Suppose someone does manage to list the features which influence- or are influenced by – a geometric parameter (see previous post here). This information by itself is not enough to allow anyone to simplify things. We would also need to know what size the influence was.
For example, in looking at horizontal radius, perhaps friction determines 30% of the final value for radius, whilst weather only for 2%. (Note – the percentage figures in this blog post are invented, and are included for illustrational purposes only).
If we knew what these percentages are we could update table 1 (taken from the previous post) and sort the factors in terms of the size of their impact, as for example in table 2. We could then say that designers could ignore factors which have only a small influence – and other people could also see why and what we have done.
Getting the balance right
One of the basic parameters in geometric design is stopping sight distance (SSD).The details which design standards prepared for Albania (ref. 1155) have on this include the following formula:
where Ra is the aero-dynamic resistance. Ra itself is dependent on the mass volume of air in standard condition.
The same document includes two charts, which are labelled:
- Figure 5.5: Required stopping sight distance on motorways
- Figure 5.6: Required stopping sight distance on other rural roads
My impression is that the formula is both over-complicated (surely the value of the mass volume of air will only have a small influence on stopping sight distance?) and that the charts are over-simplified (for example, figure 5.5 should perhaps be labelled “required stopping sight distance for cars on surfaced roads with a design speed of ≥ 80 km/hr”). Also, we know that vehicle type and road surface type both have a strong influence on SSD. Perhaps the whole standard could be revised to give SSD values separately for trucks and cars; and separately for surfaced and gravelled roads.
- For any design parameter, list the features which can influence / or be influenced it together with an estimate of the size of their influence
- Propose values for the design parameter which are based on major influencers and not minor ones
- Don’t make things more complicated than they need to be.
- Don’t simplify things by leaving out important influencers
1155 – Albania, ARDM 2 Road design manual vol. 2 / geometric Design; MPWTT, 2007 (?)