Youths reportedly jumping off a highway bridge before one of three serious crashes on major Hawke's Bay roads in 15 hours has prompted a warning from police.

District road policing manager Matt Broderick said police had received more than 25 calls in recent weeks, reporting people climbing and jumping into the river off the Twyford Hawke's Bay Expressway bridge.

The bridge is narrow, has no pedestrian walkway, and the traffic speed limit is 100km/h.

On Thursday afternoon a truck driver was trapped in the cab of his vehicle after crashing on the bridge about 4.10pm.

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A man in his 50s remained in Hawke's Bay Hospital in a stable condition on Friday afternoon, while three other male patients were assessed and later discharged.

While there was speculation that the crash was the result of distraction caused by youths leaping from the bridge, Boderick said no such behaviour was seen by police.

Police also questioned people below the bridge, but they denied any jumping activity.

Boderick said police strongly discouraged people from walking on to and jumping from the bridge, regardless of the cause of the crash.

Boderick said vehicles passing within a metre of each other on the bridge at 100km/h along with heavy vehicles travelling at speed could generate "significant air movement" which could potentially pull someone into the path of traffic.

"People walking on the bridge put not only themselves, but motorists at risk.

"A driver who swerves to avoid a person on the side of the road risks swerving into an oncoming vehicle, causing potential risk to themselves, other motorists, and pedestrians."

The crash was one of three in just 15 hours on Hawke's Bay highways.

At 6.05pm on Thursday, police were called to a crash on Pakowhai Rd where two people suffered moderate injuries and another minor injuries.

All were taken to Hawke's Bay Hospital. A hospital spokesperson said an update on their condition on Friday was not readily available.

Then on Friday morning, State Highway 50A in Hastings was closed after a serious two-car crash about 7.30am.

The road was closed between the intersections with Omahu Rd and York Rd with diversions in place for some time.

A Hawke's Bay Hospital spokesperson said a 17-year-old man was in a stable condition after the crash while a woman was assessed and discharged.

The NZ Transport Agency said extreme care on the roads was needed as the wettest and windiest weather in a few months heads in Hawke's Bay's direction.

NZTA's regional transport systems manager Oliver Postings said even the smallest amount of rain could make the roads extremely slippery after a long dry spell.

"In fact, it can be more dangerous than heavy rain because grime and exhaust particles that have built up on the road take longer to be washed away.

"The slippery film on the surface of the road is invisible, so don't assume it's not there just because you can't see it," Postings said.