Running late feels uncomfortable, so you send nearly there while still far away. The person waiting cannot tell whether to start without you or change the plan.
Send a realistic estimate as soon as you know the delay. If you do not yet know, say that and give the next update point. Use a safe method and avoid messaging while driving or doing anything that requires your attention.
Let them adjust
Explain whether the delay affects a booking, meeting or shared responsibility. Ask whether they should proceed without you where that is an option. Do not assume that the original arrangement can simply move.
Update the estimate if it changes materially. On arrival, acknowledge the wait and address any practical consequence. Later, review the cause if it is likely to recur. A travel estimate may need revision, or preparation may need to begin earlier. The immediate message should help the other person make a decision with the time they have, instead of keeping them attached to an optimistic arrival promise.
