Before revision: Achieving the right balance between planning quality, safety and runtime efficiency is a major challenge for autonomous driving research . Optimisation-based planners are typically capable of producing high-quality, safe plans, but  at the cost of efficiency.
After revision: Achieving the right balance between planning quality, safety and runtime efficiency is a major challenge for autonomous driving research . Optimisation-based motion planners are capable of producing high-quality, safe plans, but at the cost of efficiency.