from experience, someone just holding your hand and being calm has an amazingly calming effect. also there was an off duty nurse talking to me, and just having someone just distract me was calming. she asked me loads of questions, like my name, where was i going, where did i work. anything to stop me thinking about what was currently happening to me. she also managed to get my phone out of my bag and call my mum (i couldn't remembed anyones number) and i was so pleased that as she did it she walked out of earshot - i knew my mum's reaction would not be great. apparently she also didnt give my mum any real details, just that I was involved in a collision with a lorry but was talking and paramedics were on their way. i think my mum would have had a nervoud break down if she had said i was crushed under the wheel.
having someone in calm control of the situation makes a huge difference. also someone who is patient but firm with you when you try to do silly things that you shouldn't. you might get annoyed at the time, but after are so grateful. also someone to stop the driver doing somethign silly, even if they are trying to help. they are likely to be in shock and not functioning properly either.