Many of the other answers address how to account for it, or how to fix it, but you've specifically asked not to fix it, but:

What do I say to Susan to make it clear that we're not mad about the fact that she's late, but that it would be a lot easier if she'd just come clean about it?

More specifically:

How to [get someone to give] more accurate ETAs?

You've given enough background information in the question and comments to suggest strongly the possibility that she does not have the skills to accurately estimate her arrival time. I don't think it's an issue of active or malicious lie or intent to deceive, I suspect she honestly believes one of both of:

1. her estimates of traveling time are reasonable
2. her awareness of passage of time is good

These complicate matters. If she receives a text asking for a timeframe to meet, she'll fail to make a correct estimate at (1), providing an estimate that is too low, and once she's made the estimate she'll continue her task, failing at (2) by believing it'll only take "a minute" before she leaves, and then perhaps 5-10 minutes later she's out the door to travel perhaps double the time she estimated to meet, and ends up a total of 20-30 minutes late.

The real issue is that time estimation is hard, and she's not able to give correct estimates.

If you insist that she give you accurate estimates, you are insisting on her changing her skills at time estimation. Changing yourself is hard enough, but changing someone else exponentially harder.

So if 1) you don't want to change her (or recognize that you can't) and 2) you need accurate estimates, then you need to get more information.

A time estimate is a single variable, and you can't extrapolate very much information from it. If you ask for more information, though, then over time you can develop a model for turning her information into an estimate. Perhaps these questions are all you need:

• Where are you at right now?
• What are you doing?
• When will you be done?
• How long will it take you to pack up and leave?
• Do you have any stops along the way (errands, etc)?
• How long will it take you to go from where you are to the meeting point?

This should give her the ability to make better estimates, particularly if you go over the actual results with her each time, and then the next time you ask remind them that their estimate was off for a particular part of it.

This takes a long time, you are essentially giving her a new skill or toolset for time estimation, but that's what's necessary if you want her to give you accurate ETAs rather than adapting to them yourself or adopting a technological crutch.

Many of the other answers address how to account for it, or how to fix it, but you've specifically asked not to fix it, but:

What do I say to Susan to make it clear that we're not mad about the fact that she's late, but that it would be a lot easier if she'd just come clean about it?

More specifically:

How to [get someone to give] more accurate ETAs?

You've given enough background information in the question and comments to suggest strongly the possibility that she does not have the skills to accurately estimate her arrival time. I don't think it's an issue of active or malicious lie or intent to deceive, I suspect she honestly believes one or both of:

1. her estimates of traveling time are reasonable
2. her awareness of passage of time is good

These complicate matters. If she receives a text asking for a timeframe to meet, she'll fail to make a correct estimate at (1), providing an estimate that is too low, and once she's made the estimate she'll continue her task, failing at (2) by believing it'll only take "a minute" before she leaves, and then perhaps 5-10 minutes later she's out the door to travel perhaps double the time she estimated to meet, and ends up a total of 20-30 minutes late.

The real issue is that time estimation is hard, and she's not able to give correct estimates.

If you insist that she give you accurate estimates, you are insisting on her changing her skills at time estimation. Changing yourself is hard enough, but changing someone else exponentially harder.

So if 1) you don't want to change her (or recognize that you can't) and 2) you need accurate estimates, then you need to get more information.

A time estimate is a single variable, and you can't extrapolate very much information from it. If you ask for more information, though, then over time you can develop a model for turning her information into an estimate. Perhaps these questions are all you need:

• Where are you at right now?
• What are you doing?
• When will you be done?
• How long will it take you to pack up and leave?
• Do you have any stops along the way (errands, etc)?
• How long will it take you to go from where you are to the meeting point?

This should give her the ability to make better estimates, particularly if you go over the actual results with her each time, and then the next time you ask remind them that their estimate was off for a particular part of it.

This takes a long time, you are essentially giving her a new skill or toolset for time estimation, but that's what's necessary if you want her to give you accurate ETAs rather than adapting to them yourself or adopting a technological crutch.