Ask Phoebe: Mangere Bridge high enough to clear wharves

New Mangere Bridge on the South Western Motorway. Photo / Richard Robinson

Driving over the new Mangere Bridge, heading south to the airport, the other day I wondered why it has been built so high? It's not like any ships can get under it as the little old disused bridge is still in the way, and where would ships go anyway? Dean Spooner, Henderson.

The Transport Agency did look at a low-level bridge during the planning stages of the new bridge, but there were several reasons why the idea was dropped.

There would be construction difficulties and extra costs involved in connecting the motorway approaches if the bridges were of different heights. As well, the new bridge had to be high enough to clear Onehunga Harbour Rd, and the added height will now be sufficient if ever the Onehunga wharves are extended.

But most importantly, to my mind at least, is that the two motorway bridges standing side by side look great, don't they?

The stretch of Line Rd from Taniwha St to West Tamaki Rd is fairly busy, joining as it does Glen Innes with Glendowie and St Heliers. It's quite wide, but only suitable for one lane in each direction.

Why then does it have no centre line? A centre line is an important visual clue, reminding drivers to stay on their side of the road. The lack of markings of any adequacy seems to be an oversight. Are there standards for line markings, particularly centre lines? And why do other roads in the area have marked centre lines? Winston Byblow, Glen Innes.

All road markings used on the Auckland City network are defined in the Manual of Traffic Signs and Markings which states that a marked centre line is desirable on all arterial and principal urban roads, or where curves or crash records show the need for a centre line. The requirement also applies to collector roads that carry a decent amount of non-local traffic, and this is what Line Rd is.

However, Line Rd is straight and has no crash history. There is also a belief that in some cases it may in fact be safer to leave a road unmarked.

Ashby Ave has a centre line because of the bends in the road. Taniwha St and Apirana Ave have centre lines because they are classed as arterial routes.

Do you know if the Auckland Marathon on October 31 is likely to cause any disruption to traffic heading north over the Harbour Bridge that day? Lyn Lester, Birkenhead.

There will be some disruption, but it should only be minimal. The marathon has used the bridge since 2000, and it is organised to cause the least possible disturbance. The race is held early on Sunday morning when traffic is usually light, and only the two northbound clip-on lanes are involved. Some on and offramps near the bridge will also be closed, but detour signs will be in place.

The closures start about 4.30am, and runners must be off the bridge by 9.30am. All lanes and ramps will be open by 10.30am, which is when Sunday traffic starts to build.