So far I have discussed the concept of fundamental parameters in highway geometric design (for example, here) and the fundamental parameter “road type“. This post covers the fundamental parameter “vehicle type“. Vehicle type relates directly to design vehicles – the traffic which the road is to be designed for. Choice of vehicle types for a road affect such things as lane widths, cross-section and design speed(s).
On vehicle types, the 2011 edition of the USA’s AASHTO green book (ref.831) says:
“Key controls in geometric highway design are the physical characteristics and the proportions of vehicles of various sizes using the highway. Therefore, it is appropriate to examine all vehicle types, establish general class groupings, and select vehicles of representative sizes within each class for design use”.
And, as a matter of interest ref. 831 also says that:
“….the bicycle should also be considered as a design vehicle where bicycle use is allowed on a highway”.
So, where do I think current standards get it wrong?
More than one design vehicle for each vehicle type
There is no reason to stop at one design vehicle for a particular vehicle type. After all, one example will not cover the range of dimensions of the vehicles of this type which will use the road. The AASHTO document (ref. 831 table 2-1a) has a set of 19 different design vehicles, including 5 sub-types of bus (but no types for cyclists or pedestrians). And in fact the first quote above from the AASHT green book refers to vehicle class (~ type) groupings.
Another AASHTO publication (ref.985) gives dimensions for a two-wheel standard bicycle. The dimensions do not represent the wide range of bicycle types which exist – they ignores bike-trailer combinations for example. On this point the same document says:
“Modifications to facilities (e.g., widths, curve radii, superelevations, etc.) that are necessary to accommodate adult tricycles, bicycle trailers, and other special purpose humanpowered vehicles and accessories should be made in accordance with the expected use, using sound engineering judgment“
(my emphasis again), which sounds a bit like avoiding the problem.
Same country, different vehicle types
It might be thought that, once a respected organisation comes up with a set of vehicle types and design vehicles, this set would be used all over the country. This isn’t necessarily the case. To use the USA as an example again, California DOT’s Highway Design Manual 2012 (ref. 1919) refers to a STAA Design Vehicle which does not appear in the AASHTO guide; and the “Los Angeles 2010 bicycle plan, technical design handbook” (ref.917) gives details for four different design vehicles (which don’t appear in the AASHTO book) within what we could call the vehicle grouping “bicycle“.
Same vehicle types, different countries
Some countries decide to use vehicle type / design vehicle details from a respected source from another country. The Tanzania road standard apparently uses dimensions for bicycles taken from Norway. But there are exceptional types of bicycle in Tanzania (such as gupa work bikes ), so the dimension details from Norwegian practice don’t really fit. The same could well be true for AASTHO truck vehicle types if used in e.g. Pakistan.
Non- “western” vehicle types
The AASHTO set of design vehicles might be described as a “Western” set, and could well be suitable for use in countries with a similar history and use of vehice types. But the AASHTO set may not be suitable for other parts of the world, where countries may have “non-western” vehicle types. Besides Tanzania (gupa work bikes) other examples would be Peru (motocarros), Nepal (bullock carts), Bangladesh (work rickshaws).
- If an engineer does not know that a vehicle type exists, he won’t design the road for it
- It should be possible to develop a list of vehicle types which covers use in different countries
- The set of vehicle types should include more than one example per vehicle type (in other words, vehicle types > type groups > examples
- The dimensions of design vehicles should include the dimensions of the vehicle in use, not just the dimensions of the vehicle as they leave the manufacturer.
I am presently working on a listing of highway geometric design parameters. The listing has a subset for vehicle type goups, with each group having a number of examples from design guidelines from different countries.
831 USA, AASHTO, A policy on the geometric design of highways and streets 2011
917 – USA, Los Angeles 2010 bicycle plan, technical design handbook, City of Los Angeles, 2011
985 – USA, AASHTO Guide for the development of bicycle facilities, AASHTO 1999.
1919 – USA, California Highway Design Manual, California DOT, 2012