally the consequence of factors that act regularly or periodically on the transportation system, such as daily journeys to and from work places, schools etc, or weekend trips (shopping or recreation). However, even recurrent
congestion can display a large degree of randomness, especially in its duration and severity.
Thus while recurrent traffic is in most cases predictable in terms of location and time, there could be cases where this is not possible. What is also clear from an examination of the causes of “recurrent” congestion across different types of road networks is the extreme vulnerability of traffic to sudden breakdowns as demand approaches the technical maximum throughput capacity on a link (junction) or in the network.
When roads are operated at or near their maximum capacity (saturation), small changes in available capacity due to such factors as differential vehicle speeds, lane changes, and acceleration and deceleration cycles can trigger a sudden switch from flowing to stop-and-go traffic.
Likewise, saturated intersections can quickly give rise to queues whose upstream propagation can, in turn, swamp local roads and intersections, creating an extended congestion.
Non-recurrent congestion is the effect of unexpected, unplanned or large events (e.g. road works, crashes, accidents, special events (including the passage of VIPs) and so on, that affect parts of the transportation system more or less randomly and, as such, cannot be easily predicted.
The share of non-recurrent congestion varies from road network to road network and is linked to the presence and effectiveness of incident response strategies, roadwork scheduling and prevailing atmospheric conditions (eg rain, local floods, etc.). While most non-recurrent incidents have the same negative impact on roadway performance, not all incidents are purely random nor are they equally difficult to plan for.
While most crashes are unpredictable by their very nature, accident–prone segments of the roadway can be identified via statistical analysis and specific geometric or other safety treatments applied. Likewise, roadworks (including street cleaning) can be managed in such a way as to minimise their impacts on traffic (e.g. by undertaking major road works at night).
Even weather, while impossible to change, can be better managed on the roads with active speed management and can be prepared-for with contingency planning that can lessen its impact on traffic. The specific mechanisms relating to the triggering of congestion are different according to different classes of roadways.
Congestion on uninterrupted flow facilities such as inter-city highways does not occur in the same manner nor for the same proximate causes as congestion arising on interrupted flow facilities such as those found in dense urban centres. One key relationship for policy-makers to keep in mind is the relationship between the release of existing capacity or the provision of new capacity - and the subsequent demand for use of that newly available capacity.
This relationship is captured in the price-elasticity of travel and has an impact in how quickly newly available capacity is filled. In particular, there is broad evidence that newly available capacity does attract new travel on the road in question.
That is why when new highways or flyovers are constructed, it is always a matter of time before they too become congested. This is not necessarily a bad thing since travelers are able to undertake trips that they otherwise could not on those routes or at those times.
What matters however, from a policy perspective, is the likely ex-post demand for travel and not the existing level of demand. The impact of induced and/or diverted traffic should not be
underestimated – not only for road building projects but also for policies whose practical result is to free up capacity.
Finally, effective congestion management policies should seek to understand the nature of travel demand in congested conditions. While workplace- home trips may be a key factor, it is important not to overlook other types of peak-hour trips including school runs, leisure travel and freight travel that often make a substantial contribution to traffic in peak periods.
(Next: Proposal to better manage traffic congestion)
Total Comments on the above stories (2)
Prof Kironde, congratulations for a very detailed analysis of the traffic congestion problem in Dar es Salaam. Lagos has a number of flyovers and a long bridge(may be 6m) that were designed to ease traffic congestion. Today, Lagos is much more congested than Dar es Salaam. We need to look beyond the physical structures to how we organise our urban space. The lack of parking space is without doubt the most serious omission in our plans, and if looked into some space along the roads would be available to ease traffic flow.
But the human capital is also extremely wanting as solution to our traffic problem in the name of traffic police. I have seen cases where the police force works extremely hard to speed up traffic flows but unfortunately ending up with slowing traffic. Someone must alert the police force of the fact that traffic lights are just as good and the police force has to be around to see that drivers obey the lights, and only intervene when things become extremely bad. The case in point is the traffic flow at Ubungo- Morogoro/Nelson Mandela Junction and Mwenge! Thank you for addressing the issues that we needed most
Big up 4 ur educative ideas on a chronic traffic congestion problem in Dsm city. th licencing the small sized daladala has 2 be limited . even the unplanned population migration has 2 solved .thanks!
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