It happened on the B3081 near Verwood - he had to be cut free by firefighters.
Dorset 'Ready For Olympics Sailing'
Security preparations for the Olympic sailing events have been tested and are ready, according to the crime and security minister.
James Brokenshire says security officials will remain vigilant until the Paralympics are completed.
He was visiting the Olympic sailing venue at Weymouth and Portland to witness procedures carried out on the water by Dorset Police.
The venue is the largest Olympic and Paralympic site outside London and hosts the only other full-time Olympic village.
Mr Brokenshire said:
''Portland, Weymouth and Dorset are ready. Ready for the Games, ready for the sailing, ready for the Olympics and ready to welcome thousands of people to this really special event.
''There's been a huge amount of preparation to deliver on a safe and secure Games for visitors, for competitors and for VIPs who may be coming to Portland and Weymouth.
''We have taken a risk-based approach, an intelligence-led approach, to ensure that what we are seeing is, yes, visible security, but security that doesn't get in the way of the enjoyment of the Games.
''The strong message that I have got from today is that there is very good co-ordination, which I think is so essential in delivering on the security plan.
''Our plans are mature, we've been through a strong testing and exercise programme not only here in Dorset, but also across the country.
''But we remain vigilant and we constantly look at the threat, constantly look at the risk and we will continue to look at those issues all the way through the Games and we won't keep our eyes off that until the Paralympic Games have been completed.''
Unlike previous Olympic sailing events, the waters around Weymouth and Portland will not be closed, allowing the ports to stay open with restrictions during the Games.
But the fields of play will be controlled by marshals, aided by police and Navy personnel.
On Tuesday May 15th, Dorset Police's marine officers took to the water to demonstrate the stopping procedures they could implement as they patrol 50 square miles of seawater for 65 days.
They included firing a long stretch of webbing from a police boat across the front of another vessel in a similar way to how a stinger device is used by traffic police.
Chief Inspector David Dent, maritime security co-ordinator for Dorset Police, said:
''There are a range of tactics available to us to deal with any incoming craft that might pose a threat to the sailing events.
''Those range from coming alongside a vessel and asking them to stop by displaying a sign, by calling them on VHF radio and also getting in a defensive position adjacent to the craft.
''The top end of those tactical options is what was seen out on the water today, which was a thing called running gear entanglement, which basically means deploying a device in front of the vessel which will cause it to stop in a very short distance.''
Security organisations are preparing for a severe to critical threat level, although the country's current threat level is lower.
Issues they are preparing for include anything from someone going overboard to poor weather, protesters and terrorism.
Officers will patrol the water on a variety of vessels including jet-skis, inflatable boats and smaller craft and will be supported by all three military forces.
At the end of last month HMS Bulwark arrived off the Dorset coast to support a test exercise ahead of the sailing and windsurfing events this summer.
It will be based 10 miles off Weymouth and Dorset Police will use it as its command base during the Olympics.
The ship, which will have a specialist unit of Royal Marines on board, has since sailed to Plymouth, but will return in July for the Games.
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