constant speed on animation path

[whj]

Hello, By pressing 'a', my script simply starts a car in a road. My question is how to make the car move in a constant speed. I use path.constantspeed(), but seems to me that the car will accelerate when it move along the circle. Does the way I add control point have problem?

import viz
import math
import time

viz.go()


myroad = viz.add('road4.wrl')
myroad.setPosition(0,0.1,0)
myroad.setScale(3.2,3.2,3.2)

mymini = viz.add('mini.osgx')
mymini.setPosition(-10,-10,0)

y_pos_car = 0.14
path = viz.addAnimationPath()
positions = [ [2,y_pos_car,50], [1,y_pos_car,35], [0,y_pos_car,25], [-1,y_pos_car,15], 
				[-1,y_pos_car,8], [-25,y_pos_car,0], [-15,y_pos_car,-8], [-10,y_pos_car,-10], 
				[-6,y_pos_car,-10], [-3,y_pos_car,-12], [0,y_pos_car,-18], [3,y_pos_car,-17], 
				[6,y_pos_car,0], [10,y_pos_car,4], [15,y_pos_car,2], [25,y_pos_car,-8], 
				[35,y_pos_car,-16], [50,y_pos_car,-10], [60,y_pos_car,20], [80,y_pos_car,40],
				[200,y_pos_car,200] ]
for x in range(0,len(positions)):
	cp = viz.add(viz.CONTROL_POINT)
	cp.setPosition(positions[x])
	path.add(cp,x+1)

path.loop(viz.OFF)
path.translatemode(viz.BEZIER)
path.setAutoRotate(viz.ON)
mymini.link(path)

	
def mytimer(num):
	pass

def onkeydown(key):
	if key == 'a': 
		path.play()
		path.constantspeed(viz.ON,6.7)
		


viz.callback(viz.KEYBOARD_EVENT, onkeydown) #keyboard event callback
viz.callback(viz.TIMER_EVENT, mytimer) #timer event callback
viz.starttimer(0,1/60,viz.FOREVER) #timer

BTW, I cannot attach my models zip file here because it always said "Upload of file failed". What should I try?

Thanks a lot!