The simplicity of this routine is why I like it so much. You simply run the command and then select a polyline and it displays temporary arrows that show the direction of the polyline. Once you either zoom in or out, or even use RE [enter] for a REGEN, the temporary arrows will stop displaying.

All credit goes to Luis Esquival and RonJon from the.swamp.org for sharing and editing this routine.

Below – Various polylines have their directions shown

As an extra bonus, this routine works on 3DPOLYLINEs. Even though the arrows aren’t exactly aligned, it is still nice to have a way to show the direction…

~enjoy

;; Original Code by Luis Esquival http://www.theswamp.org/index.php?topic=9441.msg169894#msg169894
;; Displays the direction of polylines with temporary arrows
;;
;; Modified by RonJonP
;; http://www.theswamp.org/index.php?topic=35706.msg409414#msg409414
(vl-load-com)
(defun getarcsegment (cen r fromvertex p2 / a1 a2 d)
  (if (and fromvertex p2)
    (progn (setq a1 (angle cen fromvertex)
		 a2 (angle cen p2)
	   )
	   (if (or (< a1 a2) (equal a1 a2 0.001))
	     (setq d (* r (- a2 a1)))
	     (setq d (* r (- (+ 6.2831853 a2) a1)))
	   )
    )
    ;; is a circle
    (setq d (* r 6.2831853))
  )
)

(defun getbulgedata (bulge fromvertex p2 / dir theta beta radio dat)
  (setq	dir   (cond ((minusp bulge) -1.0)
		    (t 1.0)
	      )
	theta (* 4.0 (atan (abs bulge)))
  )
  (if (> theta pi)
    (setq theta	(- (* 2.0 pi) theta)
	  dir	(* -1.0 dir)
    )
  )
  (setq	theta (/ theta 2.0)
	radio (abs (/ (distance fromvertex p2) (* 2.0 (abs (sin theta)))))
	beta  (+ (angle fromvertex p2) (* (- (/ pi 2.0) theta) dir))
	pc    (polar fromvertex beta radio)
  )
  (getarcsegment pc radio p2 fromvertex)
)

(defun getlwpolydata
       (vla_poly / name endparam param closed fromvertex p2 midp bulge vlist)
  (setq closed (vla-get-closed vla_poly))
  (setq endparam (vlax-curve-getendparam vla_poly))
  (setq param endparam)
  (setq i 0)
  (while (> param 0)
    (setq param (1- param))
    (setq fromvertex (vlax-curve-getpointatparam vla_poly i))
    (if	(vlax-property-available-p vla_poly 'bulge)
      (setq bulge (vla-getbulge vla_poly (fix i)))
    )
    (setq nextvertex (vlax-curve-getpointatparam vla_poly (+ i 1)))
    (setq dis (distance fromvertex nextvertex))
    (setq midpt (vlax-curve-getpointatparam vla_poly (+ i 0.5)))
    (if	(and bulge (not (zerop bulge)))
      (progn (setq bulge (getbulgedata bulge fromvertex nextvertex))
	     (setq etype "ARC")
      )
      (progn bulge (setq etype "LINE"))
    )
;;;;;;    (if	(not :rcmPrefixArcText)
;;;;;;      (setq :rcmPrefixArcText "L="))
    (setq vlist	(cons (list ;; vertex number
			    (+ i 1)
			    ;; object type
			    etype
			    ;; midpoint
			    midpt
			    ;; start vertex
			    fromvertex
			    ;; ending vertex
			    nextvertex
			    ;; curved or straight length
;;;;;;	       (if (= eType "ARC")
;;;;;;		 (strcat
;;;;;;		   :rcmPrefixArcText
;;;;;;		   (rtos bulge (rcmd-getUnits-mode) :rcmPrec))
;;;;;;		 ;; is straight
;;;;;;		 (rtos dis (rcmd-getUnits-mode) :rcmPrec))
		      )
		      vlist
		)
    )
    (setq i (1+ i))
  )
  (reverse vlist)
)

(defun dib_flechdir (lst_dat / unidad angf dirf pfm pf1 pf2 pf3 pf4 pftemp)
  ;; set arrow length according to screen height
  ;; to draw the same arrows at any level of zoom
  (setq unidad (/ (getvar "VIEWSIZE") 15))
  (foreach dat lst_dat
    (setq angf (cadr dat)
	  dirf (caddr dat)
	  pfm  (polar (car dat) (+ angf (/ pi 2)) (* unidad 0.3))
	  pf1  (polar pfm (- angf pi) (/ unidad 2.0))
	  pf2  (polar pfm angf (/ unidad 2.0))
    )
    (if	(= dirf 1)
      (setq pf3	(polar pf2 (- angf (/ (* pi 5.0) 6.0)) (/ unidad 4.0))
	    pf4	(polar pf2 (+ angf (/ (* pi 5.0) 6.0)) (/ unidad 4.0))
      )
      (setq pftemp pf1
	    pf1	   pf2
	    pf2	   pftemp
	    pf3	   (polar pf2 (+ angf (/ pi 6.0)) (/ unidad 4.0))
	    pf4	   (polar pf2 (- angf (/ pi 6.0)) (/ unidad 4.0))
      )
    )
    (if	flag_dir
      (progn ;; draw green arrow
	     ;; when you are changing direction
	     (grdraw pf1 pf2 3)
	     (grdraw pf2 pf3 3)
	     (grdraw pf2 pf4 3)
      )
      (progn ;; draw arrow
	     (grdraw pf1 pf2 4)
	     (grdraw pf2 pf3 4)
	     (grdraw pf2 pf4 4)
      )
    )
  )
  (setq flag_dir nil)
)

;;; Command for test...
(defun c:PLD (/ pol obj pol_data)
  (setq	pol	 (car (entsel "\nSelect polyline: "))
	obj	 (vlax-ename->vla-object pol)
	pol_data (getlwpolydata obj)
  )
  (dib_flechdir
    (setq lst_dat
	   (vl-remove
	     nil
	     (mapcar (function (lambda (i)
				 (if (nth 2 i)
				   (list (nth 2 i) (angle (nth 3 i) (nth 4 i)) 1)
				 )
			       )
		     )
		     pol_data
	     )
	   )
    )
  )
  (princ)
)