How can I improve traction on an outdoor plywood ramp?
I think the best method is to use an anti slip tape / strip / paint

Tape



Strip



Paint



They are ordered form easiest to hardest.

The strips will be the easiest because you just stick them on. You will obviously stick them on the length where your tires will be going and not sideways (because that would be pointless). The strips are more durable, but don't be deceived by the stick on ones once they are stuck.. they are STUCK! The best, but most work is painting the whole ramp.

Preparation.

Needs to be a dry day for sticking on or painting. You need to clean the surface with paper towels - after you wiped the dirt away and the surface you want to stick/paint is dry you will use more paper towels and cheap alcohol (methanol, spirits, etc) - Using the alcohol cleans the surface and removes oil (so don't touch it after you wiped it). Clean it almost like waxing (several times the same spot), this will guarantee your surface is dirt free and guarantees best adhesion for years and years of life.

The strips you can just put down - but you need some cordless drill or elbow grease to get those screws in. Make sure they are flush (level with the strip) or you risk slashing your tires.

Here are some links

http://www.vubasupplies.co.uk/anti-slip-surfaces.html

http://www.anti-slip-paint.co.uk/

http://www.antislip.uk.com/antislip-decking-strip.html