I have some statistical data from which I want to graph the means and use the standard deviations as error bars. However this produces a graph with some of the error bars passing below zero. A negative value is silly for this data (mean trip times), so I was wondering what is a sensible way to graph the data.



Is it alright for STD error bars to be below zero?
Your error bars may be giving you a hint to look more closely at the distribution of your data: it may not be symmetric. For example, if your data is essentially log-normal you could work with the logs of your numbers and the problem will automatically go away.


I'm not a fan of error bars. In theory they let you visually do some statistical significance estimates and perhaps give some sense of the underlying data. But there are a lot of subtleties and at least one study has found that even experienced scientists often misinterpret them. This [nice blog post](https://scienceblogs.com/cognitivedaily/2008/07/31/most-researchers-dont-understa-1) discusses some of the issues.


If you do need to summarize the data with a few statistics, I'd argue for [boxplots](https://en.wikipedia.org/wiki/Box_plot) as a better way to represent asymmetric distributions, along with text/captions that highlight important statistical significance conclusions.


