Hilly, Hillier, Hilliest

Any road into or out of Wellington is going to be hilly.
There's a big difference between the three possible routes from Peka Peka to Wellington.
Two of these routes traverse the Ngauranga Gorge. This must be Wellington's most notorious road, thanks to the diabolical combination of wicked reverse camber and a very steep slope. In wet weather it is murderous.
The steep section of the Ngauranga Gorge is just over one kilometre long. It has a slope of more than 8.5 percent, which means that for every one hundred metres of horizontal travel, the road climbs (or falls) 8.5 metres.
If you live in Wellington, you’ll know it doesn't take much to cause an accident on the Ngauranga Gorge.
This road also wastes fuel, because many drivers are forced to use their brakes on their way down. If you can coast all the way down without braking, then, at least in theory, you could save enough fuel to make up for the extra fuel you needed to get up the hill in the first place.
However, if the road is too steep, then you have to use your brakes, which wastes energy warming up brake discs and brake pads.
We might improve matters by adopting vehicles with hybrid transmission systems, but I doubt that even the best hybrid systems could completely solve this problem. And of course, hybrids can't fix the safety problem.
Coastal Route
Heading south along the existing (Coastal) route from Peka Peka, the first steep section is the climb through Pukerua Bay, which reaches 83 metres above sea level. The steepest part of this climb has an eight percent slope.
The downhill section south of Pukerua Bay has a five percent slope. The rest of the route would be fairly flat, if it weren’t for the Ngauranga Gorge.
Transmission Gully
This is by far the hilliest option. From the Peka Peka end, the first steep section is the 4.5 kilometre climb from McKay’s Crossing to the Wainui Saddle, which includes two kilometres of eight percent slope. Fortunately, this will have sweeping curves, crawler lanes in both directions, and NO reverse camber. Still, if you think this part will have a 100 kph speed limit, you're dreaming. We’ll be lucky to get an 80 kph speed limit here.
The next steep bit is the climb from Pauahatanui to the James Cook Interchange behind Whitby. This has just over a kilometre of 6.5 percent, which is too steep for a motorway. And then, of course, we have the Ngauranga Gorge, which will remain the scourge of State Highway One if we build a road through Transmission Gully.
Akatarawa Motorway
This is practically flat from Peka Peka to Reikorangi (inland from Waikanae). Then it climbs up to the Menin Reiki saddle, with 3.8 kilometres of five percent slope. According to Land Transport NZ's “State Highway Geometric Design Manual” (pg. 5-3), this will be flat enough for a 120 kph speed environment.
By the way, this road is nowhere near as high as the saddle on the existing Akatarawa Road. Menin Reiki is 340-360 metres above sea level, whereas the summit of the existing Akatarawa Road is 461 metres above sea level. Fortunately, we don’t need to go there.
From the summit, the road follows the Akatarawa River down to Cloustonville, with a grade varying between three and four percent. At Cloustonville there is five hundred metres of five percent, and then from there to Wellington, the slope never exceeds three percent. You might as well call it flat.
Comparing the Options
Akatarawa is by far the flattest option, with a maximum grade of five percent. However, since it has not been surveyed in detail, this is best thought of as a theoretical possibility. The terrain looks promising but we have not yet checked it out. At the other end of the scale, Transmission Gully is by far the steepest.
Another interesting question is: Which route has the most uphill and downhill driving? We can get a simple comparison by adding up the height of all the hills: the total vertical climb. Between Peka Peka and Kaiwharawhara, the vertical climb adds up to:

• Coastal: 250 metres (Pukerua Bay, Ngauranga)
• Transmission Gully: 585 metres (Wainui Saddle, James Cook, miscellaneous hills, and Ngauranga)
• Akatarawa: 340 metres (Menin Reiki)

Hilliness is not the only important criteria. However, it has a significant influence on safety and fuel consumption. The Akatarawa Route is a lot better than Transmission Gully. And, since it avoids the steep grades of the Ngauranga Gorge, it also beats the Coastal Route.