Hi Everyone,

A client has requested a driveway design requiring 30% - 35% slopes on a very steep tropical lot in Hawaii. The site is quite narrow and does not have enough space for switchbacks. The parcel does have an existing driveway at those grades (see photos).




The client and the architect are both pushing the steep slope design due to most driveways in the neighborhood being constructed this way. However, the local standard requires a max slope of 15% or less and most colleagues I've consulted would not exceed 20% due to liability issues. The issues I've identified include vehicle damage due to rapid slope transitions, lack of emergency vehicle access, drainage issues, the slopes exceeding local codes and sound engineering judgement.

My instinct is to decline the project but would appreciate advice on:

1. What is the upper limit for a driveway slope?
2. Is there a workable solution I'm not seeing here?

Thanks,
Hawaii PE