Then there's the implications of Transmission Gully opening in 2020. With the mooted Petone-Grenada road due, in the best-case scenario, to be completed three years after the Transmission Gully route opens, we'll have to put up with an increased amount of traffic for (at best) three years.
The logical thing to do is to upgrade SH58 now. Petone-Grenada may make sense by itself, I don't dispute that. But it certainly doesn't make sense to have an unsafe road taking more traffic because the Hutt Valley's link to Transmission Gully won't be ready until three years after the new route out of Wellington opens. Luckily, NZTA is seeking public submissions on Petone-Grenada link. Why not let them know what you think.