Best way to visualize route frequency on a map?

I have a map that displays flight routes. Now I'd like to indicate that some routes are operated more frequently.

Example (flights per week):

• SFO - NRT: 27
• JFK - BER: 4
• BER - DXB: 12

How to visualize the frequency so that the user is able to distinguish between frequent and less frequent routes?

I can imagine various solutions, including

1. stroke width,
2. dotted/dashed line width varying gaps, or
3. color gradient.

Are there best practices?