Hello Glenn,
Greetings.
Christchurch has some very steep locations on the Port Hills and the older established areas and new developments alike have many of the issues we were grappling with in Auckland.
In my days as a developer's engineer in Auckland I pushed your rules to the absolute limit and then had the joy of driving on the results - in one case daily at my own home.
I believe 1 in 5 is a sensible limit.
When a designer tries to maximise the centreline length of an alignment to achieve an acceptable grade between two elevations one method is to use a horizontal curve. A simple C or even an S bend is a longer route than a straight line between two points. So taking this to an extreme if the curves are minimum radius the length may be maximised. A centreline 1 in 5 gradient now becomes 1 in 3.5 or so on the INSIDE wheel track of the minimum radius curve.
Result is loss of traction or wheel spin for a vehicle going uphill.
I achieved a private driveway which met Auckland City's maximum driveway gradient and yet an empty wheelbarrow parked on my driveway facing downhill would creep down grade. A 4 cylinder car facing downhill with the handbrake on could be moved merely with the hand pressure of a person .
Take care when shutting the boot :-)
At that grade the driveway is functional for a vehicle but not inherently safe for residents nor for unsuspecting visitors.
The other design trap is the need to maintain a safe sight line when the 1 in 5 gradient transitions to level (use an over vertical curve at the top of the drive).
This is the same danger whether at the footpath crossing for a property below the road or at the dwelling when the driveway is above the road.
A driver must be able to see over the car bonnet to avoid running over a child on the footpath or avoid hitting another car parked in the "parking space" at the top of the driveway.
I realise we do not have many HQ Holdens or P76 Leylands on the road anymore.
There may be an argument that cars are shorter, turn tighter, climb steeper and driver visibility is better by design.
The regulators are here to protect the end user (and unsuspecting purchaser) from the clever designer.
I note you do not distinguish between "private driveways" and commercial buildings where users may be staff or occasional visitors.
I welcome discussion on if it is appropriate to have the same standard for access to visitors car parks in commercial buildings or a higher standard than residential driveways.
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David Plom
Synergine Group Limited
Christchurch
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Original Message:
Sent: 21-06-2015 20:41
From: Glenn Broadbent
Subject: Driveway gradients
Hi
In the suburbs of Auckland Central we try to limit driveway gradients to a maximum gradient of 1 in 5, occasionally allowing up to 1 in 4. The expectation is that they will be concreted and on shaded steep areas we will require a rough surface. We require flatter grades at the transition toward the road and manoeuvring areas. We often have architects and their engineers wishing to exceed these gradients. I am interested to hear what gradients you may allow in your districts and any issues you may have with steep drives.
Glenn
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Glenn Broadbent
Auckland Council
Auckland
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