Help Forum

Help requested posted on 27th July 2017:

Visibility at Junctions

Y distance has been used for many years to determine visibility at junctions / access points in lightly-trafficked residential streets. This Y distance is linked to the SSD (1.5s reaction; 4.41 m/s/s braking) of an approaching vehicle. CIHT have endorsed a 'new' method using the Z distance (ie not the X or Y distance of a triangle) but with no reference to the SSD. Is this correct?

Harry Craig

Reply to this request


Response posted on 3rd August 2017 by:
David Carter

E: Ralge@sky.com
T:

Visibility at Junctions

The topic is of great interest to me in my work.
Sadly, I need a great deal of explanation to understand what you are asking. Clearly I have not been mixing in the right circles?
X Y Z distance?
SSD?
CIHT?
Thanks in advance.


Response posted on 3rd August 2017 by:
David Carter

E: Ralge@sky.com
T:

Visibility at Junctions

The topic is of great interest to me in my work.
Sadly, I need a great deal of explanation to understand what you are asking. Clearly I have not been mixing in the right circles?
X Y Z distance?
SSD?
CIHT?
Thanks in advance.


Response posted on 9th August 2017 by:
Harry Craig

E: harry@harrycraig.com
T:

Visibility at Junctions

Sorry for late reply.
Y distance is described in Manual for Streets (a DfT publication and available for download). It is the 'standard' method of measuring visibility at junctions in lighty-trafficked residential streets. CIHT is the Chartered Institution of Highways and Transportation & provides the means to become a 'chartered member' which has some status within the Transpotation Industry.
The Y distance is a very precise measurement and relates to the Sight stopping Distance (SSD) of an approaching vehicle. This SSD is also clearly defined and the constants in the formula to determine are also clearly defined.
The measurement of visiblity is contained within a triangle of 'sight lines' ie the X, Y and the 3rd sight line I call the Z line. A Chartere Member of CIHT used the Z distance to determine visibility at a recent Planning Application. This Z distance was 40% longer than the Y distance so this member concluded the visibility was safe. I do not know anyone, nor have i seen any evidence that the Z distance has any relevance to visibility - particularly as there is no relation between z distance and SSD. However CIHT seem OK with this but that is not correct - or is it?


Response posted on 9th August 2017 by:
Harry Craig

E: harry@harrycraig.com
T:

Visibility at Junctions

Sorry for late reply.
Y distance is described in Manual for Streets (a DfT publication and available for download). It is the 'standard' method of measuring visibility at junctions in lighty-trafficked residential streets. CIHT is the Chartered Institution of Highways and Transportation & provides the means to become a 'chartered member' which has some status within the Transpotation Industry.
The Y distance is a very precise measurement and relates to the Sight stopping Distance (SSD) of an approaching vehicle. This SSD is also clearly defined and the constants in the formula to determine are also clearly defined.
The measurement of visiblity is contained within a triangle of 'sight lines' ie the X, Y and the 3rd sight line I call the Z line. A Chartere Member of CIHT used the Z distance to determine visibility at a recent Planning Application. This Z distance was 40% longer than the Y distance so this member concluded the visibility was safe. I do not know anyone, nor have i seen any evidence that the Z distance has any relevance to visibility - particularly as there is no relation between z distance and SSD. However CIHT seem OK with this but that is not correct - or is it?


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