As with most moving vehicles and transportation methods its well accepted that seatbelts or support straps increase safety for the user and can also keep a person firmly in position with less unwanted movement while seated.
For wheelchairs you can purchase lap straps that usually have a lock fastener that is easy to click into place and equally as easy to undo but most importantly it will keep you safe and secure while seated.
They are adjustable too so that you can set it to just the right tightness for yourself so that you’re completely comfortable while the strap is fastened.
A lap strap will usually be quite cheap too as it is a basic addition for your wheelchair, do make sure that the strap that you purchase has been designed for use with wheelchairs though, some straps are sold that are suitable for wheelchairs and mobility scooters, these are fine also, ideally if you purchase from a mobility product retailer you will know that you’re purchasing the right type of wheelchair strap.
With prices from around £10, you can find a wide range of safety straps on Amazon.