Highways Division Junction Statement

Statement on behalf of the Highways Division, Department of Infrastructure. PA 12/0573

1. Given the role of the Highways Division in determination of this application, this statement only sets out the factual background to the proposal, advise on relevant highway policy and provide assistance to the inspector in his or her consideration of the application. This statement specifically sets out factual information on: the choice of junction to serve the proposed development; transport assessment methodology; and the impact of the proposed junction on road users.

Scoping and pre application assessment of junction design options prior to application PA11/1250.

Roundabout Option

1. The Highway Division determined at the pre application stage, based upon the Kirk Michael Relief Road Study (Atkins, 2004) and Design Manual for Roads and Bridges (DMRB) design standards, prior to application PA11/1250, that a roundabout junction would be its preferred option to accommodate the proposed new access at the junction of the A3/A4.

1. No detail design of a roundabout junction at this location had been undertaken by Atkins, only a preliminary design undertaken to establish its maximum size (see para 29, Appendix A - Route options, Highway design considerations.

1. Bryan G Hall Consulting Civil and Transportation Engineers acting for Heritage Holmes designed a compact roundabout (see Drwg No. 09/348/TR/015/1) in accordance with DMRB TD16/07 Geometric Design of Roundabouts, but taking in to account that junction lies on the TT course.

1. Bryan G Hall Consulting Civil and Transportation Engineers consulted both the Highways Division and motor racing representatives including the Clerk of the TT course regarding the proposed roundabout design. The feedback from the motor racing representatives regarding the design and layout of the roundabout was negative. A number of aspects of the design were identified as impacting on the safety of racers due to the speed at which motorcycles would approach and exit this location, including:

• the large areas of road markings - causing differential surface grip, especially in wet weather;
• change in kerb lines – cause riders sighting problems on approach to bend.

1. The Highways Division also had concerns regarding the lack deflection (to accommodate the racing line) for vehicles approaching the junction from the south, possible solutions including using demountable islands, central island and overrun areas were considered. It was felt however the use of demountable islands, central

island and overrun areas could cause road surface to deteriorate, which had both maintenance and racing implications. Also demountable over run areas may be more susceptible to damage from turning HGV's.

1. Currently there is only one roundabout on the TT course, located at Governors Hill. This junction is located on a 90 degree bend, therefore the approach and exit speed of the racers is much lower than at the junction of the A3/A4.

1. Highways Division after consultation with representatives from the TT believes that roundabout option will need to be set aside until such time that a design can found that satisfies both the Departments desire for a safe and efficient junction and the TT representative's desire for a junction that can safely accommodate racing.

Mini-roundabout option

1. Several mini-roundabout junctions exist on the TT course:

• Quaterbridge – 2
• Braddan Bridge – 2
• Signpost
• Governors Hill

1. These junctions are located on either on a 90 degree bend or on a straight section of road after a from exit from a bend, therefore the approach and exit speed of the racers is much lower than at the junction of the A3/A4.

1. In addition DMRB TD54/07 Design of Mini Roundabouts, recommends that four arm mini roundabouts should be not used where the sum of the maximum peak hour entry flows for all arms exceeds 500 veh/hr. Therefore, a mini-roundabout junction can not be used at the junction of the A3/A4.

The simply priority crossroad option:

1. DMRB TD 42/95 considers that a staggered T junction is preferable to a crossroads junction due to crossroads having generally poor safety performance. The document goes on to recommend that crossroads are considered suitable only at simply junctions where the minor flows do not warrant a ghost island or single lane duelling. The existing minor road (A4) 2 way AADT vehicular flow will exceed the 300 vehicle threshold for a ghost island. The 2 way AADT flow for the new minor road serving the development will also likely exceed the threshold, given that the predicted combined am and pm peak 2 way flow is 240 vehicles. In both cases the minor arm flow would preclude the option of a priority crossroads. Furthermore, there is also a general recommendation that it is undesirable to position a simply junction inside a sharp curve

1. Finally, the junction design would be constrained as additional safety measures such as ghost islands, hatching or physical islands which may be required would be resisted by the TT representatives.