Andy, in response
The linked reference document is part of the Smart Motorway Programme (SMP) Design Guide – that has been formally instructed for use by all programme schemes. Invariably when the ‘design’ becomes complicated and discussed with the Area team the guidance gets a wider circulation.
The documents were current at time of their writing – updating superseded references may be a future task (current SPaTS task terminates in March 2020). Language terminology generally used ‘should’ as its additional guidance but subservient to DMRB.
Your quoted example of a sign(s) within an entry/exit taper widening has occurred several times, although the conflicts can often be avoided by placement of an FTP (say) 800m upstream of an exit and the following FTP 3km (approx.) downstream that also avoids signs at the entry.
ROTTM signs are to be used with low traffic flows i.e. relaxed conditions, but HGV obscuration is still a valid consideration. The preference is to place ROTTM signs with a 2.4m mounting height to overcome white-van obscuration. Designs should try to minimise off-sets from the four main running lanes so that their conspicuosity and reading times are achieved.
Your aspiration that Designers will do the right thing has not always been borne out by experience; with some replying ‘if its within 10% its fine’ and often add ‘the local area team are content’. The difficultly has always been the suitable inclusion of ROTTM signs amongst the other smart motorway infrastructure and avoiding junctions and Emergency Areas. Often the designer assumes the proposal is optimal or change is too difficult, so unfortunately what I would consider as sub-optimal designs have been progressed. The reason for the guidance is to make optimal designs the norm and so deliver ROTTM signs that appear ‘regular’ for the customer.