Bus Sizes - the right mix

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Summary

The cost efficiency of bus transport depends on economies of scale.  Generally, “the bigger the better” – as long as the bus is consistently loaded to near 100% of its seating capacity. 

The optimum size of bus therefore varies according to the passenger loads on its route.

Where there are very high passenger volumes (major urban commuter routes and inter-city arterials) sufficient to consistently fill very large buses, far superior economy and efficiency will be achieved by using the largest possible buses – ideally buses which could carry many hundreds or even thousands of passengers per trip. 

That is why all the most economical and efficient mass transport systems in the world use trains.   Their  economies of scale are unrivalled.

Busiest routes

Therefore, in the most densely populated commuter routes/zones:

Where appropriate rail infrastructure exists, or can be readily established, the optimum form of mass passenger transport is the railway. Nairobi is not entirely without urban rail networks which either already exist or which could be readily adapted at viable capital cost. 

Where the train option is not practicable,  the road bus system should try to emulate the advantages of trains – the highest possible seating capacity per driver/engine/vehicle space, using dedicated one-way channels.

Buses with the highest seating capacity (circa 100) are double deckers and those which are articulated, towing a second passenger module.   These require roads which are especially smooth and which have other special attributes not currently abundant in Kenya.

In practice, the largest buses in Kenya’s mass transport system are likely to be 60-seaters. On routes where speeds are relatively low and distances are relatively short, they are designed to carry up to 100 people (a payload of 8 tonnes, seated and standing). 

These largest buses should be used as the only (or at least, the predominant) form of passenger transport in all areas and on all routes and at all times where most or all seats are likely to be occupied and the roads are suitable.

Lower volume routes

However, very large buses become progressively less efficient as their occupancy falls, to the extent they can be the least efficient option where occupancy levels are consistently around 50% or lower.  It follows that a medium bus (26-seater mnamba) is more economical and efficient than either a larger or smaller bus on routes where its occupancy is likely to be near 100%;  in turn, a 14-seater minibus is most economical and efficient on the lowest passenger volume routes.

Bus sizes should therefore be assigned to routes according to the passenger volumes on those routes, to achieve near full occupancy at the desired frequency of passage.

Road compatability factors

A further important consideration is the width of roads (and hard shoulders for bus stops).   There are numerous peri-urban roads which are not wide enough to properly accommodate two 60-seater or 26/29-seater buses passing in opposite directions.   Use of 26/29-seater buses in these locations is dangerous (especially to cyclists and pedestrians), disruptive to all other traffic (prevents overtaking and forces on-coming vehicles off the road) and mutilates road edges, which become both damaging and dangerous to all users.  In these locations, unless and until the roads and shoulders are widened, 14-seater buses should prevail and 26/29-seater or larger buses should be prohibited, even if passenger loads justify their use.

Similarly, 14-seater buses might justifiably be prohibited from routes where the roads and passenger loads make larger buses practical and intrinsically more economical and efficient. 

Road space utilization 

In addition to economic factors, road space is a key issue.  To illustrate: 

In motion, any vehicle (irrespective of its size) requires a space of circa 10 metres in front, 10 metres behind and two metres either side of its own body.

A 14-seater minibus therefore occupies 150 sq m of road space (more than 10 sq m per passenger).

A 60-seater bus occupies 220 sq m of road space (less than 4 sq m per passenger).

It follows that transport of large numbers of commuters by minibus requires nearly three times as much road space as transporting the same number of people in large buses.

The 26-seater falls in most respects between these two extremes and therefore offers the most economical and efficient option in the intermediate niche.  It is less wasteful than a minibus in high traffic zones, and less wasteful that a 60-seater bus in low traffic zones.  While thus very flexible, it is the optimum choice in only a very narrow band of conditions.

Size choice

A simple graph can be plotted to show the road space efficiency per passenger of the three main types of buses, depending on their occupancy.   Similar graphs can be plotted to show fuel efficiency and other capital and operating costs per passenger km.

These will illustrate the over-riding importance of occupancy levels in choosing the optimum size of bus, and that:-

  • the 60-seater, if running fully occupied,  is by far the most economical and efficient in purchase price, fuel consumption, other operating costs and road space useage.
  • The 26-seater becomes comparatively  more economical and efficient than a 60-seater filled to less than circa 50% capacity.
  • The 14-seater becomes comparatively more economical and efficient than a 26-seater filled to less than circa 50% capacity.

Outlook

In parallel, it should be noted that Kenya has a population of circa 40 million people and a vehicle population of 1 million.  Hence a people:vehicle ratio of 40:1.  The global average is 7:1, and in the most developed countries the ratio is 2:1

Kenya’s current ratio of 40:1 indicates an extreme dependence on public/mass transport.   And this overall figure in fact camouflages an even more severe position. 

In round numbers, 350,000 of Kenya’s vehicles  have no formal passenger provision.  They carry cargo.  In practice they provide mobility to less than half-a-million people.    A further 350,000 are private cars with an average capacity of fewer than five people each.  That mobilizes only 1.5 million more.

Therefore the balance of 38 million people are catered for by only 300,000 vehicles.  A ratio of  126:1.  Further, Kenya’s human population is trended to double within the next 25 years – even more rapidly in major urban areas.   To both meet that  growth and improve people/transport ratios, Kenya will need to increase its overall public/mass transport capacity by the order of 10-fold in the next two decades.     

Conclusion

In sum, Kenya needs buses of all sizes, but each size class should be used on routes where it offers optimum economy and efficiency to users, and where it is least dangerous, disruptive or damaging to other traffic and the road.

For optimum economy and efficiency and because buses cater for such an overwhelmingly high proportion of all personal mobility, buses should be allocated dedicated road channels (lanes) in urban areas where this is physically practicable.

Other measures

Other issues no less important than the right bus size include:-

  • Priority stipulation and enforcement of the Clearway principle.
  • Parallel provision of and adherence to bus stops that are off-stream, well surfaced, and sacrosanct.
  • Stipulation and enforcement of mechanical, bodywork and safety feature standards for all buses – including increasingly deterrent tests related to age (which has important safety, emissions and fuel efficiency impacts).
  • Public education campaigns to improve road conduct and use, including tuition standards and meaningful tests.
  • Equally stringent discipline on road and traffic administrators as on road users.
  • Consideration of radial arteries and concentric ring roads in urban road planning.

By combining use of trains where practicable, rational allocation of routes to appropriate bus sizes, and the foregoing systems/standards measures, optimum economy and efficiency for all road users will be achieved.

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