One way/two way roads

[Peter_Arup]

Hiya

A road is categorised as either one-way, two-way or dual carriageway.

We don't use the dual carriageway one.

What, then are the differences between the one-way and two-way ones? In terms of the effective noise source which is generated I assume there's no difference as I think it creates an area source the shape of the road.

For the gradient correction (paras 14.3 and 15 in CRTN) my reading of this is that the gradient correction would be applied to ALL roads except one-way roads going downhill. Is this, then, the only way in which one-way/two-ways differ?

Also, could you clarify the purpose of the road attribute 'Traffic Basis' and where this is changed for a road (can't see it in the road properties form).

Cheers

Peter

EDIT: just noticed you have a Support forum  - please feel free to shift this post there if you want to

[roger]

CRTN says that you should assume that the traffic is all in the nearside lane, unless it is a dual carriageway where the two carriageways are separated by at least 5 m horizontally or 1 m vertically.

This assumption has two effects: firstly that the traffic is brought closer to the receiver, and secondly, that the gradient correction only applies to uphill flows.  So if the nearside carriageway is uphill, all the traffic gets a gradient correction, but if it is downhill, none of the traffic gets a gradient correction.

If you use two one-way flows, then the traffic is not lumped into the nearer carriageway, and the gradient correction will always be applied to the uphill flow. 

I think I am right in saying that the latest revision to DMRB recommends using two one-way flows, and it is probably more accurate to do so, especially on the very wide motorways that are being built, because screening and distance effects of the two carriageways are that much greater. 

However, it is important to remember that much of the empirical work on which CRTN is based was from measurements on real roads where it would have not been possible to separate the various traffic flow directions, so although it seems weird to lump all the flows together, separating them is probably extrapolating from measured data.

Traffic Speed Basis is used to indicate whether or not the speed you have entered takes the gradient into account.  It is either corrected or not corrected for gradient.  CRTN has a built-in assumption for the effect of gradient on traffic speed, and is most applicable when using the 'prescribed speeds' in CRTN rather than any measured speed.  When you select 'not corrected for gradient' then the CRTN correction is applied to the speed.

However, if you have measured speeds to take the gradient effect into account, you do not wish to apply the built-in speed correction as well, so set the speed basis to 'corrected for gradient'.

You do this by checking the tick-box in the traffic flow parameter dialogue, because it is a property of the traffic speed, not a property of the road segment.

Hope this helps.
Roger Tompsett