The Split command on the Editor menu can split a selected line from either the start point or end point of the line by using a specified distance value, a percentage of total length, or an m-value. Split updates the shape of the existing feature and creates one or more new features using the default attribute values for the feature class.
The Split dialog box displays the length of the original feature in current map units to help you split it accurately.
Arrows are shown on the feature to indicate the line's direction, so you can choose whether you want to split the line from the start or end point of the line. These arrows will help ensure that the line is split from the proper orientation. 
- Click the Edit tool
on the Editor toolbar and click the feature.
- Click the Editor menu and click Split.
- Choose the split method you want to use:
- Split the feature at a certain distance.
- Split the feature at a certain percentage of the whole.
- Use measures (m-values) to split the line.
- Type a distance, percentage, or m-value based on the split method.
- Arrows are shown on the selected line to indicate its direction. Click From Start Point of Line if you want to split the feature starting from the first vertex, or click From End Point of Line if you want to split the feature starting from the last vertex.
- Click OK.
The line is split, where Split updates the shape of the existing feature and creates one or more new features using the default attribute values for the feature class.