In most modern vehicles, it is generally safe to switch from manual mode (where you manually select specific gears) to the "Drive" (D) mode while driving at highway speeds. The transmission system is designed to handle such transitions smoothly. However, it is always a good practice to consult your vehicle's owner's manual for specific guidance on your particular make and model.
Switching from manual mode to "Drive" mode typically means that the transmission will revert to automatic shifting, allowing the vehicle's onboard computer to manage gear changes based on factors such as speed, load, and driving conditions. This can be convenient for highway driving where consistent and efficient gear changes are often required.
It is worth noting that abrupt and frequent gear changes while driving at high speeds can put additional stress on the transmission. However, a single transition from manual mode to "Drive" mode is unlikely to cause damage. Just ensure that you are operating the vehicle within the recommended speed limits and following safe driving practices.
Always exercise caution and be aware of the conditions and traffic around you when switching modes or making any adjustments while driving. If you have any concerns or doubts, it's advisable to consult with a qualified mechanic or contact the vehicle manufacturer for specific guidance related to your vehicle.