When picking up the pieces after an accident, you may have to rely on good Samaritans for evidence. A strong case relies on as much evidence as possible.

In today’s digitally focused world, the vast majority of people carry a device capable of recording video at all times. Some even whip them out within milliseconds to record something out of the ordinary. However, they have no legal obligation to share these with you.

In the event that you have been captured on video, the only rights you have in relation to this file is to block it from being shared to the public, be it online or peer-to-peer (Bluetooth, airdrop, etc.).

If you notice someone recording an incident you have been involved in, your best course of action is to politely ask for their details and to have the evidence shared with you. 
If they are agreeable, the video or image can be shared in various ways, which I will detail below.

Email.

Most email servers limit uploads to 20mb, so sharing video over email is almost impossible.
What you can do is share email addresses and ask that the video be uploaded and shared via google drive or dropbox.
These services will enable the uploader to share a link to the file, which will be available for download.

Phone to phone, device to device.

Apple to Apple: iphone to iphone, ipad to ipad, imac to imac or any configuration of these is easy, simple and efficient. Apple products communicate with each other through both wi-fi and Bluetooth, so sharing files is possible even without an internet connection. Both devices need to have Bluetooth active and wi-fi active and should have Airdrop enabled.
Here is a more in-depth guide to Airdrop on the Apple website.

Android to Android: Devices, such as those made by Samsung, Huawei and Google, use Quick Share. It’s much the same as Airdrop but is exclusive to android devices. The two cannot be used together, so this method will require your party and the witness party to have compatible devices. Here is a more in-depth guide to Quick Share on Google’s support website.

Online video services.

Online video services, such as Youtube, allow you to share videos privately to only those who have a direct link to it.
The witness party will need to upload this file to Youtube and set it to “unlisted”.
Setting the video to “Private” will only allow the uploader to view it. Here is a guide to Youtube upload privacy settings on Google’s support website.

Messaging apps.

Text: Sharing videos through text is possible but very expensive. I would generally discourage this option just for the cost.

Whatsapp: You can alternatively use the free messaging app “Whatsapp” to transfer large files. 
The file size limit on this app is 2gb, which should be more than enough to share evidence. 
Whatsapp just requires a phone number to use and works essentially the same as text messaging. Here is a detailed guide on how to share files through Whatsapp.

Facebook messenger: Facebook messenger is another viable alternative to text, but it can’t handle files any bigger than 100mb. This should be enough for most pieces of video evidence but if the witness has recorded at 4k, the size of the file may be too big to share via this method.
Here is a video tutorial from the official Messenger account on Facebook.