Comment

Representation ID: 15872

Received: 11/05/2016

Respondent: National Highways

An assessment of the operation of the SRN junctions has not been undertaken within the Highway Modelling Report. Therefore it is not possible for AECOM to speculate on the potential operation of these junctions following Local Plan development.
Junction 17 (A1023 / Mascalls Lane / Spital Lane) and Junction 19/20 (A127 / B186) are located close to M25 Junctions 28 and 29 respectively and AECOM consider that any queuing back from
these junctions towards the motorway could compromise the effective operation of these junctions. AECOM have primarily considered the operation of the junctions for Option 1, as this is the preferred option that has been taken forward for implementation in the LP.
At Junction 17 the A1023 approach to the junction (the approach of concern to HE) is predicted to operate over capacity in the AM peak, with Mean Max Queues (MMQ) expected to reach approximately 300m back from the junction. Whilst M25 Junction 28 is approximately 500m away, as the approach is over capacity it is likely that the full extent of the queue will be approximately 600m at the end of the peak hour. This could reach back to, and through, M25 Junction 28 and could therefore affect the operation of the junction and the M25 offslips.
At Junction 19 the A127 Eastbound offslip is predicted to operate significantly over-capacity in the AM peak, with queues predicted to reach approximately 1km, which could stretch back to M25 Junction 29.

Comment

Representation ID: 15873

Received: 11/05/2016

Respondent: National Highways

Additional issues:
Appear to be mistakes in the reporting. In Tables 11-1 and 11-2, the description of junction 15 (Priority left-in left-out) is not consistent with the description in Table 2-1 (double mini-roundabout). Further descriptions that the operation of this junction is very much influenced by the level of mainline traffic suggests that this is not the junction described as Junction 15 in Section 2. Hence, it is unclear whether the reported values are meaningful. Some investigation is required as to what has been modelled as Junction 15. Junction 21 is labelled as not applicable (N/A) for options 2 and 3. While this is correct for option 2 as the junction would be closed, there should be a value for option 3. The appropriate values
should be inserted into Tables 11-1 and 11-2.

Comment

Representation ID: 15874

Received: 11/05/2016

Respondent: National Highways

Additional issues. Appear to be mistakes in reporting. Some through trips on the motorway appear to be designated as development trips. This is clear from the plots in Appendix D where there are more trips on the M25 passing northbound through Junction 29 than leave at the exit at Junction 28. Hence, there must be some trips which pass through both junctions with neither trip end in the Brentwood area. It is not clear whether this has any significance for the assessment.

Comment

Representation ID: 15875

Received: 11/05/2016

Respondent: National Highways

Additional issues. Appear to be mistakes in reporting. It is reported that there seems to be some traffic reassignment within the option tests when the development traffic is added to the baseline traffic. According to the described methodology, this should not happen. The only reason why this may happen is if routes are closed as in Thorndon Avenue in Option 2. Some investigation is recommended as to why this happens.

Comment

Representation ID: 15876

Received: 11/05/2016

Respondent: National Highways

Additional issues. Consequences of the adopted process. There are some zone pairs where there are at least two alternative routes. The choice of route will affect the junctions through which development traffic passes. Small changes in the assignment assumptions may result in significant differences in junction loading. The distribution and assignment has been based on certain assumptions for the values of time and distance. Some of the routes used by development trips in the model may not be considered to be suitable by the local highway authority. In these cases, they may take action to prohibit or deter the movement. This would have the consequence of increasing the loading on the junctions of alternative routes. In some cases, this may result in other junctions being assessed as critical. Sensitivity tests could be undertaken which apply a penalty to unsuitable routes thereby encouraging development trips onto more major roads.

Comment

Representation ID: 15877

Received: 11/05/2016

Respondent: National Highways

Additional isues. Consequences of the adopted process.
As discussed earlier in this note, no assessment is made of M25 junctions. The increase in the number of trips at M25 Junctions 28 and 29 is higher than at any of the junctions modelled. AECOM recommend that capcity tests are undertaken to determine the resultant operation performance at these junctions. This type of assessment could have used a full highway assignment model had one been available. The assignment from such a model would differ in several important ways:
The extent of increasing junction delays would affect route choice. (This has two consequences: Several routes will be used by development trips between zone pairs; and existing trips may change route as a result of congestion caused by development trips).
Increasing congestion may result in interference (blocking back) between junctions.

Comment

Representation ID: 15878

Received: 11/05/2016

Respondent: National Highways

While the methodology used would show a worst case at some junctions, allowing for reassignment of trips may indicate a worst case at other junctions. Ultimately, this may depend
on the level of mitigation. If full mitigation is provided, then the routes assigned may be used by all traffic. In reality, it is unlikely that full mitigation will be possible at these junctions with consequence reassignment to sub optimal routes. This may require mitigation measures at other junctions.
It is probable that the optimum strategy for Brentwood Borough Council would be to install some mitigation measures at the junctions assessed as being most congested but accept that it is not possible to achieve nil detriment by this method alone. Actions to reduce congestion on other routes could result in a reassignment of traffic away from the more congested routes which may be a more cost effective strategy depending on the relative costs and constraints of improvements.

Comment

Representation ID: 15879

Received: 11/05/2016

Respondent: National Highways

Conclusion
This technical note has been prepared to review the Highway Modelling approach used to support the preparation of the Brentwood Local Plan. The review was intended to establish whether PBA's (Brentwood BC's consultants) approach to transport modelling will reasonably reflect the potential
impact the Local Plan development could have on the strategic road network over the plan period. AECOM has made a number of comments and recommendations throughout the note.

Comment

Representation ID: 15880

Received: 11/05/2016

Respondent: National Highways

Conclusion (2)
In summary AECOM consider that the trip generation and distribution assumed by PBA can be considered reasonable. Some concerns have been raised regarding the assignment of trips across the network, primarily the decision to assign all trips between two zones to the same route, whereas in reality AECOM consider that a number of different routes may be used, particularly if routes become congested and users change to an alternative route to avoid the congestion.
However, AECOM consider that the current methodology could result in a robust impact at the strategic road network and therefore may be reasonable to take forward. If, however, the local highway authority plans to deter drivers from making use of certain routes or congestion hotspots cause a significant change in route choice, the total development trips on the strategic road network could alter from that presented within the Highway Modelling Report. The assessment undertaken suggests very high impacts to the SRN.
Junction capacity assessments of the strategic road network have not been presented within the Modelling Report, despite experiencing significant increases in trips at the junction as a result of development. It is recommended these are undertaken to determine the predicted operation of the junctions following Local Plan development and to determine what measures may be required to mitigate the impact. Consideration may also need to be given to undertaking merge/diverge assessments at various locations to determine whether changes are required to support the Local Plan development.