16th December 2019 at 1:04 pm #3445Simon_OgbornParticipant
For your information … Highways England’s Smart Motorway Programme (SMP) has developed a visual aid to enable the ‘regularity’ of ROTTM signs.
Temporary traffic management (TTM) on high speed dual-carriageway roads typically comprises, following a ‘Road works’ 1-mile sign, four pairs of regularly spaced approach ‘wicket’ signs in advance of the traffic cones that reduce the available traffic lanes of the carriageway, following the guidance of the Traffic Signs Manual Chapter 8 (TSM). The placement of these wicket signs is a regular 200 yards apart, with a 10% tolerance; 800 yards, 600 yards, 400 yards and 200 yards.
These signs and their associated taper point (commencement of traffic cones) should be placed on lengths of carriageway where the visibility is good and there are no impediments, for example not within a junction merge area. The taper point being placed in a ‘safe zone’; that for both its location and the preceding signs they would all be safe to install, operate and remove. A designer can therefore pre-determine these safe zones for both nearside and offside lane closures, which are often the same. From a standard TTM vehicle a cone-protected safe-works area can be provided up to 4 km in length, for maintenance of highway assets. Overlapping these 4 km TTM sets enables maintenance to be managed along each link, between junctions and at grade-separated junctions the ability to easily close the entire carriageway.
On Smart Motorways where the hard shoulder is converted to an operational lane there are numerous gantry-supported type 4 Motorway Signals (MS4s). These do display speed restrictions whilst the workforce is on the carriageway setting up the TTM. As yet, MS4s have not received DfT authorisation to display ‘wicket’ information, which is understood to be an aspiration of Highways England.
To minimise roadworker exposure whilst they deploy TTM apparatus, under ‘relaxed’ conditions it is acceptable that the signs are provided solely in the verge. Alternatively, fixed-location powered LED type signs may be used known as Remotely Operated TTM (ROTTM) signs, which further reduces TTM set-up time, roadworker exposure and increases the maintenance working window.
Positioning of ROTTM signs is complicated as there is limited suitable Smart Motorway verge for their placement, in part due to the high quantity of other infrastructure. A further consequence of providing these fixed position ROTTM signs is the ‘fix’ of its associated taper point, a Fixed taper Point (FTP). The design problem then becomes one of how to place these temporarily-used signs in physically permanent places confirming that they are ‘regularly’ spaced, rather than simply ensuring that they are within tolerances of the displayed distance.
Highways England’s Smart Motorway Programme (SMP) has developed a visual aid to enable the ‘regularity’ to be considered from the customer perspective (i.e. by drivers). The aid plots the signs’ distance to the FTP plotted against the time for each subsequent sign to become legible for a driver, travelling at 70 mph, being the function of the size of interval between them, which graphically for a (plus / minus 10%) TSM compliant sign set 200 yards apart is a horizontal line.
More information for designers may be found in Appendix E of the SMP Design Guide document E1.14 ‘Fixed Taper Points & Remotely Operated Temporary Traffic Management Signs’, accessed from the following web-link https://hebim.withbc.com/pub/english.cgi/0/196469?op=lp .16th December 2019 at 2:19 pm #3795Talk-Mobility AdministratorKeymaster
An interesting document. Presumably it has been sent to all potential SM scheme designers as well as all HE MSPs in each Area?
A few comments if I may, in no particular order:
IANs 150 and 163 have been withdrawn, so referencing them in this document might cause problems for anyone who doesn’t still have a copy. I’m also not sure what the legal position would be if we do quote withdrawn documents. However, I understand that the contents of these IANs have been subsumed into TSM Chapter 8, Part 3, currently under revision prior to hard copies being printed by TSO. Ultimately I understand that Part 3 will be wrapped into revised Parts 1 and 2, so further editing of this document may be advisable in future?
There are numerous examples of “should” and “should not” statements throughout the document, which is fine, but I’d add a cautionary note that there will inevitably be sites where whatever the advice might be just can’t be accommodated. For example, siting a wicket sign adjacent an entry or exit slip is a very common occurrence with static plate signs and given the potential difficulties of siting FTPs, I would not be at all surprised to find this to be reasonably common for ROTTM signs.
I’m struggling to understand the emphasis on “Regularity” and how it might influence the design. An array of ROTTMS typically comprises 5 nearside only signs. I fear it will be quite common for traffic travelling in anything but LBS1 to find one or more of these signs obscured by HGVs travelling in LBS1. However, when we consider that these LED ROTTMs tend to be much more visible than static plate signs, I think we can be reasonably confident the approaching drivers will be adequately warned of the closed lane(s) ahead no matter how regularly or irregularly they actually observe any unobscured signs. I note that only having signs on one side of the road may well lead to obscuration regardless of whether they are plate signs or LEDs. Depending on traffic flows, we can never be certain that placing signs on only one side of the road might result in some drivers not seeing any signs on the approach to a taper point – although I’d like to think that a taper lit by sequential lamps will be visible even if the approach signs all happen to be obscured by HGVs.
I would expect a designer to attempt to site all the wicket signs at a certain spacing, then adjust them all if it proves that one of them is in a sub-optimal location, should that one sign require moving by a significant degree, whilst still keeping within the 10% siting tolerance for all signs. This should be a rare occurrence and might mean that a totally different FTP could be called for instead. If a design had, say, the 800 and 600 signs very close together but the 400 then a “long” way after the 600, I’d hope the designer would make all attempts to make the spacing more regular without needing to worry about producing a chart to prove they’ve done so. Or simply move the FTP point a few metres if possible, to allow the approach signs to then be “correctly” located? Note that static plate signs tend to be placed 2 marker posts apart, (very approximately 200m), allowing that the distance between marker posts all too often is a movable feast. I’m not aware of anyone in the TTM industry ever siting signs on motorways based on the yardage quoted on a discrete sign, in my experience they are always simply sited based on marker posts.1st January 2020 at 2:20 pm #3796Talk-Mobility AdministratorKeymaster
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.
- You must be logged in to reply to this topic.