Many motorists struggle with variable school-zone speed limits and are going too fast through areas where children need to cross the road, pedestrian advocates say.

Reports today said more than 20,000 vehicles — or 200 a day — had been snapped speeding in NSW school zones in the first half of this year.

Most school zones are marked with a fixed sign telling drivers to slow down to 40km/h during school hours, but there are only 165 large flashing signs in NSW.

The state government has announced a program worth $46.5 million to install a further 400 flashing school zone signs over the four years.  

But motorists and safety advocates say this is not enough.

Pedestrian Council chairman Harold Scruby said many people who went too fast outside schools simply failed to notice they had entered the 40km/h zone.

"We believe that there should be flashing lights in all school zones along major streets and highways," Mr Scruby said.

"It's very difficult to remember the school zone times and remembering where every new school zone starts and ends.

"It's an absolute joke … The roads minister is sacrificing the lives of children and adults."

But the NSW Roads and Traffic Authority (RTA) disputed accusations that there weren't enough signs in place.

"We found no one treatment fits all road environments and not all environments benefit from flashing light systems," a RTA spokesman said.

"In addition to roadside signs, there are also bright yellow pavement markers on the road with the number 40 to reinforce the speed limit."

An RTA evaluation found that motorists entering school zones with flashing signs enter on average 6.1km/h slower, with reductions of up to 20km/h.

It also found that the introduction of flashing lights improved compliance.

Prior to the installation of the flashing signs, 34 percent of motorists adhered to the speed limit, but after installation compliance increased to 54 percent.

However, there are only 165 flashing signs in NSW, divided between 3000 school zones.